[LoopIdiom] 'logical right-shift until zero' ('count active bits') "on steroids" idiom recognition.

[lebedev.ri]

Summary

I think i've added exhaustive test coverage, and i have verified that alive2 is happy with all the tests,
so in principle i'm fine with landing this without review, but just in case..

This adds support for the "count active bits" pattern, i.e.:

int countActiveBits(unsigned val) {
    int cnt = 0;
    for( ; (val >> cnt) != 0; ++cnt)
        ;
    return cnt;
}

but a somewhat more general one, since that is what i need:

int countActiveBits(unsigned val, int start, int off) {
    int cnt;
    for (cnt = start; val >> (cnt + off); cnt++)
        ;
    return cnt;
}

I've followed in footstep of 'left-shift until bittest' idiom (D91038),
in the sense that iff the ctlz intrinsic is cheap, we'll transform,
regardless of all other factors.

This can have a shocking effect on certain benchmarks:

raw.pixls.us-unique/Olympus/XZ-1$ /repositories/googlebenchmark/tools/compare.py -a benchmarks ~/rawspeed/build-{old,new}/src/utilities/rsbench/rsbench --benchmark_counters_tabular=true --benchmark_min_time=0.00000001 --benchmark_repetitions=128 p1319978.orf
RUNNING: /home/lebedevri/rawspeed/build-old/src/utilities/rsbench/rsbench --benchmark_counters_tabular=true --benchmark_min_time=0.00000001 --benchmark_repetitions=128 p1319978.orf --benchmark_display_aggregates_only=true --benchmark_out=/tmp/tmp49_28zcm
2021-05-09T01:06:05+03:00
Running /home/lebedevri/rawspeed/build-old/src/utilities/rsbench/rsbench
Run on (32 X 3600.24 MHz CPU s)
CPU Caches:
  L1 Data 32 KiB (x16)
  L1 Instruction 32 KiB (x16)
  L2 Unified 512 KiB (x16)
  L3 Unified 32768 KiB (x2)
Load Average: 5.26, 6.29, 3.49
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Benchmark                                                      Time             CPU   Iterations  CPUTime,s CPUTime/WallTime     Pixels Pixels/CPUTime Pixels/WallTime Raws/CPUTime Raws/WallTime WallTime,s
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
p1319978.orf/threads:32/process_time/real_time_mean          145 ms          145 ms          128   0.145319         0.999981   10.1568M       69.8949M        69.8936M      6.88159       6.88146   0.145322
p1319978.orf/threads:32/process_time/real_time_median        145 ms          145 ms          128   0.145317         0.999986   10.1568M       69.8941M        69.8931M      6.88151       6.88141   0.145319
p1319978.orf/threads:32/process_time/real_time_stddev      0.766 ms        0.766 ms          128   766.586u         15.1302u          0       354.167k        354.098k    0.0348699     0.0348631   766.469u
RUNNING: /home/lebedevri/rawspeed/build-new/src/utilities/rsbench/rsbench --benchmark_counters_tabular=true --benchmark_min_time=0.00000001 --benchmark_repetitions=128 p1319978.orf --benchmark_display_aggregates_only=true --benchmark_out=/tmp/tmpwb9sw2x0
2021-05-09T01:06:24+03:00
Running /home/lebedevri/rawspeed/build-new/src/utilities/rsbench/rsbench
Run on (32 X 3599.95 MHz CPU s)
CPU Caches:
  L1 Data 32 KiB (x16)
  L1 Instruction 32 KiB (x16)
  L2 Unified 512 KiB (x16)
  L3 Unified 32768 KiB (x2)
Load Average: 4.05, 5.95, 3.43
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Benchmark                                                      Time             CPU   Iterations  CPUTime,s CPUTime/WallTime     Pixels Pixels/CPUTime Pixels/WallTime Raws/CPUTime Raws/WallTime WallTime,s
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
p1319978.orf/threads:32/process_time/real_time_mean         99.8 ms         99.8 ms          128  0.0997758         0.999972   10.1568M       101.797M        101.794M      10.0225       10.0222  0.0997786
p1319978.orf/threads:32/process_time/real_time_median       99.7 ms         99.7 ms          128  0.0997165         0.999985   10.1568M       101.857M        101.854M      10.0284       10.0281  0.0997195
p1319978.orf/threads:32/process_time/real_time_stddev      0.224 ms        0.224 ms          128   224.166u          34.345u          0        226.81k        227.231k    0.0223309     0.0223723   224.586u
Comparing /home/lebedevri/rawspeed/build-old/src/utilities/rsbench/rsbench to /home/lebedevri/rawspeed/build-new/src/utilities/rsbench/rsbench
Benchmark                                                               Time             CPU      Time Old      Time New       CPU Old       CPU New
----------------------------------------------------------------------------------------------------------------------------------------------------
p1319978.orf/threads:32/process_time/real_time_pvalue                 0.0000          0.0000      U Test, Repetitions: 128 vs 128
p1319978.orf/threads:32/process_time/real_time_mean                  -0.3134         -0.3134           145           100           145           100
p1319978.orf/threads:32/process_time/real_time_median                -0.3138         -0.3138           145           100           145           100
p1319978.orf/threads:32/process_time/real_time_stddev                -0.7073         -0.7078             1             0             1             0

[lebedev.ri]

Actually add changed tests.

[lebedev.ri]

Hmm, looks like i might also need at least the "[shifty] count leading zeros" variation of this.

[craig.topper]

llvm/lib/Transforms/Scalar/LoopIdiomRecognize.cpp

2605

This should check SuccessorBB?

[lebedev.ri]

@craig.topper thank you for taking a look!
Nit addressed.

[zhuhan0]

llvm/test/Transforms/LoopIdiom/X86/logical-right-shift-until-zero.ll

9–65

I could be wrong but would this mis-compile if %nbits results in unsigned overflow? For example,

%val = 0x10000000
%start = 1
%extraoffset.= 255

Loop trip count is 8 before transformation but 1 after.

[lebedev.ri]

llvm/lib/Transforms/Scalar/LoopIdiomRecognize.cpp

2605

Indeed. Same with the previous transform.

llvm/test/Transforms/LoopIdiom/X86/logical-right-shift-until-zero.ll

9–65

Could you please specify, for which bitwidth your counterexample is?
I'm going to guess i32, so we have

%iv = i32 1
%nbits = i32 256
%val.shifted = lshr i32 %val, 256

We then navigate to https://llvm.org/docs/LangRef.html#lshr-instruction

If op2 is (statically or dynamically) equal to or larger than the number of bits in op1, this instruction returns a poison value.

So i'm not seeing a miscompile.

As i have said, i've verified each of the tests here with alive2, and they are all fine.

[zhuhan0]

llvm/test/Transforms/LoopIdiom/X86/logical-right-shift-until-zero.ll

9–65

8 bits.

%start = i8 1
%extraoffset = i8 255  ; unsigned

Sorry I'm not familiar with alive2 so forgive me if I'm questioning something that's obviously proven to be correct.

[lebedev.ri]

llvm/test/Transforms/LoopIdiom/X86/logical-right-shift-until-zero.ll

9–65

Then i do not understand what %val = 0x10000000 means.
What's the decimal value of %val?

[lebedev.ri]

llvm/test/Transforms/LoopIdiom/X86/logical-right-shift-until-zero.ll

9–65

Ah, you mean %val = 128.
So we have

$ cat /tmp/test.ll 
declare void @escape_inner(i8, i8, i8, i1, i8)
declare void @escape_outer(i8, i8, i8, i1, i8)

define i8 @p0() {
entry:
  br label %loop

loop:
  %iv = phi i8 [ 1, %entry ], [ %iv.next, %loop ]
  %nbits = add nsw i8 %iv, 255
  %val.shifted = lshr i8 128, %nbits
  %val.shifted.iszero = icmp eq i8 %val.shifted, 0
  %iv.next = add i8 %iv, 1

  call void @escape_inner(i8 %iv, i8 %nbits, i8 %val.shifted, i1 %val.shifted.iszero, i8 %iv.next)

  br i1 %val.shifted.iszero, label %end, label %loop

end:
  %iv.res = phi i8 [ %iv, %loop ]
  %nbits.res = phi i8 [ %nbits, %loop ]
  %val.shifted.res = phi i8 [ %val.shifted, %loop ]
  %val.shifted.iszero.res = phi i1 [ %val.shifted.iszero, %loop ]
  %iv.next.res = phi i8 [ %iv.next, %loop ]

  call void @escape_outer(i8 %iv.res, i8 %nbits.res, i8 %val.shifted.res, i1 %val.shifted.iszero.res, i8 %iv.next.res)

  ret i8 %iv.res
}
$ ./bin/opt -loop-idiom -mtriple=x86_64 -mcpu=core-avx2 -o - -S /tmp/test.ll | ./bin/opt -O3 -S -o - -
; ModuleID = '<stdin>'
source_filename = "/tmp/test.ll"
target triple = "x86_64"

declare void @escape_inner(i8, i8, i8, i1, i8) local_unnamed_addr #0

declare void @escape_outer(i8, i8, i8, i1, i8) local_unnamed_addr #0

define i8 @p0() local_unnamed_addr #0 {
entry:
  tail call void @escape_inner(i8 1, i8 0, i8 -128, i1 false, i8 2)
  tail call void @escape_inner(i8 2, i8 1, i8 64, i1 false, i8 3)
  tail call void @escape_inner(i8 3, i8 2, i8 32, i1 false, i8 4)
  tail call void @escape_inner(i8 4, i8 3, i8 16, i1 false, i8 5)
  tail call void @escape_inner(i8 5, i8 4, i8 8, i1 false, i8 6)
  tail call void @escape_inner(i8 6, i8 5, i8 4, i1 false, i8 7)
  tail call void @escape_inner(i8 7, i8 6, i8 2, i1 false, i8 8)
  tail call void @escape_inner(i8 8, i8 7, i8 1, i1 false, i8 9)
  tail call void @escape_inner(i8 9, i8 8, i8 undef, i1 true, i8 10)
  tail call void @escape_outer(i8 9, i8 8, i8 undef, i1 true, i8 10)
  ret i8 9
}

attributes #0 = { "target-cpu"="core-avx2" }

@escape_inner() is called 9 times, not 1.

[zhuhan0]

LGTM.

llvm/test/Transforms/LoopIdiom/X86/logical-right-shift-until-zero.ll

9–65

Thanks for the test.

[lebedev.ri]

@craig.topper ?

In D102116#2757904, @zhuhan0 wrote:

LGTM.

Thanks for taking a look!

In D102116#2752152, @lebedev.ri wrote:

Hmm, looks like i might also need at least the "[shifty] count leading zeros" variation of this.

... and i guess it might be an extension of recognizeShiftUntilBitTest().

[lebedev.ri]

@craig.topper ping
Thanks!

[craig.topper]

LGTM

[lebedev.ri]

In D102116#2763934, @craig.topper wrote:

LGTM

Thank you for the review!

I plan to hopefully look into the next idiom soon after.

[uabelho]

Hi,

THere seems to be something problematic with this patch.
With

opt -enable-new-pm=0 -o /dev/null bbi-56362_2.ll -loop-idiom

I get

Instruction does not dominate all uses!
  %1 = load i16, i16* getelementptr inbounds ({ i16, i16 }, { i16, i16 }* @v_92, i32 0, i32 0), align 1
  %.numleadingzeros = call i16 @llvm.ctlz.i16(i16 %1, i1 false)
in function main

and with

opt -enable-new-pm=1 -o /dev/null bbi-56362_2.ll -loop-idiom

I get

opt: ../lib/Transforms/Scalar/LoopPassManager.cpp:253: auto llvm::FunctionToLoopPassAdaptor::run(llvm::Function &, llvm::FunctionAnalysisManager &)::(anonymous class)::operator()(llvm::StringRef, llvm::Any) const: Assertion `L->isRecursivelyLCSSAForm(LAR.DT, LI) && "Loops must remain in LCSSA form!"' failed.

bbi-56362_2.ll1 KBDownload

[lebedev.ri]

Thanks, fixed in aa3dac95edbfb892b6236341b431b222f7bd0926!
Roman

[uabelho]

In D102116#2776646, @lebedev.ri wrote:

Thanks, fixed in aa3dac95edbfb892b6236341b431b222f7bd0926!
Roman

Thanks!

llvm/lib/Transforms/Scalar/LoopIdiomRecognize.cpp

 
 #define DEBUG_TYPE "loop-idiom"
 
 STATISTIC(NumMemSet, "Number of memset's formed from loop stores");
 STATISTIC(NumMemCpy, "Number of memcpy's formed from loop load+stores");
 STATISTIC(
     NumShiftUntilBitTest,
     "Number of uncountable loops recognized as 'shift until bitttest' idiom");
+STATISTIC(NumShiftUntilZero,
+          "Number of uncountable loops recognized as 'shift until zero' idiom");
 
 bool DisableLIRP::All;
 static cl::opt<bool, true>
     DisableLIRPAll("disable-" DEBUG_TYPE "-all",
                    cl::desc("Options to disable Loop Idiom Recognize Pass."),
                    cl::location(DisableLIRP::All), cl::init(false),
                    cl::ReallyHidden);
 
private:
   bool recognizeAndInsertFFS();  /// Find First Set: ctlz or cttz
   void transformLoopToCountable(Intrinsic::ID IntrinID, BasicBlock *PreCondBB,
                                 Instruction *CntInst, PHINode *CntPhi,
                                 Value *Var, Instruction *DefX,
                                 const DebugLoc &DL, bool ZeroCheck,
                                 bool IsCntPhiUsedOutsideLoop);
 
   bool recognizeShiftUntilBitTest();
+  bool recognizeShiftUntilZero();
 
   /// @}
 };
 
 class LoopIdiomRecognizeLegacyPass : public LoopPass {
 public:
   static char ID;
 

 
 bool LoopIdiomRecognize::runOnNoncountableLoop() {
   LLVM_DEBUG(dbgs() << DEBUG_TYPE " Scanning: F["
                     << CurLoop->getHeader()->getParent()->getName()
                     << "] Noncountable Loop %"
                     << CurLoop->getHeader()->getName() << "\n");
 
   return recognizePopcount() || recognizeAndInsertFFS() ||
-         recognizeShiftUntilBitTest();
+         recognizeShiftUntilBitTest() || recognizeShiftUntilZero();
 }
 
 /// Check if the given conditional branch is based on the comparison between
 /// a variable and zero, and if the variable is non-zero or zero (JmpOnZero is
 /// true), the control yields to the loop entry. If the branch matches the
 /// behavior, the variable involved in the comparison is returned. This function
 /// will be called to see if the precondition and postcondition of the loop are
 /// in desirable form.
bool LoopIdiomRecognize::recognizeShiftUntilBitTest() {
 
   // Other passes will take care of actually deleting the loop if possible.
 
   LLVM_DEBUG(dbgs() << DEBUG_TYPE " shift-until-bittest idiom optimized!\n");
 
