Use profile info to adjust loop unroll threshold.

[danielcdh]

Summary

For flat loop, even if it is hot, it is not a good idea to unroll in runtime, thus we set a lower partial unroll threshold.
For hot loop, we set a higher unroll threshold and allows expensive tripcount computation to allow more aggressive unrolling.

[mzolotukhin]

Hi,

Thanks for working on this! Please find some comments inline.

Michael

lib/Transforms/Scalar/LoopUnrollPass.cpp

762

This looks like a magic number to me. Can we use some parameter for it (or maybe separate thresholds for 'hot' and 'cold' loops)?

test/Other/pass-pipelines.ll

49–50 ↗

(On Diff #77556)

Hmm, is loop-unroll in a separate instance of loop pass manager now?

[danielcdh]

Update the patch to remove dependency to BFI/PSI and only use trip count to evaluate if we want to unroll the loop.

Also steal Micheal's getLoopEstimatedTripCount implementation.

lib/Transforms/Scalar/LoopUnrollPass.cpp

762

Logic removed from the patch

test/Other/pass-pipelines.ll

49–50 ↗

(On Diff #77556)

Removed dependency to BFI/PSI

[mzolotukhin]

The change looks good to me, thank you! I'm assuming you and Michael will figure out which version of getLoopEstimatedTripCount you want to use, other than that I have mostly nitpicky comments below.

BTW, do you have performance testing results for this patch? I'd expect some improvements in code-size and compile-time with these changes.

Michael

lib/Transforms/Scalar/LoopUnrollPass.cpp

757

Please add some comment here.

758–759

if (auto ProfileTripCount = getLoopEstimatedTripCount(L)) ?

lib/Transforms/Utils/LoopUtils.cpp

1071

This version and the one from D25963 should eventually become the same, right?

1077–1078

Probably we also need to check that the latch is exiting (i.e. the branch is conditional).

test/Transforms/LoopUnroll/unroll-heuristics-pgo.ll

5

Please add @ to the name.

6

Is it enough to just check presence of the prologue? Maybe explicitly check that we have several copies of some instruction?

[danielcdh]

update

lib/Transforms/Utils/LoopUtils.cpp

1071

Yes, I stole the code from D25963 ;-)

[danielcdh]

The perf/size impact of this patch is small on speccpu as flat loop is rare in most of the benchmarks.

spec/2006/fp/C++/444.namd 25.47 +0.30%
spec/2006/fp/C++/447.dealII 45.46 +0.23%
spec/2006/fp/C++/450.soplex 43.38 +0.58%
spec/2006/fp/C++/453.povray 37.88 -0.78%
spec/2006/fp/C/433.milc 23.75 -0.13%
spec/2006/fp/C/470.lbm 41.53 -0.09%
spec/2006/fp/C/482.sphinx3 48.97 -0.11%
spec/2006/int/C++/471.omnetpp 22.79 -0.22%
spec/2006/int/C++/473.astar 22.99 +0.17%
spec/2006/int/C++/483.xalancbmk 38.55 -0.42%
spec/2006/int/C/400.perlbench 37.06 +1.14%
spec/2006/int/C/401.bzip2 23.38 +0.98%
spec/2006/int/C/403.gcc 34.52 -0.25%
spec/2006/int/C/429.mcf 42.28 -0.05%
spec/2006/int/C/445.gobmk 27.98 +0.60%
spec/2006/int/C/456.hmmer 26.01 -0.06%
spec/2006/int/C/458.sjeng 30.42 +0.50%
spec/2006/int/C/462.libquantum 57.48 +0.37%
spec/2006/int/C/464.h264ref 47.6 -0.70%

geometric mean +0.11%

include/llvm/Transforms/Utils/LoopUtils.h

 /// Optional's not-a-value.
 Optional<const MDOperand *> findStringMetadataForLoop(Loop *TheLoop,
                                                       StringRef Name);
 
 /// \brief Set input string into loop metadata by keeping other values intact.
 void addStringMetadataToLoop(Loop *TheLoop, const char *MDString,
                              unsigned V = 0);
 