   ++NumShiftUntilBitTest;
   return MadeChange;
 }
+
+/// Return true if the idiom is detected in the loop.
+///
+/// The core idiom we are trying to detect is:
+/// \code
+///   entry:
+///     <...>
+///     %start = <...>
+///     %extraoffset = <...>
+///     <...>
+///     br label %for.cond
+///
+///   loop:
+///     %iv = phi i8 [ %start, %entry ], [ %iv.next, %for.cond ]
+///     %nbits = add nsw i8 %iv, %extraoffset
+///     %val.shifted = lshr i8 %val, %nbits
+///     %val.shifted.iszero = icmp eq i8 %val.shifted, 0
+///     %iv.next = add i8 %iv, 1
+///     <...>
+///     br i1 %val.shifted.iszero, label %end, label %loop
+///
+///   end:
+///     %iv.res = phi i8 [ %iv, %loop ] <...>
+///     %nbits.res = phi i8 [ %nbits, %loop ] <...>
+///     %val.shifted.res = phi i8 [ %val.shifted, %loop ] <...>
+///     %val.shifted.iszero.res = phi i1 [ %val.shifted.iszero, %loop ] <...>
+///     %iv.next.res = phi i8 [ %iv.next, %loop ] <...>
+///     <...>
+/// \endcode
+static bool detectShiftUntilZeroIdiom(Loop *CurLoop, ScalarEvolution *SE,
+                                      Instruction *&ValShiftedIsZero,
+                                      Instruction *&IV, Value *&Start,
+                                      Value *&Val, const SCEV *&ExtraOffsetExpr,
+                                      bool &InvertedCond) {
+  LLVM_DEBUG(dbgs() << DEBUG_TYPE
+             " Performing shift-until-zero idiom detection.\n");
+
+  // Give up if the loop has multiple blocks or multiple backedges.
+  if (CurLoop->getNumBlocks() != 1 || CurLoop->getNumBackEdges() != 1) {
+    LLVM_DEBUG(dbgs() << DEBUG_TYPE " Bad block/backedge count.\n");
+    return false;
+  }
+
+  Instruction *ValShifted, *NBits, *IVNext;
+  Value *ExtraOffset;
+
+  BasicBlock *LoopHeaderBB = CurLoop->getHeader();
+  BasicBlock *LoopPreheaderBB = CurLoop->getLoopPreheader();
+  assert(LoopPreheaderBB && "There is always a loop preheader.");
+
+  using namespace PatternMatch;
+
+  // Step 1: Check if the loop backedge, condition is in desirable form.
+
+  ICmpInst::Predicate Pred;
+  BasicBlock *TrueBB, *FalseBB;
+  if (!match(LoopHeaderBB->getTerminator(),
+             m_Br(m_Instruction(ValShiftedIsZero), m_BasicBlock(TrueBB),
+                  m_BasicBlock(FalseBB))) ||
+      !match(ValShiftedIsZero,
+             m_ICmp(Pred, m_Instruction(ValShifted), m_Zero())) ||
+      !ICmpInst::isEquality(Pred)) {
+    LLVM_DEBUG(dbgs() << DEBUG_TYPE " Bad backedge structure.\n");
+    return false;
+  }
+
+  // Step 2: Check if the comparison's operand is in desirable form.
+
+  if (!match(ValShifted, m_LShr(m_Value(Val), m_Instruction(NBits)))) {
+    LLVM_DEBUG(dbgs() << DEBUG_TYPE " Bad comparisons value computation.\n");
+    return false;
+  }
+
+  // Step 3: Check if the shift amount is in desirable form.
+
+  if (match(NBits, m_c_Add(m_Instruction(IV),
+                           m_LoopInvariant(m_Value(ExtraOffset), CurLoop))) &&
+      (NBits->hasNoSignedWrap() || NBits->hasNoUnsignedWrap()))
+    ExtraOffsetExpr = SE->getNegativeSCEV(SE->getSCEV(ExtraOffset));
+  else if (match(NBits,
+                 m_Sub(m_Instruction(IV),
+                       m_LoopInvariant(m_Value(ExtraOffset), CurLoop))) &&
+           NBits->hasNoSignedWrap())
+    ExtraOffsetExpr = SE->getSCEV(ExtraOffset);
+  else {
+    IV = NBits;
+    ExtraOffsetExpr = SE->getZero(NBits->getType());
+  }
+
+  // Step 4: Check if the recurrence is in desirable form.
+  auto *IVPN = dyn_cast<PHINode>(IV);
+  if (!IVPN || IVPN->getParent() != LoopHeaderBB) {
+    LLVM_DEBUG(dbgs() << DEBUG_TYPE " Not an expected PHI node.\n");
+    return false;
+  }
+
+  Start = IVPN->getIncomingValueForBlock(LoopPreheaderBB);
+  IVNext = dyn_cast<Instruction>(IVPN->getIncomingValueForBlock(LoopHeaderBB));
+
+  if (!IVNext || !match(IVNext, m_Add(m_Specific(IVPN), m_One()))) {
+    LLVM_DEBUG(dbgs() << DEBUG_TYPE " Bad recurrence.\n");
+    return false;
+  }
+
+  // Step 4: Check if the backedge's destinations are in desirable form.
+
+  assert(ICmpInst::isEquality(Pred) &&
+         "Should only get equality predicates here.");
+
+  // cmp-br is commutative, so canonicalize to a single variant.
+  InvertedCond = Pred != ICmpInst::Predicate::ICMP_EQ;
+  if (InvertedCond) {
+    Pred = ICmpInst::getInversePredicate(Pred);
+    std::swap(TrueBB, FalseBB);
+  }
+
+  // We expect to exit loop when comparison yields true,
+  // so when it yields false we should branch back to loop header.
+  if (FalseBB != LoopHeaderBB) {
+    LLVM_DEBUG(dbgs() << DEBUG_TYPE " Bad backedge flow.\n");
+    return false;
+  }
+
+  // Okay, idiom checks out.
+  return true;
+}
+
+/// Look for the following loop:
+/// \code
+///   entry:
+///     <...>
+///     %start = <...>
+///     %extraoffset = <...>
+///     <...>
+///     br label %for.cond
+///
+///   loop:
+///     %iv = phi i8 [ %start, %entry ], [ %iv.next, %for.cond ]
+///     %nbits = add nsw i8 %iv, %extraoffset
+///     %val.shifted = lshr i8 %val, %nbits
+///     %val.shifted.iszero = icmp eq i8 %val.shifted, 0
+///     %iv.next = add i8 %iv, 1
+///     <...>
+///     br i1 %val.shifted.iszero, label %end, label %loop
+///
+///   end:
+///     %iv.res = phi i8 [ %iv, %loop ] <...>
+///     %nbits.res = phi i8 [ %nbits, %loop ] <...>
+///     %val.shifted.res = phi i8 [ %val.shifted, %loop ] <...>
+///     %val.shifted.iszero.res = phi i1 [ %val.shifted.iszero, %loop ] <...>
+///     %iv.next.res = phi i8 [ %iv.next, %loop ] <...>
+///     <...>
+/// \endcode
+///
+/// And transform it into:
+/// \code
+///   entry:
+///     <...>
+///     %start = <...>
+///     %extraoffset = <...>
+///     <...>
+///     %val.numleadingzeros = call i8 @llvm.ctlz.i8(i8 %val, i1 0)
+///     %val.numactivebits = sub i8 8, %val.numleadingzeros
+///     %extraoffset.neg = sub i8 0, %extraoffset
+///     %tmp = add i8 %val.numactivebits, %extraoffset.neg
+///     %iv.final = call i8 @llvm.smax.i8(i8 %tmp, i8 %start)
+///     %loop.tripcount = sub i8 %iv.final, %start
+///     br label %loop
+///
+///   loop:
+///     %loop.iv = phi i8 [ 0, %entry ], [ %loop.iv.next, %loop ]
+///     %loop.iv.next = add i8 %loop.iv, 1
+///     %loop.ivcheck = icmp eq i8 %loop.iv.next, %loop.tripcount
+///     %iv = add i8 %loop.iv, %start
+///     <...>
+///     br i1 %loop.ivcheck, label %end, label %loop
+///
+///   end:
+///     %iv.res = phi i8 [ %iv.final, %loop ] <...>
+///     <...>
+/// \endcode
+bool LoopIdiomRecognize::recognizeShiftUntilZero() {
+  bool MadeChange = false;
+
+  Instruction *ValShiftedIsZero, *IV;
+  Value *Start, *Val;
+  const SCEV *ExtraOffsetExpr;
+  bool InvertedCond;
+  if (!detectShiftUntilZeroIdiom(CurLoop, SE, ValShiftedIsZero, IV, Start, Val,
+                                 ExtraOffsetExpr, InvertedCond)) {
+    LLVM_DEBUG(dbgs() << DEBUG_TYPE
+               " shift-until-zero idiom detection failed.\n");
+    return MadeChange;
+  }
+  LLVM_DEBUG(dbgs() << DEBUG_TYPE " shift-until-zero idiom detected!\n");
+
+  // Ok, it is the idiom we were looking for, we *could* transform this loop,
+  // but is it profitable to transform?
+
+  BasicBlock *LoopHeaderBB = CurLoop->getHeader();
+  BasicBlock *LoopPreheaderBB = CurLoop->getLoopPreheader();
+  assert(LoopPreheaderBB && "There is always a loop preheader.");
+
+  BasicBlock *SuccessorBB = CurLoop->getExitBlock();
+  assert(LoopPreheaderBB && "There is only a single successor.");
+
+  IRBuilder<> Builder(LoopPreheaderBB->getTerminator());
+  Builder.SetCurrentDebugLocation(IV->getDebugLoc());
+
+  Intrinsic::ID IntrID = Intrinsic::ctlz;
+  Type *Ty = Val->getType();
+  unsigned Bitwidth = Ty->getScalarSizeInBits();
+
+  TargetTransformInfo::TargetCostKind CostKind =
+      TargetTransformInfo::TCK_SizeAndLatency;
+
+  // The rewrite is considered to be unprofitable iff and only iff the
+  // intrinsic we'll use are not cheap. Note that we are okay with *just*
+  // making the loop countable, even if nothing else changes.
+  IntrinsicCostAttributes Attrs(
+      IntrID, Ty, {UndefValue::get(Ty), /*is_zero_undef=*/Builder.getFalse()});
+  InstructionCost Cost = TTI->getIntrinsicInstrCost(Attrs, CostKind);
+  if (Cost > TargetTransformInfo::TCC_Basic) {
+    LLVM_DEBUG(dbgs() << DEBUG_TYPE
+               " Intrinsic is too costly, not beneficial\n");
+    return MadeChange;
+  }
+
+  // Ok, transform appears worthwhile.
+  MadeChange = true;
+
+  bool OffsetIsZero = false;
+  if (auto *ExtraOffsetExprC = dyn_cast<SCEVConstant>(ExtraOffsetExpr))
+    OffsetIsZero = ExtraOffsetExprC->isZero();
+
+  // Step 1: Compute the loop's final IV value / trip count.
+
+  CallInst *ValNumLeadingZeros = Builder.CreateIntrinsic(
+      IntrID, Ty, {Val, /*is_zero_undef=*/Builder.getFalse()},
+      /*FMFSource=*/nullptr, Val->getName() + ".numleadingzeros");
+  Value *ValNumActiveBits = Builder.CreateSub(
+      ConstantInt::get(Ty, Ty->getScalarSizeInBits()), ValNumLeadingZeros,
+      Val->getName() + ".numactivebits", /*HasNUW=*/true,
+      /*HasNSW=*/Bitwidth != 2);
+
+  SCEVExpander Expander(*SE, *DL, "loop-idiom");
+  Expander.setInsertPoint(&*Builder.GetInsertPoint());
+  Value *ExtraOffset = Expander.expandCodeFor(ExtraOffsetExpr);
+
+  Value *ValNumActiveBitsOffset = Builder.CreateAdd(
+      ValNumActiveBits, ExtraOffset, ValNumActiveBits->getName() + ".offset",
+      /*HasNUW=*/OffsetIsZero, /*HasNSW=*/true);
+  Value *IVFinal = Builder.CreateIntrinsic(Intrinsic::smax, {Ty},
+                                           {ValNumActiveBitsOffset, Start},
+                                           /*FMFSource=*/nullptr, "iv.final");
+
+  auto *LoopBackedgeTakenCount = cast<Instruction>(Builder.CreateSub(
+      IVFinal, Start, CurLoop->getName() + ".backedgetakencount",
+      /*HasNUW=*/OffsetIsZero, /*HasNSW=*/true));
+  // FIXME: or when the offset was `add nuw`
+
+  // We know loop's backedge-taken count, but what's loop's trip count?
+  Value *LoopTripCount =
+      Builder.CreateAdd(LoopBackedgeTakenCount, ConstantInt::get(Ty, 1),
+                        CurLoop->getName() + ".tripcount", /*HasNUW=*/true,
+                        /*HasNSW=*/Bitwidth != 2);
+
+  // Step 2: Adjust the successor basic block to recieve the original
+  //         induction variable's final value instead of the orig. IV itself.
+
+  IV->replaceUsesOutsideBlock(IVFinal, LoopHeaderBB);
+
+  // Step 3: Rewrite the loop into a countable form, with canonical IV.
+
+  // The new canonical induction variable.
+  Builder.SetInsertPoint(&LoopHeaderBB->front());
+  auto *CIV = Builder.CreatePHI(Ty, 2, CurLoop->getName() + ".iv");
+
+  // The induction itself.
+  Builder.SetInsertPoint(LoopHeaderBB->getFirstNonPHI());
+  auto *CIVNext =
+      Builder.CreateAdd(CIV, ConstantInt::get(Ty, 1), CIV->getName() + ".next",
+                        /*HasNUW=*/true, /*HasNSW=*/Bitwidth != 2);
+
+  // The loop trip count check.
+  auto *CIVCheck = Builder.CreateICmpEQ(CIVNext, LoopTripCount,
+                                        CurLoop->getName() + ".ivcheck");
+  auto *NewIVCheck = CIVCheck;
+  if (InvertedCond) {
+    NewIVCheck = Builder.CreateNot(CIVCheck);
+    NewIVCheck->takeName(ValShiftedIsZero);
+  }
+
+  // The original IV, but rebased to be an offset to the CIV.
+  auto *IVDePHId = Builder.CreateAdd(CIV, Start, "", /*HasNUW=*/false,
+                                     /*HasNSW=*/true); // FIXME: what about NUW?
+  IVDePHId->takeName(IV);
+
+  // The loop terminator.
+  Builder.SetInsertPoint(LoopHeaderBB->getTerminator());
+  Builder.CreateCondBr(CIVCheck, SuccessorBB, LoopHeaderBB);
+  LoopHeaderBB->getTerminator()->eraseFromParent();
+
+  // Populate the IV PHI.
+  CIV->addIncoming(ConstantInt::get(Ty, 0), LoopPreheaderBB);
+  CIV->addIncoming(CIVNext, LoopHeaderBB);
+
+  // Step 4: Forget the "non-computable" trip-count SCEV associated with the
+  //   loop. The loop would otherwise not be deleted even if it becomes empty.
+
+  SE->forgetLoop(CurLoop);
+
+  // Step 5: Try to cleanup the loop's body somewhat.
+  IV->replaceAllUsesWith(IVDePHId);
+  IV->eraseFromParent();
+
+  ValShiftedIsZero->replaceAllUsesWith(NewIVCheck);
+  ValShiftedIsZero->eraseFromParent();
+
+  // Other passes will take care of actually deleting the loop if possible.
+
+  LLVM_DEBUG(dbgs() << DEBUG_TYPE " shift-until-zero idiom optimized!\n");
+
+  ++NumShiftUntilZero;
+  return MadeChange;
+}

llvm/test/Transforms/LoopIdiom/X86/logical-right-shift-until-zero-debuginfo.ll

 ; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
 ; RUN: opt -debugify -loop-idiom -mtriple=x86_64 -mcpu=core-avx2 < %s -S | FileCheck %s
 
 declare void @escape_inner(i8, i8, i8, i1, i8)
 declare void @escape_outer(i8, i8, i8, i1, i8)
 
 define i8 @p(i8 %val, i8 %start, i8 %extraoffset) {
 ; CHECK-LABEL: @p(
 ; CHECK-NEXT:  entry:
-; CHECK-NEXT:    br label [[LOOP:%.*]], !dbg [[DBG20:![0-9]+]]
+; CHECK-NEXT:    [[VAL_NUMLEADINGZEROS:%.*]] = call i8 @llvm.ctlz.i8(i8 [[VAL:%.*]], i1 false), !dbg [[DBG20:![0-9]+]]
+; CHECK-NEXT:    [[VAL_NUMACTIVEBITS:%.*]] = sub nuw nsw i8 8, [[VAL_NUMLEADINGZEROS]], !dbg [[DBG20]]
+; CHECK-NEXT:    [[TMP0:%.*]] = sub i8 0, [[EXTRAOFFSET:%.*]], !dbg [[DBG21:![0-9]+]]
+; CHECK-NEXT:    [[VAL_NUMACTIVEBITS_OFFSET:%.*]] = add nsw i8 [[VAL_NUMACTIVEBITS]], [[TMP0]], !dbg [[DBG20]]
+; CHECK-NEXT:    [[IV_FINAL:%.*]] = call i8 @llvm.smax.i8(i8 [[VAL_NUMACTIVEBITS_OFFSET]], i8 [[START:%.*]]), !dbg [[DBG20]]
+; CHECK-NEXT:    [[LOOP_BACKEDGETAKENCOUNT:%.*]] = sub nsw i8 [[IV_FINAL]], [[START]], !dbg [[DBG20]]
+; CHECK-NEXT:    [[LOOP_TRIPCOUNT:%.*]] = add nuw nsw i8 [[LOOP_BACKEDGETAKENCOUNT]], 1, !dbg [[DBG20]]
+; CHECK-NEXT:    br label [[LOOP:%.*]], !dbg [[DBG21]]
 ; CHECK:       loop:
-; CHECK-NEXT:    [[IV:%.*]] = phi i8 [ [[START:%.*]], [[ENTRY:%.*]] ], [ [[IV_NEXT:%.*]], [[LOOP]] ], !dbg [[DBG21:![0-9]+]]
-; CHECK-NEXT:    call void @llvm.dbg.value(metadata i8 [[IV]], metadata [[META9:![0-9]+]], metadata !DIExpression()), !dbg [[DBG21]]
-; CHECK-NEXT:    [[NBITS:%.*]] = add nsw i8 [[IV]], [[EXTRAOFFSET:%.*]], !dbg [[DBG22:![0-9]+]]
+; CHECK-NEXT:    [[LOOP_IV:%.*]] = phi i8 [ 0, [[ENTRY:%.*]] ], [ [[LOOP_IV_NEXT:%.*]], [[LOOP]] ], !dbg [[DBG20]]
+; CHECK-NEXT:    [[LOOP_IV_NEXT]] = add nuw nsw i8 [[LOOP_IV]], 1, !dbg [[DBG20]]
+; CHECK-NEXT:    [[LOOP_IVCHECK:%.*]] = icmp eq i8 [[LOOP_IV_NEXT]], [[LOOP_TRIPCOUNT]], !dbg [[DBG20]]
+; CHECK-NEXT:    [[IV:%.*]] = add nsw i8 [[LOOP_IV]], [[START]], !dbg [[DBG20]]
+; CHECK-NEXT:    call void @llvm.dbg.value(metadata i8 [[IV]], metadata [[META9:![0-9]+]], metadata !DIExpression()), !dbg [[DBG20]]
+; CHECK-NEXT:    [[NBITS:%.*]] = add nsw i8 [[IV]], [[EXTRAOFFSET]], !dbg [[DBG22:![0-9]+]]
 ; CHECK-NEXT:    call void @llvm.dbg.value(metadata i8 [[NBITS]], metadata [[META11:![0-9]+]], metadata !DIExpression()), !dbg [[DBG22]]
-; CHECK-NEXT:    [[VAL_SHIFTED:%.*]] = lshr i8 [[VAL:%.*]], [[NBITS]], !dbg [[DBG23:![0-9]+]]
+; CHECK-NEXT:    [[VAL_SHIFTED:%.*]] = lshr i8 [[VAL]], [[NBITS]], !dbg [[DBG23:![0-9]+]]
 ; CHECK-NEXT:    call void @llvm.dbg.value(metadata i8 [[VAL_SHIFTED]], metadata [[META12:![0-9]+]], metadata !DIExpression()), !dbg [[DBG23]]
-; CHECK-NEXT:    [[VAL_SHIFTED_ISZERO:%.*]] = icmp eq i8 [[VAL_SHIFTED]], 0, !dbg [[DBG24:![0-9]+]]
-; CHECK-NEXT:    call void @llvm.dbg.value(metadata i1 [[VAL_SHIFTED_ISZERO]], metadata [[META13:![0-9]+]], metadata !DIExpression()), !dbg [[DBG24]]
-; CHECK-NEXT:    [[IV_NEXT]] = add i8 [[IV]], 1, !dbg [[DBG25:![0-9]+]]
+; CHECK-NEXT:    call void @llvm.dbg.value(metadata i1 [[LOOP_IVCHECK]], metadata [[META13:![0-9]+]], metadata !DIExpression()), !dbg [[DBG24:![0-9]+]]
+; CHECK-NEXT:    [[IV_NEXT:%.*]] = add i8 [[IV]], 1, !dbg [[DBG25:![0-9]+]]
 ; CHECK-NEXT:    call void @llvm.dbg.value(metadata i8 [[IV_NEXT]], metadata [[META14:![0-9]+]], metadata !DIExpression()), !dbg [[DBG25]]
-; CHECK-NEXT:    call void @escape_inner(i8 [[IV]], i8 [[NBITS]], i8 [[VAL_SHIFTED]], i1 [[VAL_SHIFTED_ISZERO]], i8 [[IV_NEXT]]), !dbg [[DBG26:![0-9]+]]
-; CHECK-NEXT:    br i1 [[VAL_SHIFTED_ISZERO]], label [[END:%.*]], label [[LOOP]], !dbg [[DBG27:![0-9]+]]
+; CHECK-NEXT:    call void @escape_inner(i8 [[IV]], i8 [[NBITS]], i8 [[VAL_SHIFTED]], i1 [[LOOP_IVCHECK]], i8 [[IV_NEXT]]), !dbg [[DBG26:![0-9]+]]
+; CHECK-NEXT:    br i1 [[LOOP_IVCHECK]], label [[END:%.*]], label [[LOOP]], !dbg [[DBG27:![0-9]+]]
 ; CHECK:       end:
-; CHECK-NEXT:    [[IV_RES:%.*]] = phi i8 [ [[IV]], [[LOOP]] ], !dbg [[DBG28:![0-9]+]]
+; CHECK-NEXT:    [[IV_RES:%.*]] = phi i8 [ [[IV_FINAL]], [[LOOP]] ], !dbg [[DBG28:![0-9]+]]
 ; CHECK-NEXT:    [[NBITS_RES:%.*]] = phi i8 [ [[NBITS]], [[LOOP]] ], !dbg [[DBG29:![0-9]+]]
 ; CHECK-NEXT:    [[VAL_SHIFTED_RES:%.*]] = phi i8 [ [[VAL_SHIFTED]], [[LOOP]] ], !dbg [[DBG30:![0-9]+]]
-; CHECK-NEXT:    [[VAL_SHIFTED_ISZERO_RES:%.*]] = phi i1 [ [[VAL_SHIFTED_ISZERO]], [[LOOP]] ], !dbg [[DBG31:![0-9]+]]
+; CHECK-NEXT:    [[VAL_SHIFTED_ISZERO_RES:%.*]] = phi i1 [ [[LOOP_IVCHECK]], [[LOOP]] ], !dbg [[DBG31:![0-9]+]]
 ; CHECK-NEXT:    [[IV_NEXT_RES:%.*]] = phi i8 [ [[IV_NEXT]], [[LOOP]] ], !dbg [[DBG32:![0-9]+]]
 ; CHECK-NEXT:    call void @llvm.dbg.value(metadata i8 [[IV_RES]], metadata [[META15:![0-9]+]], metadata !DIExpression()), !dbg [[DBG28]]
 ; CHECK-NEXT:    call void @llvm.dbg.value(metadata i8 [[NBITS_RES]], metadata [[META16:![0-9]+]], metadata !DIExpression()), !dbg [[DBG29]]
 ; CHECK-NEXT:    call void @llvm.dbg.value(metadata i8 [[VAL_SHIFTED_RES]], metadata [[META17:![0-9]+]], metadata !DIExpression()), !dbg [[DBG30]]
 ; CHECK-NEXT:    call void @llvm.dbg.value(metadata i1 [[VAL_SHIFTED_ISZERO_RES]], metadata [[META18:![0-9]+]], metadata !DIExpression()), !dbg [[DBG31]]
 ; CHECK-NEXT:    call void @llvm.dbg.value(metadata i8 [[IV_NEXT_RES]], metadata [[META19:![0-9]+]], metadata !DIExpression()), !dbg [[DBG32]]
 ; CHECK-NEXT:    call void @escape_outer(i8 [[IV_RES]], i8 [[NBITS_RES]], i8 [[VAL_SHIFTED_RES]], i1 [[VAL_SHIFTED_ISZERO_RES]], i8 [[IV_NEXT_RES]]), !dbg [[DBG33:![0-9]+]]
 ; CHECK-NEXT:    ret i8 [[IV_RES]], !dbg [[DBG34:![0-9]+]]

llvm/test/Transforms/LoopIdiom/X86/logical-right-shift-until-zero.ll

 ; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
 ; RUN: opt -loop-idiom -mtriple=x86_64 -mcpu=core-avx2 < %s -S | FileCheck %s
 
 declare void @escape_inner(i8, i8, i8, i1, i8)
 declare void @escape_outer(i8, i8, i8, i1, i8)
 
 declare i8 @gen.i8()
 
 ; Most basic pattern; Note that iff the shift amount is offset, said offsetting
 ; must not cause an overflow, but `add nsw` is fine.
 define i8 @p0(i8 %val, i8 %start, i8 %extraoffset) {
 ; CHECK-LABEL: @p0(
 ; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[VAL_NUMLEADINGZEROS:%.*]] = call i8 @llvm.ctlz.i8(i8 [[VAL:%.*]], i1 false)
+; CHECK-NEXT:    [[VAL_NUMACTIVEBITS:%.*]] = sub nuw nsw i8 8, [[VAL_NUMLEADINGZEROS]]
+; CHECK-NEXT:    [[TMP0:%.*]] = sub i8 0, [[EXTRAOFFSET:%.*]]
+; CHECK-NEXT:    [[VAL_NUMACTIVEBITS_OFFSET:%.*]] = add nsw i8 [[VAL_NUMACTIVEBITS]], [[TMP0]]
+; CHECK-NEXT:    [[IV_FINAL:%.*]] = call i8 @llvm.smax.i8(i8 [[VAL_NUMACTIVEBITS_OFFSET]], i8 [[START:%.*]])
+; CHECK-NEXT:    [[LOOP_BACKEDGETAKENCOUNT:%.*]] = sub nsw i8 [[IV_FINAL]], [[START]]
+; CHECK-NEXT:    [[LOOP_TRIPCOUNT:%.*]] = add nuw nsw i8 [[LOOP_BACKEDGETAKENCOUNT]], 1
 ; CHECK-NEXT:    br label [[LOOP:%.*]]
 ; CHECK:       loop:
-; CHECK-NEXT:    [[IV:%.*]] = phi i8 [ [[START:%.*]], [[ENTRY:%.*]] ], [ [[IV_NEXT:%.*]], [[LOOP]] ]
-; CHECK-NEXT:    [[NBITS:%.*]] = add nsw i8 [[IV]], [[EXTRAOFFSET:%.*]]
-; CHECK-NEXT:    [[VAL_SHIFTED:%.*]] = lshr i8 [[VAL:%.*]], [[NBITS]]
-; CHECK-NEXT:    [[VAL_SHIFTED_ISZERO:%.*]] = icmp eq i8 [[VAL_SHIFTED]], 0
-; CHECK-NEXT:    [[IV_NEXT]] = add i8 [[IV]], 1
-; CHECK-NEXT:    call void @escape_inner(i8 [[IV]], i8 [[NBITS]], i8 [[VAL_SHIFTED]], i1 [[VAL_SHIFTED_ISZERO]], i8 [[IV_NEXT]])
-; CHECK-NEXT:    br i1 [[VAL_SHIFTED_ISZERO]], label [[END:%.*]], label [[LOOP]]
+; CHECK-NEXT:    [[LOOP_IV:%.*]] = phi i8 [ 0, [[ENTRY:%.*]] ], [ [[LOOP_IV_NEXT:%.*]], [[LOOP]] ]
+; CHECK-NEXT:    [[LOOP_IV_NEXT]] = add nuw nsw i8 [[LOOP_IV]], 1
+; CHECK-NEXT:    [[LOOP_IVCHECK:%.*]] = icmp eq i8 [[LOOP_IV_NEXT]], [[LOOP_TRIPCOUNT]]
+; CHECK-NEXT:    [[IV:%.*]] = add nsw i8 [[LOOP_IV]], [[START]]
+; CHECK-NEXT:    [[NBITS:%.*]] = add nsw i8 [[IV]], [[EXTRAOFFSET]]
+; CHECK-NEXT:    [[VAL_SHIFTED:%.*]] = lshr i8 [[VAL]], [[NBITS]]
+; CHECK-NEXT:    [[IV_NEXT:%.*]] = add i8 [[IV]], 1
+; CHECK-NEXT:    call void @escape_inner(i8 [[IV]], i8 [[NBITS]], i8 [[VAL_SHIFTED]], i1 [[LOOP_IVCHECK]], i8 [[IV_NEXT]])
+; CHECK-NEXT:    br i1 [[LOOP_IVCHECK]], label [[END:%.*]], label [[LOOP]]
 ; CHECK:       end:
-; CHECK-NEXT:    [[IV_RES:%.*]] = phi i8 [ [[IV]], [[LOOP]] ]
+; CHECK-NEXT:    [[IV_RES:%.*]] = phi i8 [ [[IV_FINAL]], [[LOOP]] ]
 ; CHECK-NEXT:    [[NBITS_RES:%.*]] = phi i8 [ [[NBITS]], [[LOOP]] ]
 ; CHECK-NEXT:    [[VAL_SHIFTED_RES:%.*]] = phi i8 [ [[VAL_SHIFTED]], [[LOOP]] ]
-; CHECK-NEXT:    [[VAL_SHIFTED_ISZERO_RES:%.*]] = phi i1 [ [[VAL_SHIFTED_ISZERO]], [[LOOP]] ]
+; CHECK-NEXT:    [[VAL_SHIFTED_ISZERO_RES:%.*]] = phi i1 [ [[LOOP_IVCHECK]], [[LOOP]] ]
 ; CHECK-NEXT:    [[IV_NEXT_RES:%.*]] = phi i8 [ [[IV_NEXT]], [[LOOP]] ]
 ; CHECK-NEXT:    call void @escape_outer(i8 [[IV_RES]], i8 [[NBITS_RES]], i8 [[VAL_SHIFTED_RES]], i1 [[VAL_SHIFTED_ISZERO_RES]], i8 [[IV_NEXT_RES]])
 ; CHECK-NEXT:    ret i8 [[IV_RES]]
 ;
 entry:
   br label %loop
 
 loop:
   %iv = phi i8 [ %start, %entry ], [ %iv.next, %loop ]
   %nbits = add nsw i8 %iv, %extraoffset
   %val.shifted = lshr i8 %val, %nbits
   %val.shifted.iszero = icmp eq i8 %val.shifted, 0
   %iv.next = add i8 %iv, 1
 
   call void @escape_inner(i8 %iv, i8 %nbits, i8 %val.shifted, i1 %val.shifted.iszero, i8 %iv.next)
 
   br i1 %val.shifted.iszero, label %end, label %loop
 
 end:
   %iv.res = phi i8 [ %iv, %loop ]
   %nbits.res = phi i8 [ %nbits, %loop ]
   %val.shifted.res = phi i8 [ %val.shifted, %loop ]
   %val.shifted.iszero.res = phi i1 [ %val.shifted.iszero, %loop ]
   %iv.next.res = phi i8 [ %iv.next, %loop ]
 
   call void @escape_outer(i8 %iv.res, i8 %nbits.res, i8 %val.shifted.res, i1 %val.shifted.iszero.res, i8 %iv.next.res)
 
   ret i8 %iv.res
 }
 
 ; `add nuw` is also fine.
 define i8 @p1(i8 %val, i8 %start, i8 %extraoffset) {
 ; CHECK-LABEL: @p1(
 ; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[VAL_NUMLEADINGZEROS:%.*]] = call i8 @llvm.ctlz.i8(i8 [[VAL:%.*]], i1 false)
+; CHECK-NEXT:    [[VAL_NUMACTIVEBITS:%.*]] = sub nuw nsw i8 8, [[VAL_NUMLEADINGZEROS]]
+; CHECK-NEXT:    [[TMP0:%.*]] = sub i8 0, [[EXTRAOFFSET:%.*]]
+; CHECK-NEXT:    [[VAL_NUMACTIVEBITS_OFFSET:%.*]] = add nsw i8 [[VAL_NUMACTIVEBITS]], [[TMP0]]
+; CHECK-NEXT:    [[IV_FINAL:%.*]] = call i8 @llvm.smax.i8(i8 [[VAL_NUMACTIVEBITS_OFFSET]], i8 [[START:%.*]])
+; CHECK-NEXT:    [[LOOP_BACKEDGETAKENCOUNT:%.*]] = sub nsw i8 [[IV_FINAL]], [[START]]
+; CHECK-NEXT:    [[LOOP_TRIPCOUNT:%.*]] = add nuw nsw i8 [[LOOP_BACKEDGETAKENCOUNT]], 1
 ; CHECK-NEXT:    br label [[LOOP:%.*]]
 ; CHECK:       loop:
-; CHECK-NEXT:    [[IV:%.*]] = phi i8 [ [[START:%.*]], [[ENTRY:%.*]] ], [ [[IV_NEXT:%.*]], [[LOOP]] ]
-; CHECK-NEXT:    [[NBITS:%.*]] = add nuw i8 [[IV]], [[EXTRAOFFSET:%.*]]
-; CHECK-NEXT:    [[VAL_SHIFTED:%.*]] = lshr i8 [[VAL:%.*]], [[NBITS]]
-; CHECK-NEXT:    [[VAL_SHIFTED_ISZERO:%.*]] = icmp eq i8 [[VAL_SHIFTED]], 0
-; CHECK-NEXT:    [[IV_NEXT]] = add i8 [[IV]], 1
-; CHECK-NEXT:    call void @escape_inner(i8 [[IV]], i8 [[NBITS]], i8 [[VAL_SHIFTED]], i1 [[VAL_SHIFTED_ISZERO]], i8 [[IV_NEXT]])
-; CHECK-NEXT:    br i1 [[VAL_SHIFTED_ISZERO]], label [[END:%.*]], label [[LOOP]]
+; CHECK-NEXT:    [[LOOP_IV:%.*]] = phi i8 [ 0, [[ENTRY:%.*]] ], [ [[LOOP_IV_NEXT:%.*]], [[LOOP]] ]
+; CHECK-NEXT:    [[LOOP_IV_NEXT]] = add nuw nsw i8 [[LOOP_IV]], 1
+; CHECK-NEXT:    [[LOOP_IVCHECK:%.*]] = icmp eq i8 [[LOOP_IV_NEXT]], [[LOOP_TRIPCOUNT]]
+; CHECK-NEXT:    [[IV:%.*]] = add nsw i8 [[LOOP_IV]], [[START]]
+; CHECK-NEXT:    [[NBITS:%.*]] = add nuw i8 [[IV]], [[EXTRAOFFSET]]
+; CHECK-NEXT:    [[VAL_SHIFTED:%.*]] = lshr i8 [[VAL]], [[NBITS]]
+; CHECK-NEXT:    [[IV_NEXT:%.*]] = add i8 [[IV]], 1
+; CHECK-NEXT:    call void @escape_inner(i8 [[IV]], i8 [[NBITS]], i8 [[VAL_SHIFTED]], i1 [[LOOP_IVCHECK]], i8 [[IV_NEXT]])
+; CHECK-NEXT:    br i1 [[LOOP_IVCHECK]], label [[END:%.*]], label [[LOOP]]
 ; CHECK:       end:
-; CHECK-NEXT:    [[IV_RES:%.*]] = phi i8 [ [[IV]], [[LOOP]] ]
+; CHECK-NEXT:    [[IV_RES:%.*]] = phi i8 [ [[IV_FINAL]], [[LOOP]] ]
 ; CHECK-NEXT:    [[NBITS_RES:%.*]] = phi i8 [ [[NBITS]], [[LOOP]] ]
 ; CHECK-NEXT:    [[VAL_SHIFTED_RES:%.*]] = phi i8 [ [[VAL_SHIFTED]], [[LOOP]] ]
-; CHECK-NEXT:    [[VAL_SHIFTED_ISZERO_RES:%.*]] = phi i1 [ [[VAL_SHIFTED_ISZERO]], [[LOOP]] ]
+; CHECK-NEXT:    [[VAL_SHIFTED_ISZERO_RES:%.*]] = phi i1 [ [[LOOP_IVCHECK]], [[LOOP]] ]
 ; CHECK-NEXT:    [[IV_NEXT_RES:%.*]] = phi i8 [ [[IV_NEXT]], [[LOOP]] ]
 ; CHECK-NEXT:    call void @escape_outer(i8 [[IV_RES]], i8 [[NBITS_RES]], i8 [[VAL_SHIFTED_RES]], i1 [[VAL_SHIFTED_ISZERO_RES]], i8 [[IV_NEXT_RES]])
 ; CHECK-NEXT:    ret i8 [[IV_RES]]
 ;
 entry:
   br label %loop
 
 loop:
end:
 
   ret i8 %iv.res
 }
 
 ; `sub nsw` is also fine.
 define i8 @p2(i8 %val, i8 %start, i8 %extraoffset) {
 ; CHECK-LABEL: @p2(
 ; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[VAL_NUMLEADINGZEROS:%.*]] = call i8 @llvm.ctlz.i8(i8 [[VAL:%.*]], i1 false)
+; CHECK-NEXT:    [[VAL_NUMACTIVEBITS:%.*]] = sub nuw nsw i8 8, [[VAL_NUMLEADINGZEROS]]
+; CHECK-NEXT:    [[VAL_NUMACTIVEBITS_OFFSET:%.*]] = add nsw i8 [[VAL_NUMACTIVEBITS]], [[EXTRAOFFSET:%.*]]
+; CHECK-NEXT:    [[IV_FINAL:%.*]] = call i8 @llvm.smax.i8(i8 [[VAL_NUMACTIVEBITS_OFFSET]], i8 [[START:%.*]])
+; CHECK-NEXT:    [[LOOP_BACKEDGETAKENCOUNT:%.*]] = sub nsw i8 [[IV_FINAL]], [[START]]
+; CHECK-NEXT:    [[LOOP_TRIPCOUNT:%.*]] = add nuw nsw i8 [[LOOP_BACKEDGETAKENCOUNT]], 1
 ; CHECK-NEXT:    br label [[LOOP:%.*]]
 ; CHECK:       loop:
-; CHECK-NEXT:    [[IV:%.*]] = phi i8 [ [[START:%.*]], [[ENTRY:%.*]] ], [ [[IV_NEXT:%.*]], [[LOOP]] ]
-; CHECK-NEXT:    [[NBITS:%.*]] = sub nsw i8 [[IV]], [[EXTRAOFFSET:%.*]]
-; CHECK-NEXT:    [[VAL_SHIFTED:%.*]] = lshr i8 [[VAL:%.*]], [[NBITS]]
-; CHECK-NEXT:    [[VAL_SHIFTED_ISZERO:%.*]] = icmp eq i8 [[VAL_SHIFTED]], 0
-; CHECK-NEXT:    [[IV_NEXT]] = add i8 [[IV]], 1
-; CHECK-NEXT:    call void @escape_inner(i8 [[IV]], i8 [[NBITS]], i8 [[VAL_SHIFTED]], i1 [[VAL_SHIFTED_ISZERO]], i8 [[IV_NEXT]])
-; CHECK-NEXT:    br i1 [[VAL_SHIFTED_ISZERO]], label [[END:%.*]], label [[LOOP]]
+; CHECK-NEXT:    [[LOOP_IV:%.*]] = phi i8 [ 0, [[ENTRY:%.*]] ], [ [[LOOP_IV_NEXT:%.*]], [[LOOP]] ]
+; CHECK-NEXT:    [[LOOP_IV_NEXT]] = add nuw nsw i8 [[LOOP_IV]], 1
+; CHECK-NEXT:    [[LOOP_IVCHECK:%.*]] = icmp eq i8 [[LOOP_IV_NEXT]], [[LOOP_TRIPCOUNT]]
+; CHECK-NEXT:    [[IV:%.*]] = add nsw i8 [[LOOP_IV]], [[START]]
+; CHECK-NEXT:    [[NBITS:%.*]] = sub nsw i8 [[IV]], [[EXTRAOFFSET]]
+; CHECK-NEXT:    [[VAL_SHIFTED:%.*]] = lshr i8 [[VAL]], [[NBITS]]
+; CHECK-NEXT:    [[IV_NEXT:%.*]] = add i8 [[IV]], 1
+; CHECK-NEXT:    call void @escape_inner(i8 [[IV]], i8 [[NBITS]], i8 [[VAL_SHIFTED]], i1 [[LOOP_IVCHECK]], i8 [[IV_NEXT]])
+; CHECK-NEXT:    br i1 [[LOOP_IVCHECK]], label [[END:%.*]], label [[LOOP]]
 ; CHECK:       end:
-; CHECK-NEXT:    [[IV_RES:%.*]] = phi i8 [ [[IV]], [[LOOP]] ]
+; CHECK-NEXT:    [[IV_RES:%.*]] = phi i8 [ [[IV_FINAL]], [[LOOP]] ]
 ; CHECK-NEXT:    [[NBITS_RES:%.*]] = phi i8 [ [[NBITS]], [[LOOP]] ]
 ; CHECK-NEXT:    [[VAL_SHIFTED_RES:%.*]] = phi i8 [ [[VAL_SHIFTED]], [[LOOP]] ]
-; CHECK-NEXT:    [[VAL_SHIFTED_ISZERO_RES:%.*]] = phi i1 [ [[VAL_SHIFTED_ISZERO]], [[LOOP]] ]
+; CHECK-NEXT:    [[VAL_SHIFTED_ISZERO_RES:%.*]] = phi i1 [ [[LOOP_IVCHECK]], [[LOOP]] ]
 ; CHECK-NEXT:    [[IV_NEXT_RES:%.*]] = phi i8 [ [[IV_NEXT]], [[LOOP]] ]
 ; CHECK-NEXT:    call void @escape_outer(i8 [[IV_RES]], i8 [[NBITS_RES]], i8 [[VAL_SHIFTED_RES]], i1 [[VAL_SHIFTED_ISZERO_RES]], i8 [[IV_NEXT_RES]])
 ; CHECK-NEXT:    ret i8 [[IV_RES]]
 ;
 entry:
   br label %loop
 