+/// \brief Get a loop's estimated trip count based on branch weight metadata.
+/// Returns 0 when the count is estimated to be 0, or None when a meaningful
+/// estimate can not be made.
+Optional<unsigned> getLoopEstimatedTripCount(Loop *L);
+
 /// Helper to consistently add the set of standard passes to a loop pass's \c
 /// AnalysisUsage.
 ///
 /// All loop passes should call this as part of implementing their \c
 /// getAnalysisUsage.
 void getLoopAnalysisUsage(AnalysisUsage &AU);
 
 /// Returns true if the hoister and sinker can handle this instruction.

lib/Transforms/Scalar/LoopUnrollPass.cpp

static cl::opt<unsigned> UnrollMaxUpperBound(
     cl::desc(
         "The max of trip count upper bound that is considered in unrolling"));
 
 static cl::opt<unsigned> PragmaUnrollThreshold(
     "pragma-unroll-threshold", cl::init(16 * 1024), cl::Hidden,
     cl::desc("Unrolled size limit for loops with an unroll(full) or "
              "unroll_count pragma."));
 
+static cl::opt<unsigned> FlatLoopTripCountThreshold(
+    "flat-loop-tripcount-threshold", cl::init(5), cl::Hidden,
+    cl::desc("If the runtime tripcount for the loop is lower than the "
+             "threshold, the loop is considered as flat and will be less "
+             "aggressively unrolled."));
+
 /// A magic value for use with the Threshold parameter to indicate
 /// that the loop unroll should be performed regardless of how much
 /// code expansion would result.
 static const unsigned NoThreshold = UINT_MAX;
 
 /// Gather the various unrolling parameters based on the defaults, compiler
 /// flags, TTI overrides and user specified parameters.
 static TargetTransformInfo::UnrollingPreferences gatherUnrollingPreferences(
if (PragmaFullUnroll && TripCount != 0) {
     if (getUnrolledLoopSize(LoopSize, UP) < PragmaUnrollThreshold)
       return false;
   }
 
   bool PragmaEnableUnroll = HasUnrollEnablePragma(L);
   bool ExplicitUnroll = PragmaCount > 0 || PragmaFullUnroll ||
                         PragmaEnableUnroll || UserUnrollCount;
 
+  if (L->getHeader()->getParent()->getEntryCount() && TripCount == 0) {
+    auto ProfileTripCount = getLoopEstimatedTripCount(L);
+    if (ProfileTripCount) {
+      if (*ProfileTripCount < FlatLoopTripCountThreshold)
+        return false;
+      else
+        UP.AllowExpensiveTripCount = true;
+    }
+  }
+
   if (ExplicitUnroll && TripCount != 0) {
     // If the loop has an unrolling pragma, we want to be more aggressive with
     // unrolling limits. Set thresholds to at least the PragmaThreshold value
     // which is larger than the default limits.
     UP.Threshold = std::max<unsigned>(UP.Threshold, PragmaUnrollThreshold);
     UP.PartialThreshold =
         std::max<unsigned>(UP.PartialThreshold, PragmaUnrollThreshold);
   }

lib/Transforms/Utils/LoopUtils.cpp

bool llvm::isGuaranteedToExecute(const Instruction &Inst,
   if (ExitBlocks.empty())
     return false;
 