 loop:
end:
 
   ret i8 %iv.res
 }
 
 ; Of course, we don't have to have an offset
 define i8 @p6(i8 %val, i8 %start) {
 ; CHECK-LABEL: @p6(
 ; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[VAL_NUMLEADINGZEROS:%.*]] = call i8 @llvm.ctlz.i8(i8 [[VAL:%.*]], i1 false)
+; CHECK-NEXT:    [[VAL_NUMACTIVEBITS:%.*]] = sub nuw nsw i8 8, [[VAL_NUMLEADINGZEROS]]
+; CHECK-NEXT:    [[VAL_NUMACTIVEBITS_OFFSET:%.*]] = add nuw nsw i8 [[VAL_NUMACTIVEBITS]], 0
+; CHECK-NEXT:    [[IV_FINAL:%.*]] = call i8 @llvm.smax.i8(i8 [[VAL_NUMACTIVEBITS_OFFSET]], i8 [[START:%.*]])
+; CHECK-NEXT:    [[LOOP_BACKEDGETAKENCOUNT:%.*]] = sub nuw nsw i8 [[IV_FINAL]], [[START]]
+; CHECK-NEXT:    [[LOOP_TRIPCOUNT:%.*]] = add nuw nsw i8 [[LOOP_BACKEDGETAKENCOUNT]], 1
 ; CHECK-NEXT:    br label [[LOOP:%.*]]
 ; CHECK:       loop:
-; CHECK-NEXT:    [[IV:%.*]] = phi i8 [ [[START:%.*]], [[ENTRY:%.*]] ], [ [[IV_NEXT:%.*]], [[LOOP]] ]
-; CHECK-NEXT:    [[VAL_SHIFTED:%.*]] = lshr i8 [[VAL:%.*]], [[IV]]
-; CHECK-NEXT:    [[VAL_SHIFTED_ISZERO:%.*]] = icmp eq i8 [[VAL_SHIFTED]], 0
-; CHECK-NEXT:    [[IV_NEXT]] = add i8 [[IV]], 1
-; CHECK-NEXT:    call void @escape_inner(i8 [[IV]], i8 [[IV]], i8 [[VAL_SHIFTED]], i1 [[VAL_SHIFTED_ISZERO]], i8 [[IV_NEXT]])
-; CHECK-NEXT:    br i1 [[VAL_SHIFTED_ISZERO]], label [[END:%.*]], label [[LOOP]]
+; CHECK-NEXT:    [[LOOP_IV:%.*]] = phi i8 [ 0, [[ENTRY:%.*]] ], [ [[LOOP_IV_NEXT:%.*]], [[LOOP]] ]
+; CHECK-NEXT:    [[LOOP_IV_NEXT]] = add nuw nsw i8 [[LOOP_IV]], 1
+; CHECK-NEXT:    [[LOOP_IVCHECK:%.*]] = icmp eq i8 [[LOOP_IV_NEXT]], [[LOOP_TRIPCOUNT]]
+; CHECK-NEXT:    [[IV:%.*]] = add nsw i8 [[LOOP_IV]], [[START]]
+; CHECK-NEXT:    [[VAL_SHIFTED:%.*]] = lshr i8 [[VAL]], [[IV]]
+; CHECK-NEXT:    [[IV_NEXT:%.*]] = add i8 [[IV]], 1
+; CHECK-NEXT:    call void @escape_inner(i8 [[IV]], i8 [[IV]], i8 [[VAL_SHIFTED]], i1 [[LOOP_IVCHECK]], i8 [[IV_NEXT]])
+; CHECK-NEXT:    br i1 [[LOOP_IVCHECK]], label [[END:%.*]], label [[LOOP]]
 ; CHECK:       end:
-; CHECK-NEXT:    [[IV_RES:%.*]] = phi i8 [ [[IV]], [[LOOP]] ]
+; CHECK-NEXT:    [[IV_RES:%.*]] = phi i8 [ [[IV_FINAL]], [[LOOP]] ]
 ; CHECK-NEXT:    [[VAL_SHIFTED_RES:%.*]] = phi i8 [ [[VAL_SHIFTED]], [[LOOP]] ]
-; CHECK-NEXT:    [[VAL_SHIFTED_ISZERO_RES:%.*]] = phi i1 [ [[VAL_SHIFTED_ISZERO]], [[LOOP]] ]
+; CHECK-NEXT:    [[VAL_SHIFTED_ISZERO_RES:%.*]] = phi i1 [ [[LOOP_IVCHECK]], [[LOOP]] ]
 ; CHECK-NEXT:    [[IV_NEXT_RES:%.*]] = phi i8 [ [[IV_NEXT]], [[LOOP]] ]
 ; CHECK-NEXT:    call void @escape_outer(i8 [[IV_RES]], i8 [[IV_RES]], i8 [[VAL_SHIFTED_RES]], i1 [[VAL_SHIFTED_ISZERO_RES]], i8 [[IV_NEXT_RES]])
 ; CHECK-NEXT:    ret i8 [[IV_RES]]
 ;
 entry:
   br label %loop
 
 loop:

 
 declare void @escape_inner.i7(i7, i7, i7, i1, i7)
 declare void @escape_outer.i7(i7, i7, i7, i1, i7)
 
 ; Other bitwidths are fine also
 define i7 @p7(i7 %val, i7 %start, i7 %extraoffset) {
 ; CHECK-LABEL: @p7(
 ; CHECK-NEXT:  entry:
-; CHECK-NEXT:    br label [[LOOP:%.*]]
-; CHECK:       loop:
-; CHECK-NEXT:    [[IV:%.*]] = phi i7 [ [[START:%.*]], [[ENTRY:%.*]] ], [ [[IV_NEXT:%.*]], [[LOOP]] ]
-; CHECK-NEXT:    [[NBITS:%.*]] = add nsw i7 [[IV]], [[EXTRAOFFSET:%.*]]
-; CHECK-NEXT:    [[VAL_SHIFTED:%.*]] = lshr i7 [[VAL:%.*]], [[NBITS]]
-; CHECK-NEXT:    [[VAL_SHIFTED_ISZERO:%.*]] = icmp eq i7 [[VAL_SHIFTED]], 0
-; CHECK-NEXT:    [[IV_NEXT]] = add i7 [[IV]], 1
-; CHECK-NEXT:    call void @escape_inner.i7(i7 [[IV]], i7 [[NBITS]], i7 [[VAL_SHIFTED]], i1 [[VAL_SHIFTED_ISZERO]], i7 [[IV_NEXT]])
-; CHECK-NEXT:    br i1 [[VAL_SHIFTED_ISZERO]], label [[END:%.*]], label [[LOOP]]
+; CHECK-NEXT:    [[VAL_NUMLEADINGZEROS:%.*]] = call i7 @llvm.ctlz.i7(i7 [[VAL:%.*]], i1 false)
+; CHECK-NEXT:    [[VAL_NUMACTIVEBITS:%.*]] = sub nuw nsw i7 7, [[VAL_NUMLEADINGZEROS]]
+; CHECK-NEXT:    [[TMP0:%.*]] = sub i7 0, [[EXTRAOFFSET:%.*]]
+; CHECK-NEXT:    [[VAL_NUMACTIVEBITS_OFFSET:%.*]] = add nsw i7 [[VAL_NUMACTIVEBITS]], [[TMP0]]
+; CHECK-NEXT:    [[IV_FINAL:%.*]] = call i7 @llvm.smax.i7(i7 [[VAL_NUMACTIVEBITS_OFFSET]], i7 [[START:%.*]])
+; CHECK-NEXT:    [[LOOP_BACKEDGETAKENCOUNT:%.*]] = sub nsw i7 [[IV_FINAL]], [[START]]
+; CHECK-NEXT:    [[LOOP_TRIPCOUNT:%.*]] = add nuw nsw i7 [[LOOP_BACKEDGETAKENCOUNT]], 1
+; CHECK-NEXT:    br label [[LOOP:%.*]]
+; CHECK:       loop:
+; CHECK-NEXT:    [[LOOP_IV:%.*]] = phi i7 [ 0, [[ENTRY:%.*]] ], [ [[LOOP_IV_NEXT:%.*]], [[LOOP]] ]
+; CHECK-NEXT:    [[LOOP_IV_NEXT]] = add nuw nsw i7 [[LOOP_IV]], 1
+; CHECK-NEXT:    [[LOOP_IVCHECK:%.*]] = icmp eq i7 [[LOOP_IV_NEXT]], [[LOOP_TRIPCOUNT]]
+; CHECK-NEXT:    [[IV:%.*]] = add nsw i7 [[LOOP_IV]], [[START]]
+; CHECK-NEXT:    [[NBITS:%.*]] = add nsw i7 [[IV]], [[EXTRAOFFSET]]
+; CHECK-NEXT:    [[VAL_SHIFTED:%.*]] = lshr i7 [[VAL]], [[NBITS]]
+; CHECK-NEXT:    [[IV_NEXT:%.*]] = add i7 [[IV]], 1
+; CHECK-NEXT:    call void @escape_inner.i7(i7 [[IV]], i7 [[NBITS]], i7 [[VAL_SHIFTED]], i1 [[LOOP_IVCHECK]], i7 [[IV_NEXT]])
+; CHECK-NEXT:    br i1 [[LOOP_IVCHECK]], label [[END:%.*]], label [[LOOP]]
 ; CHECK:       end:
-; CHECK-NEXT:    [[IV_RES:%.*]] = phi i7 [ [[IV]], [[LOOP]] ]
+; CHECK-NEXT:    [[IV_RES:%.*]] = phi i7 [ [[IV_FINAL]], [[LOOP]] ]
 ; CHECK-NEXT:    [[NBITS_RES:%.*]] = phi i7 [ [[NBITS]], [[LOOP]] ]
 ; CHECK-NEXT:    [[VAL_SHIFTED_RES:%.*]] = phi i7 [ [[VAL_SHIFTED]], [[LOOP]] ]
-; CHECK-NEXT:    [[VAL_SHIFTED_ISZERO_RES:%.*]] = phi i1 [ [[VAL_SHIFTED_ISZERO]], [[LOOP]] ]
+; CHECK-NEXT:    [[VAL_SHIFTED_ISZERO_RES:%.*]] = phi i1 [ [[LOOP_IVCHECK]], [[LOOP]] ]
 ; CHECK-NEXT:    [[IV_NEXT_RES:%.*]] = phi i7 [ [[IV_NEXT]], [[LOOP]] ]
 ; CHECK-NEXT:    call void @escape_outer.i7(i7 [[IV_RES]], i7 [[NBITS_RES]], i7 [[VAL_SHIFTED_RES]], i1 [[VAL_SHIFTED_ISZERO_RES]], i7 [[IV_NEXT_RES]])
 ; CHECK-NEXT:    ret i7 [[IV_RES]]
 ;
 entry:
   br label %loop
 
 loop:
end:
 
   ret i8 %iv.res
 }
 
 ; Cmp-br are commutable
 define i8 @t9(i8 %val, i8 %start, i8 %extraoffset) {
 ; CHECK-LABEL: @t9(
 ; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[VAL_NUMLEADINGZEROS:%.*]] = call i8 @llvm.ctlz.i8(i8 [[VAL:%.*]], i1 false)
+; CHECK-NEXT:    [[VAL_NUMACTIVEBITS:%.*]] = sub nuw nsw i8 8, [[VAL_NUMLEADINGZEROS]]
+; CHECK-NEXT:    [[TMP0:%.*]] = sub i8 0, [[EXTRAOFFSET:%.*]]
+; CHECK-NEXT:    [[VAL_NUMACTIVEBITS_OFFSET:%.*]] = add nsw i8 [[VAL_NUMACTIVEBITS]], [[TMP0]]
+; CHECK-NEXT:    [[IV_FINAL:%.*]] = call i8 @llvm.smax.i8(i8 [[VAL_NUMACTIVEBITS_OFFSET]], i8 [[START:%.*]])
+; CHECK-NEXT:    [[LOOP_BACKEDGETAKENCOUNT:%.*]] = sub nsw i8 [[IV_FINAL]], [[START]]
+; CHECK-NEXT:    [[LOOP_TRIPCOUNT:%.*]] = add nuw nsw i8 [[LOOP_BACKEDGETAKENCOUNT]], 1
 ; CHECK-NEXT:    br label [[LOOP:%.*]]
 ; CHECK:       loop:
-; CHECK-NEXT:    [[IV:%.*]] = phi i8 [ [[START:%.*]], [[ENTRY:%.*]] ], [ [[IV_NEXT:%.*]], [[LOOP]] ]
-; CHECK-NEXT:    [[NBITS:%.*]] = add nsw i8 [[IV]], [[EXTRAOFFSET:%.*]]
-; CHECK-NEXT:    [[VAL_SHIFTED:%.*]] = lshr i8 [[VAL:%.*]], [[NBITS]]
-; CHECK-NEXT:    [[VAL_SHIFTED_ISNOTZERO:%.*]] = icmp ne i8 [[VAL_SHIFTED]], 0
-; CHECK-NEXT:    [[IV_NEXT]] = add i8 [[IV]], 1
+; CHECK-NEXT:    [[LOOP_IV:%.*]] = phi i8 [ 0, [[ENTRY:%.*]] ], [ [[LOOP_IV_NEXT:%.*]], [[LOOP]] ]
+; CHECK-NEXT:    [[LOOP_IV_NEXT]] = add nuw nsw i8 [[LOOP_IV]], 1
+; CHECK-NEXT:    [[LOOP_IVCHECK:%.*]] = icmp eq i8 [[LOOP_IV_NEXT]], [[LOOP_TRIPCOUNT]]
+; CHECK-NEXT:    [[VAL_SHIFTED_ISNOTZERO:%.*]] = xor i1 [[LOOP_IVCHECK]], true
+; CHECK-NEXT:    [[IV:%.*]] = add nsw i8 [[LOOP_IV]], [[START]]
+; CHECK-NEXT:    [[NBITS:%.*]] = add nsw i8 [[IV]], [[EXTRAOFFSET]]
+; CHECK-NEXT:    [[VAL_SHIFTED:%.*]] = lshr i8 [[VAL]], [[NBITS]]
+; CHECK-NEXT:    [[IV_NEXT:%.*]] = add i8 [[IV]], 1
 ; CHECK-NEXT:    call void @escape_inner(i8 [[IV]], i8 [[NBITS]], i8 [[VAL_SHIFTED]], i1 [[VAL_SHIFTED_ISNOTZERO]], i8 [[IV_NEXT]])
-; CHECK-NEXT:    br i1 [[VAL_SHIFTED_ISNOTZERO]], label [[LOOP]], label [[END:%.*]]
+; CHECK-NEXT:    br i1 [[LOOP_IVCHECK]], label [[END:%.*]], label [[LOOP]]
 ; CHECK:       end:
-; CHECK-NEXT:    [[IV_RES:%.*]] = phi i8 [ [[IV]], [[LOOP]] ]
+; CHECK-NEXT:    [[IV_RES:%.*]] = phi i8 [ [[IV_FINAL]], [[LOOP]] ]
 ; CHECK-NEXT:    [[NBITS_RES:%.*]] = phi i8 [ [[NBITS]], [[LOOP]] ]
 ; CHECK-NEXT:    [[VAL_SHIFTED_RES:%.*]] = phi i8 [ [[VAL_SHIFTED]], [[LOOP]] ]
 ; CHECK-NEXT:    [[VAL_SHIFTED_ISNOTZERO_RES:%.*]] = phi i1 [ [[VAL_SHIFTED_ISNOTZERO]], [[LOOP]] ]
 ; CHECK-NEXT:    [[IV_NEXT_RES:%.*]] = phi i8 [ [[IV_NEXT]], [[LOOP]] ]
 ; CHECK-NEXT:    call void @escape_outer(i8 [[IV_RES]], i8 [[NBITS_RES]], i8 [[VAL_SHIFTED_RES]], i1 [[VAL_SHIFTED_ISNOTZERO_RES]], i8 [[IV_NEXT_RES]])
 ; CHECK-NEXT:    ret i8 [[IV_RES]]
 ;
 entry:
end:
 
   ret i8 %iv.res
 }
 
 ; offset computation can be commuted
 define i8 @t17(i8 %val, i8 %start, i8 %extraoffset) {
 ; CHECK-LABEL: @t17(
 ; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[VAL_NUMLEADINGZEROS:%.*]] = call i8 @llvm.ctlz.i8(i8 [[VAL:%.*]], i1 false)
+; CHECK-NEXT:    [[VAL_NUMACTIVEBITS:%.*]] = sub nuw nsw i8 8, [[VAL_NUMLEADINGZEROS]]
+; CHECK-NEXT:    [[TMP0:%.*]] = sub i8 0, [[EXTRAOFFSET:%.*]]
+; CHECK-NEXT:    [[VAL_NUMACTIVEBITS_OFFSET:%.*]] = add nsw i8 [[VAL_NUMACTIVEBITS]], [[TMP0]]
+; CHECK-NEXT:    [[IV_FINAL:%.*]] = call i8 @llvm.smax.i8(i8 [[VAL_NUMACTIVEBITS_OFFSET]], i8 [[START:%.*]])
+; CHECK-NEXT:    [[LOOP_BACKEDGETAKENCOUNT:%.*]] = sub nsw i8 [[IV_FINAL]], [[START]]
+; CHECK-NEXT:    [[LOOP_TRIPCOUNT:%.*]] = add nuw nsw i8 [[LOOP_BACKEDGETAKENCOUNT]], 1
 ; CHECK-NEXT:    br label [[LOOP:%.*]]
 ; CHECK:       loop:
-; CHECK-NEXT:    [[IV:%.*]] = phi i8 [ [[START:%.*]], [[ENTRY:%.*]] ], [ [[IV_NEXT:%.*]], [[LOOP]] ]
-; CHECK-NEXT:    [[NBITS:%.*]] = add nsw i8 [[EXTRAOFFSET:%.*]], [[IV]]
-; CHECK-NEXT:    [[VAL_SHIFTED:%.*]] = lshr i8 [[VAL:%.*]], [[NBITS]]
-; CHECK-NEXT:    [[VAL_SHIFTED_ISZERO:%.*]] = icmp eq i8 [[VAL_SHIFTED]], 0
-; CHECK-NEXT:    [[IV_NEXT]] = add i8 [[IV]], 1
-; CHECK-NEXT:    call void @escape_inner(i8 [[IV]], i8 [[NBITS]], i8 [[VAL_SHIFTED]], i1 [[VAL_SHIFTED_ISZERO]], i8 [[IV_NEXT]])
-; CHECK-NEXT:    br i1 [[VAL_SHIFTED_ISZERO]], label [[END:%.*]], label [[LOOP]]
+; CHECK-NEXT:    [[LOOP_IV:%.*]] = phi i8 [ 0, [[ENTRY:%.*]] ], [ [[LOOP_IV_NEXT:%.*]], [[LOOP]] ]
+; CHECK-NEXT:    [[LOOP_IV_NEXT]] = add nuw nsw i8 [[LOOP_IV]], 1
+; CHECK-NEXT:    [[LOOP_IVCHECK:%.*]] = icmp eq i8 [[LOOP_IV_NEXT]], [[LOOP_TRIPCOUNT]]
+; CHECK-NEXT:    [[IV:%.*]] = add nsw i8 [[LOOP_IV]], [[START]]
+; CHECK-NEXT:    [[NBITS:%.*]] = add nsw i8 [[EXTRAOFFSET]], [[IV]]
+; CHECK-NEXT:    [[VAL_SHIFTED:%.*]] = lshr i8 [[VAL]], [[NBITS]]
+; CHECK-NEXT:    [[IV_NEXT:%.*]] = add i8 [[IV]], 1
+; CHECK-NEXT:    call void @escape_inner(i8 [[IV]], i8 [[NBITS]], i8 [[VAL_SHIFTED]], i1 [[LOOP_IVCHECK]], i8 [[IV_NEXT]])
+; CHECK-NEXT:    br i1 [[LOOP_IVCHECK]], label [[END:%.*]], label [[LOOP]]
 ; CHECK:       end:
-; CHECK-NEXT:    [[IV_RES:%.*]] = phi i8 [ [[IV]], [[LOOP]] ]
+; CHECK-NEXT:    [[IV_RES:%.*]] = phi i8 [ [[IV_FINAL]], [[LOOP]] ]
 ; CHECK-NEXT:    [[NBITS_RES:%.*]] = phi i8 [ [[NBITS]], [[LOOP]] ]
 ; CHECK-NEXT:    [[VAL_SHIFTED_RES:%.*]] = phi i8 [ [[VAL_SHIFTED]], [[LOOP]] ]
-; CHECK-NEXT:    [[VAL_SHIFTED_ISZERO_RES:%.*]] = phi i1 [ [[VAL_SHIFTED_ISZERO]], [[LOOP]] ]
+; CHECK-NEXT:    [[VAL_SHIFTED_ISZERO_RES:%.*]] = phi i1 [ [[LOOP_IVCHECK]], [[LOOP]] ]
 ; CHECK-NEXT:    [[IV_NEXT_RES:%.*]] = phi i8 [ [[IV_NEXT]], [[LOOP]] ]
 ; CHECK-NEXT:    call void @escape_outer(i8 [[IV_RES]], i8 [[NBITS_RES]], i8 [[VAL_SHIFTED_RES]], i1 [[VAL_SHIFTED_ISZERO_RES]], i8 [[IV_NEXT_RES]])
 ; CHECK-NEXT:    ret i8 [[IV_RES]]
 ;
 entry:
   br label %loop
 
 loop:
end:
 
   ret i8 %iv.res
 }
 
 ; We should not just blindly look for add, we should look what IV actually uses.
 define i8 @n24(i8 %val, i8 %start, i8 %extraoffset) {
 ; CHECK-LABEL: @n24(
 ; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[VAL_NUMLEADINGZEROS:%.*]] = call i8 @llvm.ctlz.i8(i8 [[VAL:%.*]], i1 false)
+; CHECK-NEXT:    [[VAL_NUMACTIVEBITS:%.*]] = sub nuw nsw i8 8, [[VAL_NUMLEADINGZEROS]]
+; CHECK-NEXT:    [[TMP0:%.*]] = sub i8 0, [[EXTRAOFFSET:%.*]]
+; CHECK-NEXT:    [[VAL_NUMACTIVEBITS_OFFSET:%.*]] = add nsw i8 [[VAL_NUMACTIVEBITS]], [[TMP0]]
+; CHECK-NEXT:    [[IV_FINAL:%.*]] = call i8 @llvm.smax.i8(i8 [[VAL_NUMACTIVEBITS_OFFSET]], i8 [[START:%.*]])
+; CHECK-NEXT:    [[LOOP_BACKEDGETAKENCOUNT:%.*]] = sub nsw i8 [[IV_FINAL]], [[START]]
+; CHECK-NEXT:    [[LOOP_TRIPCOUNT:%.*]] = add nuw nsw i8 [[LOOP_BACKEDGETAKENCOUNT]], 1
 ; CHECK-NEXT:    br label [[LOOP:%.*]]
 ; CHECK:       loop:
-; CHECK-NEXT:    [[IV:%.*]] = phi i8 [ [[START:%.*]], [[ENTRY:%.*]] ], [ [[IV_NEXT:%.*]], [[LOOP]] ]
-; CHECK-NEXT:    [[NBITS:%.*]] = add nsw i8 [[IV]], [[EXTRAOFFSET:%.*]]
-; CHECK-NEXT:    [[VAL_SHIFTED:%.*]] = lshr i8 [[VAL:%.*]], [[NBITS]]
-; CHECK-NEXT:    [[VAL_SHIFTED_ISZERO:%.*]] = icmp eq i8 [[VAL_SHIFTED]], 0
+; CHECK-NEXT:    [[LOOP_IV:%.*]] = phi i8 [ 0, [[ENTRY:%.*]] ], [ [[LOOP_IV_NEXT:%.*]], [[LOOP]] ]
+; CHECK-NEXT:    [[LOOP_IV_NEXT]] = add nuw nsw i8 [[LOOP_IV]], 1
+; CHECK-NEXT:    [[LOOP_IVCHECK:%.*]] = icmp eq i8 [[LOOP_IV_NEXT]], [[LOOP_TRIPCOUNT]]
+; CHECK-NEXT:    [[IV:%.*]] = add nsw i8 [[LOOP_IV]], [[START]]
+; CHECK-NEXT:    [[NBITS:%.*]] = add nsw i8 [[IV]], [[EXTRAOFFSET]]
+; CHECK-NEXT:    [[VAL_SHIFTED:%.*]] = lshr i8 [[VAL]], [[NBITS]]
 ; CHECK-NEXT:    [[NOT_IV_NEXT:%.*]] = add i8 [[IV]], 1
-; CHECK-NEXT:    [[IV_NEXT]] = add i8 [[IV]], 1
+; CHECK-NEXT:    [[IV_NEXT:%.*]] = add i8 [[IV]], 1
 ; CHECK-NEXT:    [[ALSO_IV_NEXT:%.*]] = add i8 [[IV]], 1
-; CHECK-NEXT:    call void @escape_inner(i8 [[IV]], i8 [[NBITS]], i8 [[VAL_SHIFTED]], i1 [[VAL_SHIFTED_ISZERO]], i8 [[IV_NEXT]])
-; CHECK-NEXT:    br i1 [[VAL_SHIFTED_ISZERO]], label [[END:%.*]], label [[LOOP]]
+; CHECK-NEXT:    call void @escape_inner(i8 [[IV]], i8 [[NBITS]], i8 [[VAL_SHIFTED]], i1 [[LOOP_IVCHECK]], i8 [[IV_NEXT]])
+; CHECK-NEXT:    br i1 [[LOOP_IVCHECK]], label [[END:%.*]], label [[LOOP]]
 ; CHECK:       end:
-; CHECK-NEXT:    [[IV_RES:%.*]] = phi i8 [ [[IV]], [[LOOP]] ]
+; CHECK-NEXT:    [[IV_RES:%.*]] = phi i8 [ [[IV_FINAL]], [[LOOP]] ]
 ; CHECK-NEXT:    [[NBITS_RES:%.*]] = phi i8 [ [[NBITS]], [[LOOP]] ]
 ; CHECK-NEXT:    [[VAL_SHIFTED_RES:%.*]] = phi i8 [ [[VAL_SHIFTED]], [[LOOP]] ]
-; CHECK-NEXT:    [[VAL_SHIFTED_ISZERO_RES:%.*]] = phi i1 [ [[VAL_SHIFTED_ISZERO]], [[LOOP]] ]
+; CHECK-NEXT:    [[VAL_SHIFTED_ISZERO_RES:%.*]] = phi i1 [ [[LOOP_IVCHECK]], [[LOOP]] ]
 ; CHECK-NEXT:    [[IV_NEXT_RES:%.*]] = phi i8 [ [[IV_NEXT]], [[LOOP]] ]
 ; CHECK-NEXT:    call void @escape_outer(i8 [[IV_RES]], i8 [[NBITS_RES]], i8 [[VAL_SHIFTED_RES]], i1 [[VAL_SHIFTED_ISZERO_RES]], i8 [[IV_NEXT_RES]])
 ; CHECK-NEXT:    ret i8 [[IV_RES]]
 ;
 entry:
   br label %loop
 
 loop:

 declare void @escape_inner.i2(i2, i2, i2, i1, i2)
 declare void @escape_outer.i2(i2, i2, i2, i1, i2)
 declare void @escape_inner.i3(i3, i3, i3, i1, i3)
 declare void @escape_outer.i3(i3, i3, i3, i1, i3)
 
 define i1 @t25_nooffset_i1(i1 %val, i1 %start) {
 ; CHECK-LABEL: @t25_nooffset_i1(
 ; CHECK-NEXT:  entry:
-; CHECK-NEXT:    br label [[LOOP:%.*]]
-; CHECK:       loop:
-; CHECK-NEXT:    [[IV:%.*]] = phi i1 [ [[START:%.*]], [[ENTRY:%.*]] ], [ [[IV_NEXT:%.*]], [[LOOP]] ]
-; CHECK-NEXT:    [[VAL_SHIFTED:%.*]] = lshr i1 [[VAL:%.*]], [[IV]]
-; CHECK-NEXT:    [[VAL_SHIFTED_ISZERO:%.*]] = icmp eq i1 [[VAL_SHIFTED]], false
-; CHECK-NEXT:    [[IV_NEXT]] = add i1 [[IV]], true
-; CHECK-NEXT:    call void @escape_inner.i1(i1 [[IV]], i1 [[IV]], i1 [[VAL_SHIFTED]], i1 [[VAL_SHIFTED_ISZERO]], i1 [[IV_NEXT]])
-; CHECK-NEXT:    br i1 [[VAL_SHIFTED_ISZERO]], label [[END:%.*]], label [[LOOP]]
+; CHECK-NEXT:    [[VAL_NUMLEADINGZEROS:%.*]] = call i1 @llvm.ctlz.i1(i1 [[VAL:%.*]], i1 false)
+; CHECK-NEXT:    [[VAL_NUMACTIVEBITS:%.*]] = sub nuw nsw i1 true, [[VAL_NUMLEADINGZEROS]]
+; CHECK-NEXT:    [[VAL_NUMACTIVEBITS_OFFSET:%.*]] = add nuw nsw i1 [[VAL_NUMACTIVEBITS]], false
+; CHECK-NEXT:    [[IV_FINAL:%.*]] = call i1 @llvm.smax.i1(i1 [[VAL_NUMACTIVEBITS_OFFSET]], i1 [[START:%.*]])
+; CHECK-NEXT:    [[LOOP_BACKEDGETAKENCOUNT:%.*]] = sub nuw nsw i1 [[IV_FINAL]], [[START]]
+; CHECK-NEXT:    [[LOOP_TRIPCOUNT:%.*]] = add nuw nsw i1 [[LOOP_BACKEDGETAKENCOUNT]], true
+; CHECK-NEXT:    br label [[LOOP:%.*]]
+; CHECK:       loop:
+; CHECK-NEXT:    [[LOOP_IV:%.*]] = phi i1 [ false, [[ENTRY:%.*]] ], [ [[LOOP_IV_NEXT:%.*]], [[LOOP]] ]
+; CHECK-NEXT:    [[LOOP_IV_NEXT]] = add nuw nsw i1 [[LOOP_IV]], true
+; CHECK-NEXT:    [[LOOP_IVCHECK:%.*]] = icmp eq i1 [[LOOP_IV_NEXT]], [[LOOP_TRIPCOUNT]]
+; CHECK-NEXT:    [[IV:%.*]] = add nsw i1 [[LOOP_IV]], [[START]]
+; CHECK-NEXT:    [[VAL_SHIFTED:%.*]] = lshr i1 [[VAL]], [[IV]]
+; CHECK-NEXT:    [[IV_NEXT:%.*]] = add i1 [[IV]], true
+; CHECK-NEXT:    call void @escape_inner.i1(i1 [[IV]], i1 [[IV]], i1 [[VAL_SHIFTED]], i1 [[LOOP_IVCHECK]], i1 [[IV_NEXT]])
+; CHECK-NEXT:    br i1 [[LOOP_IVCHECK]], label [[END:%.*]], label [[LOOP]]
 ; CHECK:       end:
-; CHECK-NEXT:    [[IV_RES:%.*]] = phi i1 [ [[IV]], [[LOOP]] ]
+; CHECK-NEXT:    [[IV_RES:%.*]] = phi i1 [ [[IV_FINAL]], [[LOOP]] ]
 ; CHECK-NEXT:    [[VAL_SHIFTED_RES:%.*]] = phi i1 [ [[VAL_SHIFTED]], [[LOOP]] ]
-; CHECK-NEXT:    [[VAL_SHIFTED_ISZERO_RES:%.*]] = phi i1 [ [[VAL_SHIFTED_ISZERO]], [[LOOP]] ]
+; CHECK-NEXT:    [[VAL_SHIFTED_ISZERO_RES:%.*]] = phi i1 [ [[LOOP_IVCHECK]], [[LOOP]] ]
 ; CHECK-NEXT:    [[IV_NEXT_RES:%.*]] = phi i1 [ [[IV_NEXT]], [[LOOP]] ]
 ; CHECK-NEXT:    call void @escape_outer.i1(i1 [[IV_RES]], i1 [[IV_RES]], i1 [[VAL_SHIFTED_RES]], i1 [[VAL_SHIFTED_ISZERO_RES]], i1 [[IV_NEXT_RES]])
 ; CHECK-NEXT:    ret i1 [[IV_RES]]
 ;
 entry:
   br label %loop
 
 loop:
end:
 
   call void @escape_outer.i1(i1 %iv.res, i1 %iv.res, i1 %val.shifted.res, i1 %val.shifted.iszero.res, i1 %iv.next.res)
 
   ret i1 %iv.res
 }
 define i2 @t26_nooffset_i2(i2 %val, i2 %start) {
 ; CHECK-LABEL: @t26_nooffset_i2(
 ; CHECK-NEXT:  entry:
-; CHECK-NEXT:    br label [[LOOP:%.*]]
-; CHECK:       loop:
-; CHECK-NEXT:    [[IV:%.*]] = phi i2 [ [[START:%.*]], [[ENTRY:%.*]] ], [ [[IV_NEXT:%.*]], [[LOOP]] ]
-; CHECK-NEXT:    [[VAL_SHIFTED:%.*]] = lshr i2 [[VAL:%.*]], [[IV]]
-; CHECK-NEXT:    [[VAL_SHIFTED_ISZERO:%.*]] = icmp eq i2 [[VAL_SHIFTED]], 0
-; CHECK-NEXT:    [[IV_NEXT]] = add i2 [[IV]], 1
-; CHECK-NEXT:    call void @escape_inner.i2(i2 [[IV]], i2 [[IV]], i2 [[VAL_SHIFTED]], i1 [[VAL_SHIFTED_ISZERO]], i2 [[IV_NEXT]])
-; CHECK-NEXT:    br i1 [[VAL_SHIFTED_ISZERO]], label [[END:%.*]], label [[LOOP]]
+; CHECK-NEXT:    [[VAL_NUMLEADINGZEROS:%.*]] = call i2 @llvm.ctlz.i2(i2 [[VAL:%.*]], i1 false)
+; CHECK-NEXT:    [[VAL_NUMACTIVEBITS:%.*]] = sub nuw i2 -2, [[VAL_NUMLEADINGZEROS]]
+; CHECK-NEXT:    [[VAL_NUMACTIVEBITS_OFFSET:%.*]] = add nuw nsw i2 [[VAL_NUMACTIVEBITS]], 0
+; CHECK-NEXT:    [[IV_FINAL:%.*]] = call i2 @llvm.smax.i2(i2 [[VAL_NUMACTIVEBITS_OFFSET]], i2 [[START:%.*]])
+; CHECK-NEXT:    [[LOOP_BACKEDGETAKENCOUNT:%.*]] = sub nuw nsw i2 [[IV_FINAL]], [[START]]
+; CHECK-NEXT:    [[LOOP_TRIPCOUNT:%.*]] = add nuw i2 [[LOOP_BACKEDGETAKENCOUNT]], 1
+; CHECK-NEXT:    br label [[LOOP:%.*]]
+; CHECK:       loop:
+; CHECK-NEXT:    [[LOOP_IV:%.*]] = phi i2 [ 0, [[ENTRY:%.*]] ], [ [[LOOP_IV_NEXT:%.*]], [[LOOP]] ]
+; CHECK-NEXT:    [[LOOP_IV_NEXT]] = add nuw i2 [[LOOP_IV]], 1
+; CHECK-NEXT:    [[LOOP_IVCHECK:%.*]] = icmp eq i2 [[LOOP_IV_NEXT]], [[LOOP_TRIPCOUNT]]
+; CHECK-NEXT:    [[IV:%.*]] = add nsw i2 [[LOOP_IV]], [[START]]
+; CHECK-NEXT:    [[VAL_SHIFTED:%.*]] = lshr i2 [[VAL]], [[IV]]
+; CHECK-NEXT:    [[IV_NEXT:%.*]] = add i2 [[IV]], 1
+; CHECK-NEXT:    call void @escape_inner.i2(i2 [[IV]], i2 [[IV]], i2 [[VAL_SHIFTED]], i1 [[LOOP_IVCHECK]], i2 [[IV_NEXT]])
+; CHECK-NEXT:    br i1 [[LOOP_IVCHECK]], label [[END:%.*]], label [[LOOP]]
 ; CHECK:       end:
-; CHECK-NEXT:    [[IV_RES:%.*]] = phi i2 [ [[IV]], [[LOOP]] ]
+; CHECK-NEXT:    [[IV_RES:%.*]] = phi i2 [ [[IV_FINAL]], [[LOOP]] ]
 ; CHECK-NEXT:    [[VAL_SHIFTED_RES:%.*]] = phi i2 [ [[VAL_SHIFTED]], [[LOOP]] ]
-; CHECK-NEXT:    [[VAL_SHIFTED_ISZERO_RES:%.*]] = phi i1 [ [[VAL_SHIFTED_ISZERO]], [[LOOP]] ]
+; CHECK-NEXT:    [[VAL_SHIFTED_ISZERO_RES:%.*]] = phi i1 [ [[LOOP_IVCHECK]], [[LOOP]] ]
 ; CHECK-NEXT:    [[IV_NEXT_RES:%.*]] = phi i2 [ [[IV_NEXT]], [[LOOP]] ]
 ; CHECK-NEXT:    call void @escape_outer.i2(i2 [[IV_RES]], i2 [[IV_RES]], i2 [[VAL_SHIFTED_RES]], i1 [[VAL_SHIFTED_ISZERO_RES]], i2 [[IV_NEXT_RES]])
 ; CHECK-NEXT:    ret i2 [[IV_RES]]
 ;
 entry:
   br label %loop
 