   // FIXME: In general, we have to prove that the loop isn't an infinite loop.
   // See http::llvm.org/PR24078 .  (The "ExitBlocks.empty()" check above is
   // just a special case of this.)
   return true;
 }
+
+Optional<unsigned> llvm::getLoopEstimatedTripCount(Loop *L) {
+  // Only support loops with a unique exiting block, and a latch.
+  if (!L->getExitingBlock())
+    return None;
+
+  // Get the branch weights for the the loop's backedge.
+  BranchInst *LatchBR =
+      dyn_cast<BranchInst>(L->getLoopLatch()->getTerminator());
+  if (!LatchBR)
+    return None;
+
+  assert((LatchBR->getSuccessor(0) == L->getHeader() ||
+          LatchBR->getSuccessor(1) == L->getHeader()) &&
+         "At least one edge out of the latch must go to the header");
+
+  // To estimate the number of times the loop body was executed, we want to
+  // know the number of times the backedge was taken, vs. the number of times
+  // we exited the loop.
+  // The branch weights give us almost what we want, since they were adjusted
+  // from the raw counts to provide a better probability estimate. Remove
+  // the adjustment by subtracting 1 from both weights.
+  uint64_t TrueVal, FalseVal;
+  if (!LatchBR->extractProfMetadata(TrueVal, FalseVal) || (TrueVal <= 1) ||
+      (FalseVal <= 1))
+    return None;
+
+  TrueVal -= 1;
+  FalseVal -= 1;
+
+  // Divide the count of the backedge by the count of the edge exiting the loop.
+  if (LatchBR->getSuccessor(0) == L->getHeader())
+    return TrueVal / FalseVal;
+  else
+    return FalseVal / TrueVal;
+}

test/Transforms/LoopUnroll/unroll-heuristics-pgo.ll

+; RUN: opt < %s -S -loop-unroll -unroll-runtime -unroll-threshold=40 -unroll-dynamic-cost-savings-discount=0 | FileCheck %s
+
+@known_constant = internal unnamed_addr constant [9 x i32] [i32 0, i32 -1, i32 0, i32 -1, i32 5, i32 -1, i32 0, i32 -1, i32 0], align 16
+
+; CHECK-LABEL: bar_prof
+; CHECK: loop.prol
+define i32 @bar_prof(i32* noalias nocapture readonly %src, i64 %c) !prof !1 {
+entry:
+  br label %loop
+
+loop:
+  %iv = phi i64 [ 0, %entry ], [ %inc, %loop ]
+  %r  = phi i32 [ 0, %entry ], [ %add, %loop ]
+  %arrayidx = getelementptr inbounds i32, i32* %src, i64 %iv
+  %src_element = load i32, i32* %arrayidx, align 4
+  %array_const_idx = getelementptr inbounds [9 x i32], [9 x i32]* @known_constant, i64 0, i64 %iv
+  %const_array_element = load i32, i32* %array_const_idx, align 4
+  %mul = mul nsw i32 %src_element, %const_array_element
+  %add = add nsw i32 %mul, %r
+  %inc = add nuw nsw i64 %iv, 1
+  %exitcond86.i = icmp eq i64 %inc, %c
+  br i1 %exitcond86.i, label %loop.end, label %loop, !prof !2
+
+loop.end:
+  %r.lcssa = phi i32 [ %r, %loop ]
+  ret i32 %r.lcssa
+}
+
+; CHECK-LABEL: bar_prof_flat
+; CHECK-NOT: loop.prol
+define i32 @bar_prof_flat(i32* noalias nocapture readonly %src, i64 %c) !prof !1 {
+entry:
+  br label %loop
+
+loop:
+  %iv = phi i64 [ 0, %entry ], [ %inc, %loop ]
+  %r  = phi i32 [ 0, %entry ], [ %add, %loop ]
+  %arrayidx = getelementptr inbounds i32, i32* %src, i64 %iv
+  %src_element = load i32, i32* %arrayidx, align 4
+  %array_const_idx = getelementptr inbounds [9 x i32], [9 x i32]* @known_constant, i64 0, i64 %iv
+  %const_array_element = load i32, i32* %array_const_idx, align 4
+  %mul = mul nsw i32 %src_element, %const_array_element
+  %add = add nsw i32 %mul, %r
+  %inc = add nuw nsw i64 %iv, 1
+  %exitcond86.i = icmp eq i64 %inc, %c
+  br i1 %exitcond86.i, label %loop, label %loop.end, !prof !2
+
+loop.end:
+  %r.lcssa = phi i32 [ %r, %loop ]
+  ret i32 %r.lcssa
+}
+
+!1 = !{!"function_entry_count", i64 1}
+!2 = !{!"branch_weights", i32 1, i32 1000}