 loop:
end:
 
   call void @escape_outer.i2(i2 %iv.res, i2 %iv.res, i2 %val.shifted.res, i1 %val.shifted.iszero.res, i2 %iv.next.res)
 
   ret i2 %iv.res
 }
 define i3 @t27_nooffset_i3(i3 %val, i3 %start) {
 ; CHECK-LABEL: @t27_nooffset_i3(
 ; CHECK-NEXT:  entry:
-; CHECK-NEXT:    br label [[LOOP:%.*]]
-; CHECK:       loop:
-; CHECK-NEXT:    [[IV:%.*]] = phi i3 [ [[START:%.*]], [[ENTRY:%.*]] ], [ [[IV_NEXT:%.*]], [[LOOP]] ]
-; CHECK-NEXT:    [[VAL_SHIFTED:%.*]] = lshr i3 [[VAL:%.*]], [[IV]]
-; CHECK-NEXT:    [[VAL_SHIFTED_ISZERO:%.*]] = icmp eq i3 [[VAL_SHIFTED]], 0
-; CHECK-NEXT:    [[IV_NEXT]] = add i3 [[IV]], 1
-; CHECK-NEXT:    call void @escape_inner.i3(i3 [[IV]], i3 [[IV]], i3 [[VAL_SHIFTED]], i1 [[VAL_SHIFTED_ISZERO]], i3 [[IV_NEXT]])
-; CHECK-NEXT:    br i1 [[VAL_SHIFTED_ISZERO]], label [[END:%.*]], label [[LOOP]]
+; CHECK-NEXT:    [[VAL_NUMLEADINGZEROS:%.*]] = call i3 @llvm.ctlz.i3(i3 [[VAL:%.*]], i1 false)
+; CHECK-NEXT:    [[VAL_NUMACTIVEBITS:%.*]] = sub nuw nsw i3 3, [[VAL_NUMLEADINGZEROS]]
+; CHECK-NEXT:    [[VAL_NUMACTIVEBITS_OFFSET:%.*]] = add nuw nsw i3 [[VAL_NUMACTIVEBITS]], 0
+; CHECK-NEXT:    [[IV_FINAL:%.*]] = call i3 @llvm.smax.i3(i3 [[VAL_NUMACTIVEBITS_OFFSET]], i3 [[START:%.*]])
+; CHECK-NEXT:    [[LOOP_BACKEDGETAKENCOUNT:%.*]] = sub nuw nsw i3 [[IV_FINAL]], [[START]]
+; CHECK-NEXT:    [[LOOP_TRIPCOUNT:%.*]] = add nuw nsw i3 [[LOOP_BACKEDGETAKENCOUNT]], 1
+; CHECK-NEXT:    br label [[LOOP:%.*]]
+; CHECK:       loop:
+; CHECK-NEXT:    [[LOOP_IV:%.*]] = phi i3 [ 0, [[ENTRY:%.*]] ], [ [[LOOP_IV_NEXT:%.*]], [[LOOP]] ]
+; CHECK-NEXT:    [[LOOP_IV_NEXT]] = add nuw nsw i3 [[LOOP_IV]], 1
+; CHECK-NEXT:    [[LOOP_IVCHECK:%.*]] = icmp eq i3 [[LOOP_IV_NEXT]], [[LOOP_TRIPCOUNT]]
+; CHECK-NEXT:    [[IV:%.*]] = add nsw i3 [[LOOP_IV]], [[START]]
+; CHECK-NEXT:    [[VAL_SHIFTED:%.*]] = lshr i3 [[VAL]], [[IV]]
+; CHECK-NEXT:    [[IV_NEXT:%.*]] = add i3 [[IV]], 1
+; CHECK-NEXT:    call void @escape_inner.i3(i3 [[IV]], i3 [[IV]], i3 [[VAL_SHIFTED]], i1 [[LOOP_IVCHECK]], i3 [[IV_NEXT]])
+; CHECK-NEXT:    br i1 [[LOOP_IVCHECK]], label [[END:%.*]], label [[LOOP]]
 ; CHECK:       end:
-; CHECK-NEXT:    [[IV_RES:%.*]] = phi i3 [ [[IV]], [[LOOP]] ]
+; CHECK-NEXT:    [[IV_RES:%.*]] = phi i3 [ [[IV_FINAL]], [[LOOP]] ]
 ; CHECK-NEXT:    [[VAL_SHIFTED_RES:%.*]] = phi i3 [ [[VAL_SHIFTED]], [[LOOP]] ]
-; CHECK-NEXT:    [[VAL_SHIFTED_ISZERO_RES:%.*]] = phi i1 [ [[VAL_SHIFTED_ISZERO]], [[LOOP]] ]
+; CHECK-NEXT:    [[VAL_SHIFTED_ISZERO_RES:%.*]] = phi i1 [ [[LOOP_IVCHECK]], [[LOOP]] ]
 ; CHECK-NEXT:    [[IV_NEXT_RES:%.*]] = phi i3 [ [[IV_NEXT]], [[LOOP]] ]
 ; CHECK-NEXT:    call void @escape_outer.i3(i3 [[IV_RES]], i3 [[IV_RES]], i3 [[VAL_SHIFTED_RES]], i1 [[VAL_SHIFTED_ISZERO_RES]], i3 [[IV_NEXT_RES]])
 ; CHECK-NEXT:    ret i3 [[IV_RES]]
 ;
 entry:
   br label %loop
 
 loop:
end:
   call void @escape_outer.i3(i3 %iv.res, i3 %iv.res, i3 %val.shifted.res, i1 %val.shifted.iszero.res, i3 %iv.next.res)
 
   ret i3 %iv.res
 }
 
 define i1 @t27_addnsw_i1(i1 %val, i1 %start, i1 %extraoffset) {
 ; CHECK-LABEL: @t27_addnsw_i1(
 ; CHECK-NEXT:  entry:
-; CHECK-NEXT:    br label [[LOOP:%.*]]
-; CHECK:       loop:
-; CHECK-NEXT:    [[IV:%.*]] = phi i1 [ [[START:%.*]], [[ENTRY:%.*]] ], [ [[IV_NEXT:%.*]], [[LOOP]] ]
-; CHECK-NEXT:    [[NBITS:%.*]] = add nsw i1 [[IV]], [[EXTRAOFFSET:%.*]]
-; CHECK-NEXT:    [[VAL_SHIFTED:%.*]] = lshr i1 [[VAL:%.*]], [[NBITS]]
-; CHECK-NEXT:    [[VAL_SHIFTED_ISZERO:%.*]] = icmp eq i1 [[VAL_SHIFTED]], false
-; CHECK-NEXT:    [[IV_NEXT]] = add i1 [[IV]], true
-; CHECK-NEXT:    call void @escape_inner.i1(i1 [[IV]], i1 [[NBITS]], i1 [[VAL_SHIFTED]], i1 [[VAL_SHIFTED_ISZERO]], i1 [[IV_NEXT]])
-; CHECK-NEXT:    br i1 [[VAL_SHIFTED_ISZERO]], label [[END:%.*]], label [[LOOP]]
+; CHECK-NEXT:    [[VAL_NUMLEADINGZEROS:%.*]] = call i1 @llvm.ctlz.i1(i1 [[VAL:%.*]], i1 false)
+; CHECK-NEXT:    [[VAL_NUMACTIVEBITS:%.*]] = sub nuw nsw i1 true, [[VAL_NUMLEADINGZEROS]]
+; CHECK-NEXT:    [[VAL_NUMACTIVEBITS_OFFSET:%.*]] = add nsw i1 [[VAL_NUMACTIVEBITS]], [[EXTRAOFFSET:%.*]]
+; CHECK-NEXT:    [[IV_FINAL:%.*]] = call i1 @llvm.smax.i1(i1 [[VAL_NUMACTIVEBITS_OFFSET]], i1 [[START:%.*]])
+; CHECK-NEXT:    [[LOOP_BACKEDGETAKENCOUNT:%.*]] = sub nsw i1 [[IV_FINAL]], [[START]]
+; CHECK-NEXT:    [[LOOP_TRIPCOUNT:%.*]] = add nuw nsw i1 [[LOOP_BACKEDGETAKENCOUNT]], true
+; CHECK-NEXT:    br label [[LOOP:%.*]]
+; CHECK:       loop:
+; CHECK-NEXT:    [[LOOP_IV:%.*]] = phi i1 [ false, [[ENTRY:%.*]] ], [ [[LOOP_IV_NEXT:%.*]], [[LOOP]] ]
+; CHECK-NEXT:    [[LOOP_IV_NEXT]] = add nuw nsw i1 [[LOOP_IV]], true
+; CHECK-NEXT:    [[LOOP_IVCHECK:%.*]] = icmp eq i1 [[LOOP_IV_NEXT]], [[LOOP_TRIPCOUNT]]
+; CHECK-NEXT:    [[IV:%.*]] = add nsw i1 [[LOOP_IV]], [[START]]
+; CHECK-NEXT:    [[NBITS:%.*]] = add nsw i1 [[IV]], [[EXTRAOFFSET]]
+; CHECK-NEXT:    [[VAL_SHIFTED:%.*]] = lshr i1 [[VAL]], [[NBITS]]
+; CHECK-NEXT:    [[IV_NEXT:%.*]] = add i1 [[IV]], true
+; CHECK-NEXT:    call void @escape_inner.i1(i1 [[IV]], i1 [[NBITS]], i1 [[VAL_SHIFTED]], i1 [[LOOP_IVCHECK]], i1 [[IV_NEXT]])
+; CHECK-NEXT:    br i1 [[LOOP_IVCHECK]], label [[END:%.*]], label [[LOOP]]
 ; CHECK:       end:
-; CHECK-NEXT:    [[IV_RES:%.*]] = phi i1 [ [[IV]], [[LOOP]] ]
+; CHECK-NEXT:    [[IV_RES:%.*]] = phi i1 [ [[IV_FINAL]], [[LOOP]] ]
 ; CHECK-NEXT:    [[NBITS_RES:%.*]] = phi i1 [ [[NBITS]], [[LOOP]] ]
 ; CHECK-NEXT:    [[VAL_SHIFTED_RES:%.*]] = phi i1 [ [[VAL_SHIFTED]], [[LOOP]] ]
-; CHECK-NEXT:    [[VAL_SHIFTED_ISZERO_RES:%.*]] = phi i1 [ [[VAL_SHIFTED_ISZERO]], [[LOOP]] ]
+; CHECK-NEXT:    [[VAL_SHIFTED_ISZERO_RES:%.*]] = phi i1 [ [[LOOP_IVCHECK]], [[LOOP]] ]
 ; CHECK-NEXT:    [[IV_NEXT_RES:%.*]] = phi i1 [ [[IV_NEXT]], [[LOOP]] ]
 ; CHECK-NEXT:    call void @escape_outer.i1(i1 [[IV_RES]], i1 [[NBITS_RES]], i1 [[VAL_SHIFTED_RES]], i1 [[VAL_SHIFTED_ISZERO_RES]], i1 [[IV_NEXT_RES]])
 ; CHECK-NEXT:    ret i1 [[IV_RES]]
 ;
 entry:
   br label %loop
 
 loop:
end:
 
   call void @escape_outer.i1(i1 %iv.res, i1 %nbits.res, i1 %val.shifted.res, i1 %val.shifted.iszero.res, i1 %iv.next.res)
 
   ret i1 %iv.res
 }
 define i2 @t28_addnsw_i2(i2 %val, i2 %start, i2 %extraoffset) {
 ; CHECK-LABEL: @t28_addnsw_i2(
 ; CHECK-NEXT:  entry:
-; CHECK-NEXT:    br label [[LOOP:%.*]]
-; CHECK:       loop:
-; CHECK-NEXT:    [[IV:%.*]] = phi i2 [ [[START:%.*]], [[ENTRY:%.*]] ], [ [[IV_NEXT:%.*]], [[LOOP]] ]
-; CHECK-NEXT:    [[NBITS:%.*]] = add nsw i2 [[IV]], [[EXTRAOFFSET:%.*]]
-; CHECK-NEXT:    [[VAL_SHIFTED:%.*]] = lshr i2 [[VAL:%.*]], [[NBITS]]
-; CHECK-NEXT:    [[VAL_SHIFTED_ISZERO:%.*]] = icmp eq i2 [[VAL_SHIFTED]], 0
-; CHECK-NEXT:    [[IV_NEXT]] = add i2 [[IV]], 1
-; CHECK-NEXT:    call void @escape_inner.i2(i2 [[IV]], i2 [[NBITS]], i2 [[VAL_SHIFTED]], i1 [[VAL_SHIFTED_ISZERO]], i2 [[IV_NEXT]])
-; CHECK-NEXT:    br i1 [[VAL_SHIFTED_ISZERO]], label [[END:%.*]], label [[LOOP]]
+; CHECK-NEXT:    [[VAL_NUMLEADINGZEROS:%.*]] = call i2 @llvm.ctlz.i2(i2 [[VAL:%.*]], i1 false)
+; CHECK-NEXT:    [[VAL_NUMACTIVEBITS:%.*]] = sub nuw i2 -2, [[VAL_NUMLEADINGZEROS]]
+; CHECK-NEXT:    [[TMP0:%.*]] = sub i2 0, [[EXTRAOFFSET:%.*]]
+; CHECK-NEXT:    [[VAL_NUMACTIVEBITS_OFFSET:%.*]] = add nsw i2 [[VAL_NUMACTIVEBITS]], [[TMP0]]
+; CHECK-NEXT:    [[IV_FINAL:%.*]] = call i2 @llvm.smax.i2(i2 [[VAL_NUMACTIVEBITS_OFFSET]], i2 [[START:%.*]])
+; CHECK-NEXT:    [[LOOP_BACKEDGETAKENCOUNT:%.*]] = sub nsw i2 [[IV_FINAL]], [[START]]
+; CHECK-NEXT:    [[LOOP_TRIPCOUNT:%.*]] = add nuw i2 [[LOOP_BACKEDGETAKENCOUNT]], 1
+; CHECK-NEXT:    br label [[LOOP:%.*]]
+; CHECK:       loop:
+; CHECK-NEXT:    [[LOOP_IV:%.*]] = phi i2 [ 0, [[ENTRY:%.*]] ], [ [[LOOP_IV_NEXT:%.*]], [[LOOP]] ]
+; CHECK-NEXT:    [[LOOP_IV_NEXT]] = add nuw i2 [[LOOP_IV]], 1
+; CHECK-NEXT:    [[LOOP_IVCHECK:%.*]] = icmp eq i2 [[LOOP_IV_NEXT]], [[LOOP_TRIPCOUNT]]
+; CHECK-NEXT:    [[IV:%.*]] = add nsw i2 [[LOOP_IV]], [[START]]
+; CHECK-NEXT:    [[NBITS:%.*]] = add nsw i2 [[IV]], [[EXTRAOFFSET]]
+; CHECK-NEXT:    [[VAL_SHIFTED:%.*]] = lshr i2 [[VAL]], [[NBITS]]
+; CHECK-NEXT:    [[IV_NEXT:%.*]] = add i2 [[IV]], 1
+; CHECK-NEXT:    call void @escape_inner.i2(i2 [[IV]], i2 [[NBITS]], i2 [[VAL_SHIFTED]], i1 [[LOOP_IVCHECK]], i2 [[IV_NEXT]])
+; CHECK-NEXT:    br i1 [[LOOP_IVCHECK]], label [[END:%.*]], label [[LOOP]]
 ; CHECK:       end:
-; CHECK-NEXT:    [[IV_RES:%.*]] = phi i2 [ [[IV]], [[LOOP]] ]
+; CHECK-NEXT:    [[IV_RES:%.*]] = phi i2 [ [[IV_FINAL]], [[LOOP]] ]
 ; CHECK-NEXT:    [[NBITS_RES:%.*]] = phi i2 [ [[NBITS]], [[LOOP]] ]
 ; CHECK-NEXT:    [[VAL_SHIFTED_RES:%.*]] = phi i2 [ [[VAL_SHIFTED]], [[LOOP]] ]
-; CHECK-NEXT:    [[VAL_SHIFTED_ISZERO_RES:%.*]] = phi i1 [ [[VAL_SHIFTED_ISZERO]], [[LOOP]] ]
+; CHECK-NEXT:    [[VAL_SHIFTED_ISZERO_RES:%.*]] = phi i1 [ [[LOOP_IVCHECK]], [[LOOP]] ]
 ; CHECK-NEXT:    [[IV_NEXT_RES:%.*]] = phi i2 [ [[IV_NEXT]], [[LOOP]] ]
 ; CHECK-NEXT:    call void @escape_outer.i2(i2 [[IV_RES]], i2 [[NBITS_RES]], i2 [[VAL_SHIFTED_RES]], i1 [[VAL_SHIFTED_ISZERO_RES]], i2 [[IV_NEXT_RES]])
 ; CHECK-NEXT:    ret i2 [[IV_RES]]
 ;
 entry:
   br label %loop
 
 loop:
end:
 
   call void @escape_outer.i2(i2 %iv.res, i2 %nbits.res, i2 %val.shifted.res, i1 %val.shifted.iszero.res, i2 %iv.next.res)
 
   ret i2 %iv.res
 }
 define i3 @t29_addnsw_i3(i3 %val, i3 %start, i3 %extraoffset) {
 ; CHECK-LABEL: @t29_addnsw_i3(
 ; CHECK-NEXT:  entry:
-; CHECK-NEXT:    br label [[LOOP:%.*]]
-; CHECK:       loop:
-; CHECK-NEXT:    [[IV:%.*]] = phi i3 [ [[START:%.*]], [[ENTRY:%.*]] ], [ [[IV_NEXT:%.*]], [[LOOP]] ]
-; CHECK-NEXT:    [[NBITS:%.*]] = add nsw i3 [[IV]], [[EXTRAOFFSET:%.*]]
-; CHECK-NEXT:    [[VAL_SHIFTED:%.*]] = lshr i3 [[VAL:%.*]], [[NBITS]]
-; CHECK-NEXT:    [[VAL_SHIFTED_ISZERO:%.*]] = icmp eq i3 [[VAL_SHIFTED]], 0
-; CHECK-NEXT:    [[IV_NEXT]] = add i3 [[IV]], 1
-; CHECK-NEXT:    call void @escape_inner.i3(i3 [[IV]], i3 [[NBITS]], i3 [[VAL_SHIFTED]], i1 [[VAL_SHIFTED_ISZERO]], i3 [[IV_NEXT]])
-; CHECK-NEXT:    br i1 [[VAL_SHIFTED_ISZERO]], label [[END:%.*]], label [[LOOP]]
+; CHECK-NEXT:    [[VAL_NUMLEADINGZEROS:%.*]] = call i3 @llvm.ctlz.i3(i3 [[VAL:%.*]], i1 false)
+; CHECK-NEXT:    [[VAL_NUMACTIVEBITS:%.*]] = sub nuw nsw i3 3, [[VAL_NUMLEADINGZEROS]]
+; CHECK-NEXT:    [[TMP0:%.*]] = sub i3 0, [[EXTRAOFFSET:%.*]]
+; CHECK-NEXT:    [[VAL_NUMACTIVEBITS_OFFSET:%.*]] = add nsw i3 [[VAL_NUMACTIVEBITS]], [[TMP0]]
+; CHECK-NEXT:    [[IV_FINAL:%.*]] = call i3 @llvm.smax.i3(i3 [[VAL_NUMACTIVEBITS_OFFSET]], i3 [[START:%.*]])
+; CHECK-NEXT:    [[LOOP_BACKEDGETAKENCOUNT:%.*]] = sub nsw i3 [[IV_FINAL]], [[START]]
+; CHECK-NEXT:    [[LOOP_TRIPCOUNT:%.*]] = add nuw nsw i3 [[LOOP_BACKEDGETAKENCOUNT]], 1
+; CHECK-NEXT:    br label [[LOOP:%.*]]
+; CHECK:       loop:
+; CHECK-NEXT:    [[LOOP_IV:%.*]] = phi i3 [ 0, [[ENTRY:%.*]] ], [ [[LOOP_IV_NEXT:%.*]], [[LOOP]] ]
+; CHECK-NEXT:    [[LOOP_IV_NEXT]] = add nuw nsw i3 [[LOOP_IV]], 1
+; CHECK-NEXT:    [[LOOP_IVCHECK:%.*]] = icmp eq i3 [[LOOP_IV_NEXT]], [[LOOP_TRIPCOUNT]]
+; CHECK-NEXT:    [[IV:%.*]] = add nsw i3 [[LOOP_IV]], [[START]]
+; CHECK-NEXT:    [[NBITS:%.*]] = add nsw i3 [[IV]], [[EXTRAOFFSET]]
+; CHECK-NEXT:    [[VAL_SHIFTED:%.*]] = lshr i3 [[VAL]], [[NBITS]]
+; CHECK-NEXT:    [[IV_NEXT:%.*]] = add i3 [[IV]], 1
+; CHECK-NEXT:    call void @escape_inner.i3(i3 [[IV]], i3 [[NBITS]], i3 [[VAL_SHIFTED]], i1 [[LOOP_IVCHECK]], i3 [[IV_NEXT]])
+; CHECK-NEXT:    br i1 [[LOOP_IVCHECK]], label [[END:%.*]], label [[LOOP]]
 ; CHECK:       end:
-; CHECK-NEXT:    [[IV_RES:%.*]] = phi i3 [ [[IV]], [[LOOP]] ]
+; CHECK-NEXT:    [[IV_RES:%.*]] = phi i3 [ [[IV_FINAL]], [[LOOP]] ]
 ; CHECK-NEXT:    [[NBITS_RES:%.*]] = phi i3 [ [[NBITS]], [[LOOP]] ]
 ; CHECK-NEXT:    [[VAL_SHIFTED_RES:%.*]] = phi i3 [ [[VAL_SHIFTED]], [[LOOP]] ]
-; CHECK-NEXT:    [[VAL_SHIFTED_ISZERO_RES:%.*]] = phi i1 [ [[VAL_SHIFTED_ISZERO]], [[LOOP]] ]
+; CHECK-NEXT:    [[VAL_SHIFTED_ISZERO_RES:%.*]] = phi i1 [ [[LOOP_IVCHECK]], [[LOOP]] ]
 ; CHECK-NEXT:    [[IV_NEXT_RES:%.*]] = phi i3 [ [[IV_NEXT]], [[LOOP]] ]
 ; CHECK-NEXT:    call void @escape_outer.i3(i3 [[IV_RES]], i3 [[NBITS_RES]], i3 [[VAL_SHIFTED_RES]], i1 [[VAL_SHIFTED_ISZERO_RES]], i3 [[IV_NEXT_RES]])
 ; CHECK-NEXT:    ret i3 [[IV_RES]]
 ;
 entry:
   br label %loop
 
 loop:
end:
   call void @escape_outer.i3(i3 %iv.res, i3 %nbits.res, i3 %val.shifted.res, i1 %val.shifted.iszero.res, i3 %iv.next.res)
 
   ret i3 %iv.res
 }
 
 define i1 @t30_addnuw_i1(i1 %val, i1 %start, i1 %extraoffset) {
 ; CHECK-LABEL: @t30_addnuw_i1(
 ; CHECK-NEXT:  entry:
-; CHECK-NEXT:    br label [[LOOP:%.*]]
-; CHECK:       loop:
-; CHECK-NEXT:    [[IV:%.*]] = phi i1 [ [[START:%.*]], [[ENTRY:%.*]] ], [ [[IV_NEXT:%.*]], [[LOOP]] ]
-; CHECK-NEXT:    [[NBITS:%.*]] = add nuw i1 [[IV]], [[EXTRAOFFSET:%.*]]
-; CHECK-NEXT:    [[VAL_SHIFTED:%.*]] = lshr i1 [[VAL:%.*]], [[NBITS]]
-; CHECK-NEXT:    [[VAL_SHIFTED_ISZERO:%.*]] = icmp eq i1 [[VAL_SHIFTED]], false
-; CHECK-NEXT:    [[IV_NEXT]] = add i1 [[IV]], true
-; CHECK-NEXT:    call void @escape_inner.i1(i1 [[IV]], i1 [[NBITS]], i1 [[VAL_SHIFTED]], i1 [[VAL_SHIFTED_ISZERO]], i1 [[IV_NEXT]])
-; CHECK-NEXT:    br i1 [[VAL_SHIFTED_ISZERO]], label [[END:%.*]], label [[LOOP]]
+; CHECK-NEXT:    [[VAL_NUMLEADINGZEROS:%.*]] = call i1 @llvm.ctlz.i1(i1 [[VAL:%.*]], i1 false)
+; CHECK-NEXT:    [[VAL_NUMACTIVEBITS:%.*]] = sub nuw nsw i1 true, [[VAL_NUMLEADINGZEROS]]
+; CHECK-NEXT:    [[VAL_NUMACTIVEBITS_OFFSET:%.*]] = add nsw i1 [[VAL_NUMACTIVEBITS]], [[EXTRAOFFSET:%.*]]
+; CHECK-NEXT:    [[IV_FINAL:%.*]] = call i1 @llvm.smax.i1(i1 [[VAL_NUMACTIVEBITS_OFFSET]], i1 [[START:%.*]])
+; CHECK-NEXT:    [[LOOP_BACKEDGETAKENCOUNT:%.*]] = sub nsw i1 [[IV_FINAL]], [[START]]
+; CHECK-NEXT:    [[LOOP_TRIPCOUNT:%.*]] = add nuw nsw i1 [[LOOP_BACKEDGETAKENCOUNT]], true
+; CHECK-NEXT:    br label [[LOOP:%.*]]
+; CHECK:       loop:
+; CHECK-NEXT:    [[LOOP_IV:%.*]] = phi i1 [ false, [[ENTRY:%.*]] ], [ [[LOOP_IV_NEXT:%.*]], [[LOOP]] ]
+; CHECK-NEXT:    [[LOOP_IV_NEXT]] = add nuw nsw i1 [[LOOP_IV]], true
+; CHECK-NEXT:    [[LOOP_IVCHECK:%.*]] = icmp eq i1 [[LOOP_IV_NEXT]], [[LOOP_TRIPCOUNT]]
+; CHECK-NEXT:    [[IV:%.*]] = add nsw i1 [[LOOP_IV]], [[START]]
+; CHECK-NEXT:    [[NBITS:%.*]] = add nuw i1 [[IV]], [[EXTRAOFFSET]]
+; CHECK-NEXT:    [[VAL_SHIFTED:%.*]] = lshr i1 [[VAL]], [[NBITS]]
+; CHECK-NEXT:    [[IV_NEXT:%.*]] = add i1 [[IV]], true
+; CHECK-NEXT:    call void @escape_inner.i1(i1 [[IV]], i1 [[NBITS]], i1 [[VAL_SHIFTED]], i1 [[LOOP_IVCHECK]], i1 [[IV_NEXT]])
+; CHECK-NEXT:    br i1 [[LOOP_IVCHECK]], label [[END:%.*]], label [[LOOP]]
 ; CHECK:       end:
-; CHECK-NEXT:    [[IV_RES:%.*]] = phi i1 [ [[IV]], [[LOOP]] ]
+; CHECK-NEXT:    [[IV_RES:%.*]] = phi i1 [ [[IV_FINAL]], [[LOOP]] ]
 ; CHECK-NEXT:    [[NBITS_RES:%.*]] = phi i1 [ [[NBITS]], [[LOOP]] ]
 ; CHECK-NEXT:    [[VAL_SHIFTED_RES:%.*]] = phi i1 [ [[VAL_SHIFTED]], [[LOOP]] ]
-; CHECK-NEXT:    [[VAL_SHIFTED_ISZERO_RES:%.*]] = phi i1 [ [[VAL_SHIFTED_ISZERO]], [[LOOP]] ]
+; CHECK-NEXT:    [[VAL_SHIFTED_ISZERO_RES:%.*]] = phi i1 [ [[LOOP_IVCHECK]], [[LOOP]] ]
 ; CHECK-NEXT:    [[IV_NEXT_RES:%.*]] = phi i1 [ [[IV_NEXT]], [[LOOP]] ]
 ; CHECK-NEXT:    call void @escape_outer.i1(i1 [[IV_RES]], i1 [[NBITS_RES]], i1 [[VAL_SHIFTED_RES]], i1 [[VAL_SHIFTED_ISZERO_RES]], i1 [[IV_NEXT_RES]])
 ; CHECK-NEXT:    ret i1 [[IV_RES]]
 ;
 entry:
   br label %loop
 
 loop:
end:
 
   call void @escape_outer.i1(i1 %iv.res, i1 %nbits.res, i1 %val.shifted.res, i1 %val.shifted.iszero.res, i1 %iv.next.res)
 
   ret i1 %iv.res
 }
 define i2 @t31_addnuw_i2(i2 %val, i2 %start, i2 %extraoffset) {
 ; CHECK-LABEL: @t31_addnuw_i2(
 ; CHECK-NEXT:  entry:
-; CHECK-NEXT:    br label [[LOOP:%.*]]
-; CHECK:       loop:
-; CHECK-NEXT:    [[IV:%.*]] = phi i2 [ [[START:%.*]], [[ENTRY:%.*]] ], [ [[IV_NEXT:%.*]], [[LOOP]] ]
-; CHECK-NEXT:    [[NBITS:%.*]] = add nuw i2 [[IV]], [[EXTRAOFFSET:%.*]]
-; CHECK-NEXT:    [[VAL_SHIFTED:%.*]] = lshr i2 [[VAL:%.*]], [[NBITS]]
-; CHECK-NEXT:    [[VAL_SHIFTED_ISZERO:%.*]] = icmp eq i2 [[VAL_SHIFTED]], 0
-; CHECK-NEXT:    [[IV_NEXT]] = add i2 [[IV]], 1
-; CHECK-NEXT:    call void @escape_inner.i2(i2 [[IV]], i2 [[NBITS]], i2 [[VAL_SHIFTED]], i1 [[VAL_SHIFTED_ISZERO]], i2 [[IV_NEXT]])
-; CHECK-NEXT:    br i1 [[VAL_SHIFTED_ISZERO]], label [[END:%.*]], label [[LOOP]]
+; CHECK-NEXT:    [[VAL_NUMLEADINGZEROS:%.*]] = call i2 @llvm.ctlz.i2(i2 [[VAL:%.*]], i1 false)
+; CHECK-NEXT:    [[VAL_NUMACTIVEBITS:%.*]] = sub nuw i2 -2, [[VAL_NUMLEADINGZEROS]]
+; CHECK-NEXT:    [[TMP0:%.*]] = sub i2 0, [[EXTRAOFFSET:%.*]]
+; CHECK-NEXT:    [[VAL_NUMACTIVEBITS_OFFSET:%.*]] = add nsw i2 [[VAL_NUMACTIVEBITS]], [[TMP0]]
+; CHECK-NEXT:    [[IV_FINAL:%.*]] = call i2 @llvm.smax.i2(i2 [[VAL_NUMACTIVEBITS_OFFSET]], i2 [[START:%.*]])
+; CHECK-NEXT:    [[LOOP_BACKEDGETAKENCOUNT:%.*]] = sub nsw i2 [[IV_FINAL]], [[START]]
+; CHECK-NEXT:    [[LOOP_TRIPCOUNT:%.*]] = add nuw i2 [[LOOP_BACKEDGETAKENCOUNT]], 1
+; CHECK-NEXT:    br label [[LOOP:%.*]]
+; CHECK:       loop:
+; CHECK-NEXT:    [[LOOP_IV:%.*]] = phi i2 [ 0, [[ENTRY:%.*]] ], [ [[LOOP_IV_NEXT:%.*]], [[LOOP]] ]
+; CHECK-NEXT:    [[LOOP_IV_NEXT]] = add nuw i2 [[LOOP_IV]], 1
+; CHECK-NEXT:    [[LOOP_IVCHECK:%.*]] = icmp eq i2 [[LOOP_IV_NEXT]], [[LOOP_TRIPCOUNT]]
+; CHECK-NEXT:    [[IV:%.*]] = add nsw i2 [[LOOP_IV]], [[START]]
+; CHECK-NEXT:    [[NBITS:%.*]] = add nuw i2 [[IV]], [[EXTRAOFFSET]]
+; CHECK-NEXT:    [[VAL_SHIFTED:%.*]] = lshr i2 [[VAL]], [[NBITS]]
+; CHECK-NEXT:    [[IV_NEXT:%.*]] = add i2 [[IV]], 1
+; CHECK-NEXT:    call void @escape_inner.i2(i2 [[IV]], i2 [[NBITS]], i2 [[VAL_SHIFTED]], i1 [[LOOP_IVCHECK]], i2 [[IV_NEXT]])
+; CHECK-NEXT:    br i1 [[LOOP_IVCHECK]], label [[END:%.*]], label [[LOOP]]
 ; CHECK:       end:
-; CHECK-NEXT:    [[IV_RES:%.*]] = phi i2 [ [[IV]], [[LOOP]] ]
+; CHECK-NEXT:    [[IV_RES:%.*]] = phi i2 [ [[IV_FINAL]], [[LOOP]] ]
 ; CHECK-NEXT:    [[NBITS_RES:%.*]] = phi i2 [ [[NBITS]], [[LOOP]] ]
 ; CHECK-NEXT:    [[VAL_SHIFTED_RES:%.*]] = phi i2 [ [[VAL_SHIFTED]], [[LOOP]] ]
-; CHECK-NEXT:    [[VAL_SHIFTED_ISZERO_RES:%.*]] = phi i1 [ [[VAL_SHIFTED_ISZERO]], [[LOOP]] ]
+; CHECK-NEXT:    [[VAL_SHIFTED_ISZERO_RES:%.*]] = phi i1 [ [[LOOP_IVCHECK]], [[LOOP]] ]
 ; CHECK-NEXT:    [[IV_NEXT_RES:%.*]] = phi i2 [ [[IV_NEXT]], [[LOOP]] ]
 ; CHECK-NEXT:    call void @escape_outer.i2(i2 [[IV_RES]], i2 [[NBITS_RES]], i2 [[VAL_SHIFTED_RES]], i1 [[VAL_SHIFTED_ISZERO_RES]], i2 [[IV_NEXT_RES]])
 ; CHECK-NEXT:    ret i2 [[IV_RES]]
 ;
 entry:
   br label %loop
 
 loop:
end:
 
   call void @escape_outer.i2(i2 %iv.res, i2 %nbits.res, i2 %val.shifted.res, i1 %val.shifted.iszero.res, i2 %iv.next.res)
 
   ret i2 %iv.res
 }
 define i3 @t32_addnuw_i3(i3 %val, i3 %start, i3 %extraoffset) {
 ; CHECK-LABEL: @t32_addnuw_i3(
 ; CHECK-NEXT:  entry:
-; CHECK-NEXT:    br label [[LOOP:%.*]]
-; CHECK:       loop:
-; CHECK-NEXT:    [[IV:%.*]] = phi i3 [ [[START:%.*]], [[ENTRY:%.*]] ], [ [[IV_NEXT:%.*]], [[LOOP]] ]
-; CHECK-NEXT:    [[NBITS:%.*]] = add nuw i3 [[IV]], [[EXTRAOFFSET:%.*]]
-; CHECK-NEXT:    [[VAL_SHIFTED:%.*]] = lshr i3 [[VAL:%.*]], [[NBITS]]
-; CHECK-NEXT:    [[VAL_SHIFTED_ISZERO:%.*]] = icmp eq i3 [[VAL_SHIFTED]], 0
-; CHECK-NEXT:    [[IV_NEXT]] = add i3 [[IV]], 1
-; CHECK-NEXT:    call void @escape_inner.i3(i3 [[IV]], i3 [[NBITS]], i3 [[VAL_SHIFTED]], i1 [[VAL_SHIFTED_ISZERO]], i3 [[IV_NEXT]])
-; CHECK-NEXT:    br i1 [[VAL_SHIFTED_ISZERO]], label [[END:%.*]], label [[LOOP]]
+; CHECK-NEXT:    [[VAL_NUMLEADINGZEROS:%.*]] = call i3 @llvm.ctlz.i3(i3 [[VAL:%.*]], i1 false)
+; CHECK-NEXT:    [[VAL_NUMACTIVEBITS:%.*]] = sub nuw nsw i3 3, [[VAL_NUMLEADINGZEROS]]
+; CHECK-NEXT:    [[TMP0:%.*]] = sub i3 0, [[EXTRAOFFSET:%.*]]
+; CHECK-NEXT:    [[VAL_NUMACTIVEBITS_OFFSET:%.*]] = add nsw i3 [[VAL_NUMACTIVEBITS]], [[TMP0]]
+; CHECK-NEXT:    [[IV_FINAL:%.*]] = call i3 @llvm.smax.i3(i3 [[VAL_NUMACTIVEBITS_OFFSET]], i3 [[START:%.*]])
+; CHECK-NEXT:    [[LOOP_BACKEDGETAKENCOUNT:%.*]] = sub nsw i3 [[IV_FINAL]], [[START]]
+; CHECK-NEXT:    [[LOOP_TRIPCOUNT:%.*]] = add nuw nsw i3 [[LOOP_BACKEDGETAKENCOUNT]], 1
+; CHECK-NEXT:    br label [[LOOP:%.*]]
+; CHECK:       loop:
+; CHECK-NEXT:    [[LOOP_IV:%.*]] = phi i3 [ 0, [[ENTRY:%.*]] ], [ [[LOOP_IV_NEXT:%.*]], [[LOOP]] ]
+; CHECK-NEXT:    [[LOOP_IV_NEXT]] = add nuw nsw i3 [[LOOP_IV]], 1
+; CHECK-NEXT:    [[LOOP_IVCHECK:%.*]] = icmp eq i3 [[LOOP_IV_NEXT]], [[LOOP_TRIPCOUNT]]
+; CHECK-NEXT:    [[IV:%.*]] = add nsw i3 [[LOOP_IV]], [[START]]
+; CHECK-NEXT:    [[NBITS:%.*]] = add nuw i3 [[IV]], [[EXTRAOFFSET]]
+; CHECK-NEXT:    [[VAL_SHIFTED:%.*]] = lshr i3 [[VAL]], [[NBITS]]
+; CHECK-NEXT:    [[IV_NEXT:%.*]] = add i3 [[IV]], 1
+; CHECK-NEXT:    call void @escape_inner.i3(i3 [[IV]], i3 [[NBITS]], i3 [[VAL_SHIFTED]], i1 [[LOOP_IVCHECK]], i3 [[IV_NEXT]])
+; CHECK-NEXT:    br i1 [[LOOP_IVCHECK]], label [[END:%.*]], label [[LOOP]]
 ; CHECK:       end:
-; CHECK-NEXT:    [[IV_RES:%.*]] = phi i3 [ [[IV]], [[LOOP]] ]
+; CHECK-NEXT:    [[IV_RES:%.*]] = phi i3 [ [[IV_FINAL]], [[LOOP]] ]
 ; CHECK-NEXT:    [[NBITS_RES:%.*]] = phi i3 [ [[NBITS]], [[LOOP]] ]
 ; CHECK-NEXT:    [[VAL_SHIFTED_RES:%.*]] = phi i3 [ [[VAL_SHIFTED]], [[LOOP]] ]
-; CHECK-NEXT:    [[VAL_SHIFTED_ISZERO_RES:%.*]] = phi i1 [ [[VAL_SHIFTED_ISZERO]], [[LOOP]] ]
+; CHECK-NEXT:    [[VAL_SHIFTED_ISZERO_RES:%.*]] = phi i1 [ [[LOOP_IVCHECK]], [[LOOP]] ]
 ; CHECK-NEXT:    [[IV_NEXT_RES:%.*]] = phi i3 [ [[IV_NEXT]], [[LOOP]] ]
 ; CHECK-NEXT:    call void @escape_outer.i3(i3 [[IV_RES]], i3 [[NBITS_RES]], i3 [[VAL_SHIFTED_RES]], i1 [[VAL_SHIFTED_ISZERO_RES]], i3 [[IV_NEXT_RES]])
 ; CHECK-NEXT:    ret i3 [[IV_RES]]
 ;
 entry:
   br label %loop
 
 loop:
end:
 
   ret i3 %iv.res
 }
 
 
 define i1 @t33_subnsw_i1(i1 %val, i1 %start, i1 %extraoffset) {
 ; CHECK-LABEL: @t33_subnsw_i1(
 ; CHECK-NEXT:  entry:
-; CHECK-NEXT:    br label [[LOOP:%.*]]
-; CHECK:       loop:
-; CHECK-NEXT:    [[IV:%.*]] = phi i1 [ [[START:%.*]], [[ENTRY:%.*]] ], [ [[IV_NEXT:%.*]], [[LOOP]] ]
-; CHECK-NEXT:    [[NBITS:%.*]] = sub nsw i1 [[IV]], [[EXTRAOFFSET:%.*]]
-; CHECK-NEXT:    [[VAL_SHIFTED:%.*]] = lshr i1 [[VAL:%.*]], [[NBITS]]
-; CHECK-NEXT:    [[VAL_SHIFTED_ISZERO:%.*]] = icmp eq i1 [[VAL_SHIFTED]], false
-; CHECK-NEXT:    [[IV_NEXT]] = add i1 [[IV]], true
-; CHECK-NEXT:    call void @escape_inner.i1(i1 [[IV]], i1 [[NBITS]], i1 [[VAL_SHIFTED]], i1 [[VAL_SHIFTED_ISZERO]], i1 [[IV_NEXT]])
-; CHECK-NEXT:    br i1 [[VAL_SHIFTED_ISZERO]], label [[END:%.*]], label [[LOOP]]
+; CHECK-NEXT:    [[VAL_NUMLEADINGZEROS:%.*]] = call i1 @llvm.ctlz.i1(i1 [[VAL:%.*]], i1 false)
+; CHECK-NEXT:    [[VAL_NUMACTIVEBITS:%.*]] = sub nuw nsw i1 true, [[VAL_NUMLEADINGZEROS]]
+; CHECK-NEXT:    [[VAL_NUMACTIVEBITS_OFFSET:%.*]] = add nsw i1 [[VAL_NUMACTIVEBITS]], [[EXTRAOFFSET:%.*]]
+; CHECK-NEXT:    [[IV_FINAL:%.*]] = call i1 @llvm.smax.i1(i1 [[VAL_NUMACTIVEBITS_OFFSET]], i1 [[START:%.*]])
+; CHECK-NEXT:    [[LOOP_BACKEDGETAKENCOUNT:%.*]] = sub nsw i1 [[IV_FINAL]], [[START]]
+; CHECK-NEXT:    [[LOOP_TRIPCOUNT:%.*]] = add nuw nsw i1 [[LOOP_BACKEDGETAKENCOUNT]], true
+; CHECK-NEXT:    br label [[LOOP:%.*]]
+; CHECK:       loop:
+; CHECK-NEXT:    [[LOOP_IV:%.*]] = phi i1 [ false, [[ENTRY:%.*]] ], [ [[LOOP_IV_NEXT:%.*]], [[LOOP]] ]
+; CHECK-NEXT:    [[LOOP_IV_NEXT]] = add nuw nsw i1 [[LOOP_IV]], true
+; CHECK-NEXT:    [[LOOP_IVCHECK:%.*]] = icmp eq i1 [[LOOP_IV_NEXT]], [[LOOP_TRIPCOUNT]]
+; CHECK-NEXT:    [[IV:%.*]] = add nsw i1 [[LOOP_IV]], [[START]]
+; CHECK-NEXT:    [[NBITS:%.*]] = sub nsw i1 [[IV]], [[EXTRAOFFSET]]
+; CHECK-NEXT:    [[VAL_SHIFTED:%.*]] = lshr i1 [[VAL]], [[NBITS]]
+; CHECK-NEXT:    [[IV_NEXT:%.*]] = add i1 [[IV]], true
+; CHECK-NEXT:    call void @escape_inner.i1(i1 [[IV]], i1 [[NBITS]], i1 [[VAL_SHIFTED]], i1 [[LOOP_IVCHECK]], i1 [[IV_NEXT]])
+; CHECK-NEXT:    br i1 [[LOOP_IVCHECK]], label [[END:%.*]], label [[LOOP]]
 ; CHECK:       end:
-; CHECK-NEXT:    [[IV_RES:%.*]] = phi i1 [ [[IV]], [[LOOP]] ]
+; CHECK-NEXT:    [[IV_RES:%.*]] = phi i1 [ [[IV_FINAL]], [[LOOP]] ]
 ; CHECK-NEXT:    [[NBITS_RES:%.*]] = phi i1 [ [[NBITS]], [[LOOP]] ]
 ; CHECK-NEXT:    [[VAL_SHIFTED_RES:%.*]] = phi i1 [ [[VAL_SHIFTED]], [[LOOP]] ]
-; CHECK-NEXT:    [[VAL_SHIFTED_ISZERO_RES:%.*]] = phi i1 [ [[VAL_SHIFTED_ISZERO]], [[LOOP]] ]
+; CHECK-NEXT:    [[VAL_SHIFTED_ISZERO_RES:%.*]] = phi i1 [ [[LOOP_IVCHECK]], [[LOOP]] ]
 ; CHECK-NEXT:    [[IV_NEXT_RES:%.*]] = phi i1 [ [[IV_NEXT]], [[LOOP]] ]
 ; CHECK-NEXT:    call void @escape_outer.i1(i1 [[IV_RES]], i1 [[NBITS_RES]], i1 [[VAL_SHIFTED_RES]], i1 [[VAL_SHIFTED_ISZERO_RES]], i1 [[IV_NEXT_RES]])
 ; CHECK-NEXT:    ret i1 [[IV_RES]]
 ;
 entry:
   br label %loop
 
 loop:
end:
 
   call void @escape_outer.i1(i1 %iv.res, i1 %nbits.res, i1 %val.shifted.res, i1 %val.shifted.iszero.res, i1 %iv.next.res)
 
   ret i1 %iv.res
 }
 define i2 @t34_addnuw_i2(i2 %val, i2 %start, i2 %extraoffset) {
 ; CHECK-LABEL: @t34_addnuw_i2(
 ; CHECK-NEXT:  entry:
-; CHECK-NEXT:    br label [[LOOP:%.*]]
-; CHECK:       loop:
-; CHECK-NEXT:    [[IV:%.*]] = phi i2 [ [[START:%.*]], [[ENTRY:%.*]] ], [ [[IV_NEXT:%.*]], [[LOOP]] ]
-; CHECK-NEXT:    [[NBITS:%.*]] = sub nsw i2 [[IV]], [[EXTRAOFFSET:%.*]]
-; CHECK-NEXT:    [[VAL_SHIFTED:%.*]] = lshr i2 [[VAL:%.*]], [[NBITS]]
-; CHECK-NEXT:    [[VAL_SHIFTED_ISZERO:%.*]] = icmp eq i2 [[VAL_SHIFTED]], 0
-; CHECK-NEXT:    [[IV_NEXT]] = add i2 [[IV]], 1
-; CHECK-NEXT:    call void @escape_inner.i2(i2 [[IV]], i2 [[NBITS]], i2 [[VAL_SHIFTED]], i1 [[VAL_SHIFTED_ISZERO]], i2 [[IV_NEXT]])
-; CHECK-NEXT:    br i1 [[VAL_SHIFTED_ISZERO]], label [[END:%.*]], label [[LOOP]]
+; CHECK-NEXT:    [[VAL_NUMLEADINGZEROS:%.*]] = call i2 @llvm.ctlz.i2(i2 [[VAL:%.*]], i1 false)
+; CHECK-NEXT:    [[VAL_NUMACTIVEBITS:%.*]] = sub nuw i2 -2, [[VAL_NUMLEADINGZEROS]]
+; CHECK-NEXT:    [[VAL_NUMACTIVEBITS_OFFSET:%.*]] = add nsw i2 [[VAL_NUMACTIVEBITS]], [[EXTRAOFFSET:%.*]]
+; CHECK-NEXT:    [[IV_FINAL:%.*]] = call i2 @llvm.smax.i2(i2 [[VAL_NUMACTIVEBITS_OFFSET]], i2 [[START:%.*]])
+; CHECK-NEXT:    [[LOOP_BACKEDGETAKENCOUNT:%.*]] = sub nsw i2 [[IV_FINAL]], [[START]]
+; CHECK-NEXT:    [[LOOP_TRIPCOUNT:%.*]] = add nuw i2 [[LOOP_BACKEDGETAKENCOUNT]], 1
+; CHECK-NEXT:    br label [[LOOP:%.*]]
+; CHECK:       loop:
+; CHECK-NEXT:    [[LOOP_IV:%.*]] = phi i2 [ 0, [[ENTRY:%.*]] ], [ [[LOOP_IV_NEXT:%.*]], [[LOOP]] ]
+; CHECK-NEXT:    [[LOOP_IV_NEXT]] = add nuw i2 [[LOOP_IV]], 1
+; CHECK-NEXT:    [[LOOP_IVCHECK:%.*]] = icmp eq i2 [[LOOP_IV_NEXT]], [[LOOP_TRIPCOUNT]]
+; CHECK-NEXT:    [[IV:%.*]] = add nsw i2 [[LOOP_IV]], [[START]]
+; CHECK-NEXT:    [[NBITS:%.*]] = sub nsw i2 [[IV]], [[EXTRAOFFSET]]
+; CHECK-NEXT:    [[VAL_SHIFTED:%.*]] = lshr i2 [[VAL]], [[NBITS]]
+; CHECK-NEXT:    [[IV_NEXT:%.*]] = add i2 [[IV]], 1
+; CHECK-NEXT:    call void @escape_inner.i2(i2 [[IV]], i2 [[NBITS]], i2 [[VAL_SHIFTED]], i1 [[LOOP_IVCHECK]], i2 [[IV_NEXT]])
+; CHECK-NEXT:    br i1 [[LOOP_IVCHECK]], label [[END:%.*]], label [[LOOP]]
 ; CHECK:       end:
-; CHECK-NEXT:    [[IV_RES:%.*]] = phi i2 [ [[IV]], [[LOOP]] ]
+; CHECK-NEXT:    [[IV_RES:%.*]] = phi i2 [ [[IV_FINAL]], [[LOOP]] ]
 ; CHECK-NEXT:    [[NBITS_RES:%.*]] = phi i2 [ [[NBITS]], [[LOOP]] ]
 ; CHECK-NEXT:    [[VAL_SHIFTED_RES:%.*]] = phi i2 [ [[VAL_SHIFTED]], [[LOOP]] ]
-; CHECK-NEXT:    [[VAL_SHIFTED_ISZERO_RES:%.*]] = phi i1 [ [[VAL_SHIFTED_ISZERO]], [[LOOP]] ]
+; CHECK-NEXT:    [[VAL_SHIFTED_ISZERO_RES:%.*]] = phi i1 [ [[LOOP_IVCHECK]], [[LOOP]] ]
 ; CHECK-NEXT:    [[IV_NEXT_RES:%.*]] = phi i2 [ [[IV_NEXT]], [[LOOP]] ]
 ; CHECK-NEXT:    call void @escape_outer.i2(i2 [[IV_RES]], i2 [[NBITS_RES]], i2 [[VAL_SHIFTED_RES]], i1 [[VAL_SHIFTED_ISZERO_RES]], i2 [[IV_NEXT_RES]])
 ; CHECK-NEXT:    ret i2 [[IV_RES]]
 ;
 entry:
   br label %loop
 
 loop:
end:
 
   call void @escape_outer.i2(i2 %iv.res, i2 %nbits.res, i2 %val.shifted.res, i1 %val.shifted.iszero.res, i2 %iv.next.res)
 
   ret i2 %iv.res
 }
 define i3 @t35_addnuw_i3(i3 %val, i3 %start, i3 %extraoffset) {
 ; CHECK-LABEL: @t35_addnuw_i3(
 ; CHECK-NEXT:  entry:
-; CHECK-NEXT:    br label [[LOOP:%.*]]
-; CHECK:       loop:
-; CHECK-NEXT:    [[IV:%.*]] = phi i3 [ [[START:%.*]], [[ENTRY:%.*]] ], [ [[IV_NEXT:%.*]], [[LOOP]] ]
-; CHECK-NEXT:    [[NBITS:%.*]] = sub nsw i3 [[IV]], [[EXTRAOFFSET:%.*]]
-; CHECK-NEXT:    [[VAL_SHIFTED:%.*]] = lshr i3 [[VAL:%.*]], [[NBITS]]
-; CHECK-NEXT:    [[VAL_SHIFTED_ISZERO:%.*]] = icmp eq i3 [[VAL_SHIFTED]], 0
-; CHECK-NEXT:    [[IV_NEXT]] = add i3 [[IV]], 1
-; CHECK-NEXT:    call void @escape_inner.i3(i3 [[IV]], i3 [[NBITS]], i3 [[VAL_SHIFTED]], i1 [[VAL_SHIFTED_ISZERO]], i3 [[IV_NEXT]])
-; CHECK-NEXT:    br i1 [[VAL_SHIFTED_ISZERO]], label [[END:%.*]], label [[LOOP]]
+; CHECK-NEXT:    [[VAL_NUMLEADINGZEROS:%.*]] = call i3 @llvm.ctlz.i3(i3 [[VAL:%.*]], i1 false)
+; CHECK-NEXT:    [[VAL_NUMACTIVEBITS:%.*]] = sub nuw nsw i3 3, [[VAL_NUMLEADINGZEROS]]
+; CHECK-NEXT:    [[VAL_NUMACTIVEBITS_OFFSET:%.*]] = add nsw i3 [[VAL_NUMACTIVEBITS]], [[EXTRAOFFSET:%.*]]
+; CHECK-NEXT:    [[IV_FINAL:%.*]] = call i3 @llvm.smax.i3(i3 [[VAL_NUMACTIVEBITS_OFFSET]], i3 [[START:%.*]])
+; CHECK-NEXT:    [[LOOP_BACKEDGETAKENCOUNT:%.*]] = sub nsw i3 [[IV_FINAL]], [[START]]
+; CHECK-NEXT:    [[LOOP_TRIPCOUNT:%.*]] = add nuw nsw i3 [[LOOP_BACKEDGETAKENCOUNT]], 1
+; CHECK-NEXT:    br label [[LOOP:%.*]]
+; CHECK:       loop:
+; CHECK-NEXT:    [[LOOP_IV:%.*]] = phi i3 [ 0, [[ENTRY:%.*]] ], [ [[LOOP_IV_NEXT:%.*]], [[LOOP]] ]
+; CHECK-NEXT:    [[LOOP_IV_NEXT]] = add nuw nsw i3 [[LOOP_IV]], 1
+; CHECK-NEXT:    [[LOOP_IVCHECK:%.*]] = icmp eq i3 [[LOOP_IV_NEXT]], [[LOOP_TRIPCOUNT]]
+; CHECK-NEXT:    [[IV:%.*]] = add nsw i3 [[LOOP_IV]], [[START]]
+; CHECK-NEXT:    [[NBITS:%.*]] = sub nsw i3 [[IV]], [[EXTRAOFFSET]]
+; CHECK-NEXT:    [[VAL_SHIFTED:%.*]] = lshr i3 [[VAL]], [[NBITS]]
+; CHECK-NEXT:    [[IV_NEXT:%.*]] = add i3 [[IV]], 1
+; CHECK-NEXT:    call void @escape_inner.i3(i3 [[IV]], i3 [[NBITS]], i3 [[VAL_SHIFTED]], i1 [[LOOP_IVCHECK]], i3 [[IV_NEXT]])
+; CHECK-NEXT:    br i1 [[LOOP_IVCHECK]], label [[END:%.*]], label [[LOOP]]
 ; CHECK:       end:
-; CHECK-NEXT:    [[IV_RES:%.*]] = phi i3 [ [[IV]], [[LOOP]] ]
+; CHECK-NEXT:    [[IV_RES:%.*]] = phi i3 [ [[IV_FINAL]], [[LOOP]] ]
 ; CHECK-NEXT:    [[NBITS_RES:%.*]] = phi i3 [ [[NBITS]], [[LOOP]] ]
 ; CHECK-NEXT:    [[VAL_SHIFTED_RES:%.*]] = phi i3 [ [[VAL_SHIFTED]], [[LOOP]] ]
-; CHECK-NEXT:    [[VAL_SHIFTED_ISZERO_RES:%.*]] = phi i1 [ [[VAL_SHIFTED_ISZERO]], [[LOOP]] ]
+; CHECK-NEXT:    [[VAL_SHIFTED_ISZERO_RES:%.*]] = phi i1 [ [[LOOP_IVCHECK]], [[LOOP]] ]
 ; CHECK-NEXT:    [[IV_NEXT_RES:%.*]] = phi i3 [ [[IV_NEXT]], [[LOOP]] ]
 ; CHECK-NEXT:    call void @escape_outer.i3(i3 [[IV_RES]], i3 [[NBITS_RES]], i3 [[VAL_SHIFTED_RES]], i1 [[VAL_SHIFTED_ISZERO_RES]], i3 [[IV_NEXT_RES]])
 ; CHECK-NEXT:    ret i3 [[IV_RES]]
 ;
 entry:
   br label %loop
 
 loop: