[LV] Vectorizer should adjust trip count in profile information

[ebrevnov]

Summary

Vectorized loop processes VFxUF number of elements in one iteration thus total number of iterations decreases proportionally. In addition epilog loop may not have more than VFxUF - 1 iterations. This patch updates profile information accordingly.

[ebrevnov]

Minor test update

[ebrevnov]

ping

[DaniilSuchkov]

LGTM

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

3975

Nit: VFxUF - 1

3981–3983

Style: it is usually advised to turn such conditions into early exits, it would reduce required indentation and slightly improve readability.

4002

Maybe introduce a new variable for this value (like EpilogueTakenCount)? Right now it's a bit surprising that OrigSomething is being changed. Same goes to OrigFallThroughCount.

[ebrevnov]

Minor fixes as requested by reviewer.

[ebrevnov]

I realized that current implementation has a flaw and and we should take into account that actual number of iterations is one greater than back edge taken count. In addition I believe that current structuring of calculations is easier for understanding.

[DaniilSuchkov]

LGTM

[Ayal]

Adding a few comments. Would be good to generalize and apply also to loop unroll (and jam).

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

3984

OrigFallThroughCount can still be either the exit count or the continue-to-next-iteration count, according to the code below. Wait to test if its zero until we know what it stands for?

3996

Better use distinct names, e.g., OrigExitCount and OrigBackedgeTakenCount, than continue to call them Taken and FallThrough. Perhaps use Weight instead of Count, to denote total profile frequencies, as the latter is used elsewhere to denote the actual per-invocation TripCount.

3999

bel[l]ow

4001

How about "OrigAverageTripCount"?

Explanation about its computation:
OrigAverageTripCount = (number of times header block was executed) / (number of times header was reached from pre-header == number of times latch exited)
== (OrigTakenCount + OrigFallThroughCount) / OrigFallThroughCount
== OrigTakenCount / OrigFallThroughCount + 1.

4003

How about VecAverageTripCount = OrigAverageTripCount / (VF * UF);

4007

Just to clarify, maintaining branch frequencies through optimizations is best-effort and imprecise - a total weight that does not divide VF*UF implies that the trip count of at-least one invocation did not divide VF*UF, not necessarily all of them; w/o considering also the distribution of trip counts in addition to their sum.

Setting PRIterCount = 0 and VecAverageTipCount = round(OrigAverageTripCount / (VF*UV)) when Cost->foldTailByMasking() is probably the best that can be done. The former is redundant given that it applies to dead code, and the latter should perhaps apply to all cases, in general.

4011

There's also the special case of requiresScalarEpiloque() where 0 < PEIterCount <= VF*UF for each invocation of the loop, and hence the average is also strictly positive FWIW. But best keep the approximation general instead of trying to improve it, given general lack of information.

4018

This assumes the number of times the vector loop will be reached is equal to the number of times the original scalar loop was reached (OrigFallThrougCount). This holds is Cost->foldTailByMasking(), but otherwise invocations whose trip count < VF*UF will bypass the vector loop (and also == VF*UF if requireScalarEpilogue()), plus other run time guards.

4027

Similar to above comment, invocations whose trip count divides VF*UF will bypass the scalar remainder loop (w/o foldTailByMasking nor requireScalarEpilogue), so in general PEFallThroughCount <= OrigFallThroughCount.

llvm/test/Transforms/LoopVectorize/check-prof-info.ll

4

May want to also check with UF>1.

[ebrevnov]

Addressed Ayal's comments

[ebrevnov]

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

3984

Good catch. Thanks!

3996

I fixed names. But I don't see reasons to use different variables here (if this is what you meant)

3999

fixed

4001

Ok. Turned your explanation to a comment.

4011

Agree. Let me remove this special case then.

4018

Please note that VecFallThrough is zero initially and set to OrigFallThrougCount only if vector loop is expected to be executed (VecIterCount > 0)

4027

Same explanation as for the above.

llvm/test/Transforms/LoopVectorize/check-prof-info.ll

4

I replaced masked case since we don't do anything special for it now.

[ebrevnov]

ping @Ayal

[Ayal]

Still think it would be better to provide this as a standalone function in Transforms/Utils/LoopUtils, for potential benefit of loop unroll (and jam) passes in addition to LV. Having agreed to ignore foldTail and requiresScalarEpilog, there's nothing vectorization-specific to do here. There's still an issue though with the fact that LV may use the scalar loop for both the remaining TC%(VF*UF) iterations when running the vector loop, and for all TC iterations when runtime guards bypass the vector loop. In absence of information, each such guard could be assigned 0.5 probability, or one could be aggressively optimistic and hope vector loop is always reached. In any case this deserves a comment.

Suggesting further variable name changes for the three Orig, Unrolled, and Remainder Loops, each having a LoopEntry==LoopExit edge weight, a Backedge weight, a HeaderBlock weight, and an AverageTripCount. The actual weights are recorded as TrueVal and FalseVal of the latch branches.

Patch needs to be clang-format'ed.

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

3973

"is less ... than original TC" >> "is smaller than the original TC by a factor of VFxUF"

3980

OrigTakenWeight >> OrigBackedgeTakenWeight or OrigBackedgeWeight ?
OrigExitWeight >> OrigLoopExitWeight?
May help to also set OrigLoopEntryWeight = OrigLoopExitWeight?

3983

OrigBackBranchI >> OrigLoopLatchBranch ?

3986

It seems clearer to call extractProfMetadata(TrueVal, FalseVal) and then set BackedgeTaken and LoopExit/Entry weights according to if (OrigLoopLatchBranch->getSuccessor(0) == OrigLoop->getHeader()), following LoopUtil's getLoopEstimatedTripCount(). Analogously for createBranchWeights(TrueVal, FalseVal).

In any case, better rename "IsTrueBackEdge*Loop".

4000

Patch needs to be clang-format'ed

4007

Instead of providing the explanation in a comment, seems better to implement the code this way, leaving the +1 for the compiler to optimize. I.e.,

const uint64_t OrigHeaderBlockWeight = OrigBackedgeTakenWeight + OrigLoopEntryWeight;
const unit64_t OrigAverageTripCount = OrigHeaderBlockWeight / OrigLoopEntryWeight;

4014

Better rename/expand "PE".
PEIterCount >> RemainderLoopAverageTripCount?

4018

In the general context, "Vec" >> "UnrolledLoop".
VecTakenCount >> UnrolledLoopBackedgeWeight
VecFallThrough >> UnrolledLoopExitWeight and/or UnrolledLoopEntryWeight

4020

How about

if (UnrolledLoopAverageTripCount > 0) {
  UnrolledLoopEntryWeight = OrigLoopEntryWeight;
  uint64_t UnrolledLoopHeaderWeight = UnrolledLoopAverageTripCount * UnrolledLoopEntryWeight; // Analogous to computing OrigLoopAverageTripCount from Header and Entry weights above.
  UnrolledLoopBackedgeWeight = UnrolledLoopHeaderWeight - UnrolledLoopEntryWeight;
}

leaving the -1 optimization to the compiler.

4026

PETakenCount >> RemainderLoopBackedgeWeight
PEFallThroughCount >> RemainderLoopExitWeight and/or RemainderLoopEntryWeight

[ebrevnov]

In D67905#1770563, @Ayal wrote:

Still think it would be better to provide this as a standalone function in Transforms/Utils/LoopUtils, for potential benefit of loop unroll (and jam) passes in addition to LV. Having agreed to ignore foldTail and requiresScalarEpilog, there's nothing vectorization-specific to do here. There's still an issue though with the fact that LV may use the scalar loop for both the remaining TC%(VF*UF) iterations when running the vector loop, and for all TC iterations when runtime guards bypass the vector loop. In absence of information, each such guard could be assigned 0.5 probability, or one could be aggressively optimistic and hope vector loop is always reached. In any case this deserves a comment.

Suggesting further variable name changes for the three Orig, Unrolled, and Remainder Loops, each having a LoopEntry==LoopExit edge weight, a Backedge weight, a HeaderBlock weight, and an AverageTripCount. The actual weights are recorded as TrueVal and FalseVal of the latch branches.

Patch needs to be clang-format'ed.

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

3986

Don't feel convinced. My point would be that extra variables and conditional reassignments make the code less readable. I think this is very subjective thing.

4020

That will make computations less stable to overflow. Personally I feel the way it's written today has the same level of complexity for understanding.

[ebrevnov]

Addressing issues raised by Ayal.

[ebrevnov]

Typo fixed.

[ebrevnov]

ping @Ayal

[Ayal]

llvm/include/llvm/Transforms/Utils/LoopUtils.h

361

The fact that OrigLoop is both the original loop containing the original profile weights, and acts as the RemainderLoop dedicated to leftover iterations, should be clarified. Alternatively, this utility can receive three loops: OrigLoop, UnrolledLoop and RemainderLoop, leaving it to the caller to decide if to pass OrigLoop also as RemainderLoop.

Would probably be clearer to start with UnrolledLoop receiving weights that reflect TC/UF iterations, and then OrigLoop which receives weights that reflect the remaining TC%UF iterations.

363

U[n]rolledLoop, several occurrences.

"\UF" >> "\p UF"

llvm/lib/Transforms/Utils/LoopUtils.cpp

1040

Worth commenting that OrigLoopEntryWeight also holds OrigLoopExitWeight, which is more clearly the weight associated with the (exit direction of the) latch branch.

1048

UnrolledBBI >> Unrolled[Loop]LatchBranch, as in OrigLoopLatchBranch.

As the names end up overflowing lines, can use Orig, Unrolled and Remainder to stand for OrigLoop, UnrolledLoop and RemainderLoop; i.e., taking "Loop" out.

1051

VecLoop >> UnrolledLoop

1061

Can drop the 'const', for consistency; these temporaries are obviously const's.

1064

Note that this is rounding down. Can add half of the denominator to the nominator before dividing in order to round more accurately; this is what getLoopEstimatedTripCount() does, but it seems to be off by 1 as it computes BackEdgeTakenWeight / LoopEntryWeight rounded to nearest, instead of HeaderBlockWeight / LoopEntryWeight rounded to nearest...

Simply call OrigAverageTripCount = getLoopEstimatedTripCount(OrigLoop)?

Perhaps having a setLoopEstimatedTripCount(Loop, EstimatedTripCount, EstimatedEntryWeight) would help fold the identical treatment of UnrolledLoop and RemainderLoop into one function, which also takes care of figuring out the True/False vs. Backedge/Exit directions?

1066

U[n]rolledAverageTripCount

1076

Seems slightly more logical to first set UnrolledLoopEntryWeight, and then using it set UnrolledLoopBackedgeWeight.

1085

ditto

1098

(This actually replaces the old profile metadata with the new one.)

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

118

Is this include still needed here?

3456

Comment below should start with a short sentence explaining that profile weights associated with the original loop are now distributed among the vector and scalar loops.

[ebrevnov]

llvm/lib/Transforms/Utils/LoopUtils.cpp

1040

IMHO instead of trying to clarify with a comment we better find self descriptive name for such a simple and commonly used thing. Strictly speaking OrigLoopEntryWeight != OrigLoopExitWeight. Do you find OrigBackEdgeExitWeight good enough?

1048

Removed "Loop" from most names to make them a little shorter.

1064

This "off by 1" stopped me from using it in the first place since that could be important in some cases. OK, let's reuse getLoopEstimatedTripCount. To be able to do that there are some changes to getLoopEstimatedTripCount.

[ebrevnov]

Updated as requested.

[ebrevnov]

Hi @Ayal. Thanks for you input. I fixed all places as you suggested. Please check.

[Ayal]

Thanks for making all the changes! More comments inline.

llvm/lib/Transforms/Utils/LoopUtils.cpp

680–683

Comment what this new function is for. Rename (see below)? Retain "Support loops ..." comment, added in D64553?

705

dyn_cast >> cast

Perhaps update above function to do here something like
BranchInst *LatchBR = getExpectedExitLoopLatchBranch(L)
checking if it returned nullptr or not?

722

Thanks for taking care of this fix to improve accuracy of estimated TC!
But doing so deserves a separate patch, and tests. Note that it also effects loop unrolling, i.e., its effects are beyond LV. This part can be introduced either before or after the part that teaches LV to maintain profiling info.

743

ditto (dyn_cast >> cast, ...)

744

Better check similar to above if (LatchBR->getSuccessor(0) != L->getHeader())

1036

"the \p UnrolledLoop \p RemainderLoop" >>
"\p UnrolledLoop and \p RemainderLoop"

1040

May also be worthwhile asserting that UF is positive (or greater than 1?)

1040

IMHO instead of trying to clarify with a comment we better find self descriptive name for such a simple and commonly used thing.

Definitely agree with (the preference of) finding self descriptive variable names.

Strictly speaking OrigLoopEntryWeight != OrigLoopExitWeight"

How so, given that OrigLoop has a single-entry, and an "expected" single-exit: there may be other "side/deopt" exits, but these are expected to have zero weight. E.g., when computing LoopHeaderWeight above, adding LoopEntryWeight (instead of LoopExitWeight) to BackEdgeTakenWeight seems more logical.

Do you find OrigBackEdgeExitWeight good enough?

Strictly speaking, a BackEdge cannot exit: it is an edge going from a latch block to a header block, and "BackEdgeTaken" is the number of times this edge is taken=traversed, which equals the number of times its latch branch is "taken" rather than "falls-thru" (if it's true direction points to the header). Perhaps LoopEntryExitWeight or LoopInvocationWeight could be used instead/in addition.

1041

are expected to be distinct

1064

This "off by 1" stopped me from using it in the first place since that could be important in some cases. OK, let's reuse getLoopEstimatedTripCount. To be able to do that there are some changes to getLoopEstimatedTripCount.

Indeed, best fix and reuse, in a dedicated patch as raised above, thereby isolating impact on such "some cases".

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

3465

"is not taken into account as unlikely case" >>
"is ignored, assigning all the weight to the vector loop, optimistically"

llvm/test/Transforms/LoopVectorize/check-prof-info.ll

7

Tests targeting x86 need to reside in LoopVectorize/X86

[ebrevnov]

One more round of updates.

[ebrevnov]

llvm/lib/Transforms/Utils/LoopUtils.cpp

680–683

The comment is related to get/setLoopEstimatedTripCount and still there...

705

I was thinking about that in the first place but didn't come up with a "good" enough name. I can go with that name if you like :-)

722

Ok.

1040

I think we better support 1 which could be used in some corner cases....

1040

How so, given that OrigLoop has a single-entry, and an "expected" single-exit: there may be other "side/deopt" exits, but these are expected to have zero weight. E.g., when computing LoopHeaderWeight above, adding LoopEntryWeight (instead of LoopExitWeight) to BackEdgeTakenWeight seems more logical.

For infinite loop entry is 1 while exit is 0 :-). I understand this is extreme we will never meet but still....

Strictly speaking, a BackEdge cannot exit: it is an edge going from a latch block to a header block, and "BackEdgeTaken" is the number of times this edge is taken=traversed, which equals the number of times its latch branch is "taken" rather than "falls-thru" (if it's true direction points to the header).

That's why I used TakenCount and FallThroughCount in the very first version what perfectly matches your description. I don't feel we are getting any better names with more iterations.... Perhaps LatchCycleWeight - number of times we go to loop header from the latch and LatchExitWeight - number of times we go to loop exit from the latch?

Perhaps LoopEntryExitWeight or LoopInvocationWeight could be used instead/in addition.

I think LoopEntryExitWeight may be confusing....
I think it makes sense to use EstimatedLoopInvocationWeight in conjunction with EstimatedTripCount as parameters to get/setEstimatedTripCount interface while LatchCycleWeight and LatchExitWeight in the implementation as they are little-bit more low level.

[ebrevnov]

Rebase

[ebrevnov]

Rebase

[ebrevnov]

ping @Ayal

[Ayal]

This looks good to me, thanks!

llvm/lib/Transforms/Utils/LoopUtils.cpp

748

The last part could call fixupBranchWeights() if moved here from Transforms/Utils/LoopUnrollPeel.cpp

llvm/test/Transforms/LoopVectorize/tripcount.ll

211

Following this, to clarify:
original loop has latchExitWeight=10 and backedgeTakenWeight=10,000, therefore estimatedBackedgeTakenCount=1,000 and estimatedTripCount=1,001.
Vectorizing by 4 produces
estimatedTripCounts of 1,001/4=250 and 1,001%4=1 for vectorized and remainder loops, respectively, therefore their
estimatedBackedgeTakenCounts are 249 and 0, and so the weights recorded with loop invocation weights of 10 are the above {10, 2490} and {10, 0}.

[ebrevnov]

llvm/lib/Transforms/Utils/LoopUtils.cpp

748

That would require to change implementation for fixupBranchWeights since it disregards to update when back edge taken count is zero.

llvm/test/Transforms/LoopVectorize/tripcount.ll

211

I will add this text to the test. I that what you wanted (just not sure :-))?

[ebrevnov]

Rebase

[Ayal]

llvm/lib/Transforms/Utils/LoopUtils.cpp

748

Agreed. Regarding disregarding to update when backedge taken weight is zero, note that in fixupBranchWeights(), BackedgeTakenWeight is called FallThroughWeight(?) "The weight of the edge from Latch to Header", and that

// FallThroughWeight is 0 means that there is no branch weights on original
// latch block or estimated trip count is zero.

Regarding the first meaning of 0, whoever calls fixupBranchWeights() should do so only if there were such weights on the original latch block, similar to the caller of setLoopEstimatedTripCount(). Regarding the second meaning of 0, it seem fixupBranchWeights() suffers from same +1 issue: estimating trip count to be zero when backedge taken weight is zero.

It would be good to fix and centralize the support for updating weights of loops, but such refactoring can be done as a separate follow-up patch, after landing this (accepted) patch.

llvm/include/llvm/Transforms/Utils/LoopUtils.h

 /// Returns true if \p S is defined and never is equal to signed/unsigned max.
 bool cannotBeMaxInLoop(const SCEV *S, const Loop *L, ScalarEvolution &SE,
                        bool Signed);
 
 /// Returns true if \p S is defined and never is equal to signed/unsigned min.
 bool cannotBeMinInLoop(const SCEV *S, const Loop *L, ScalarEvolution &SE,
                        bool Signed);
 
+/// Update profile info for the \p OrigLoop and \p UnrolledLoop so that original
+/// number of iterations in the \p OrigLoop (TC) are distributed as follows.
+/// \p OrigLoop gets TC%UF iterations, while rest iterations are executed as
+/// part of \p UrolledLoop. In addition, \p UrolledLoop executes blocks of \UF
+/// original iterations thus will do TC/UF iterations in total.
+///
+/// This utility may be useful for such optimizations as unroller and
+/// vectorizer as it's typical transformation for them.
+///
+/// If \p OrigLoop has no profile info associated nothing is done.
+void fixProfileInfoAfterUnrolling(Loop *OrigLoop, Loop *UrolledLoop,
+                                  uint64_t UF);
+
 } // end namespace llvm
 
 #endif // LLVM_TRANSFORMS_UTILS_LOOPUTILS_H

llvm/lib/Transforms/Utils/LoopUtils.cpp

 #include "llvm/Analysis/ScalarEvolutionExpander.h"
 #include "llvm/Analysis/ScalarEvolutionExpressions.h"
 #include "llvm/Analysis/TargetTransformInfo.h"
 #include "llvm/Analysis/ValueTracking.h"
 #include "llvm/IR/DIBuilder.h"
 #include "llvm/IR/Dominators.h"
 #include "llvm/IR/Instructions.h"
 #include "llvm/IR/IntrinsicInst.h"
+#include "llvm/IR/MDBuilder.h"
 #include "llvm/IR/Module.h"
 #include "llvm/IR/PatternMatch.h"
 #include "llvm/IR/ValueHandle.h"
 #include "llvm/InitializePasses.h"
 #include "llvm/Pass.h"
 #include "llvm/Support/Debug.h"
 #include "llvm/Support/KnownBits.h"
 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
if (LI) {
     blocks.insert(L->block_begin(), L->block_end());
     for (BasicBlock *BB : blocks)
       LI->removeBlock(BB);
 
     // The last step is to update LoopInfo now that we've eliminated this loop.
     LI->erase(L);
   }
 }
 
 Optional<unsigned> llvm::getLoopEstimatedTripCount(Loop *L) {
   // Support loops with an exiting latch and other existing exists only
   // deoptimize.
 
   // Get the branch weights for the loop's backedge.
   BasicBlock *Latch = L->getLoopLatch();
   if (!Latch)
     return None;
   BranchInst *LatchBR = dyn_cast<BranchInst>(Latch->getTerminator());
   if (!LatchBR || LatchBR->getNumSuccessors() != 2 || !L->isLoopExiting(Latch))
     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");
 
   SmallVector<BasicBlock *, 4> ExitBlocks;
   L->getUniqueNonLatchExitBlocks(ExitBlocks);
   if (any_of(ExitBlocks, [](const BasicBlock *EB) {
         return !EB->getTerminatingDeoptimizeCall();
       }))
     return None;
 
   // 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.
   uint64_t TrueVal, FalseVal;
   if (!LatchBR->extractProfMetadata(TrueVal, FalseVal))
     return None;
 
   if (!TrueVal || !FalseVal)
     return 0;
 
   // Divide the count of the backedge by the count of the edge exiting the loop,
   // rounding to nearest.
   if (LatchBR->getSuccessor(0) == L->getHeader())
     return (TrueVal + (FalseVal / 2)) / FalseVal;
   else
     return (FalseVal + (TrueVal / 2)) / TrueVal;
 }
 
 bool llvm::hasIterationCountInvariantInParent(Loop *InnerLoop,
                                               ScalarEvolution &SE) {
   Loop *OuterL = InnerLoop->getParentLoop();
   if (!OuterL)
     return true;
 
   // Get the backedge taken count for the inner loop
   BasicBlock *InnerLoopLatch = InnerLoop->getLoopLatch();
   const SCEV *InnerLoopBECountSC = SE.getExitCount(InnerLoop, InnerLoopLatch);
   if (isa<SCEVCouldNotCompute>(InnerLoopBECountSC) ||
       !InnerLoopBECountSC->getType()->isIntegerTy())
     return false;
 
   // Get whether count is invariant to the outer loop
   ScalarEvolution::LoopDisposition LD =
       SE.getLoopDisposition(InnerLoopBECountSC, OuterL);
   if (LD != ScalarEvolution::LoopInvariant)
     return false;
 
   return true;
 }
 
 Value *llvm::createMinMaxOp(IRBuilder<> &Builder,
                             RecurrenceDescriptor::MinMaxRecurrenceKind RK,
                             Value *Left, Value *Right) {
   CmpInst::Predicate P = CmpInst::ICMP_NE;
   switch (RK) {
   default:
     llvm_unreachable("Unknown min/max recurrence kind");
   case RecurrenceDescriptor::MRK_UIntMin:
     P = CmpInst::ICMP_ULT;
     break;
   case RecurrenceDescriptor::MRK_UIntMax:
     P = CmpInst::ICMP_UGT;
     break;
bool llvm::cannotBeMaxInLoop(const SCEV *S, const Loop *L, ScalarEvolution &SE,
   unsigned BitWidth = cast<IntegerType>(S->getType())->getBitWidth();
   APInt Max = Signed ? APInt::getSignedMaxValue(BitWidth) :
     APInt::getMaxValue(BitWidth);
   auto Predicate = Signed ? ICmpInst::ICMP_SLT : ICmpInst::ICMP_ULT;
   return SE.isAvailableAtLoopEntry(S, L) &&
          SE.isLoopEntryGuardedByCond(L, Predicate, S,
                                      SE.getConstant(Max));
 }
+
+/// Update profile info for the \p OrigLoop and \p UnrolledLoop.
+void llvm::fixProfileInfoAfterUnrolling(Loop *OrigLoop, Loop *UnrolledLoop,
+                                        uint64_t UF) {
+  uint64_t OrigBackedgeTakenWeight = 0;
+  uint64_t OrigLoopEntryWeight = 0;
+  auto *OrigLoopLatchBranch = OrigLoop->getLoopLatch()->getTerminator();
+
+  if (!OrigLoopLatchBranch->extractProfMetadata(OrigBackedgeTakenWeight,
+                                                OrigLoopEntryWeight))
+    return;
+
+  MDBuilder MDB(OrigLoopLatchBranch->getContext());
+  auto *UnrolledBBI = UnrolledLoop->getLoopLatch()->getTerminator();
+  bool IsTrueBackEdgeOrigLoop =
+      OrigLoop->contains(*succ_begin(OrigLoop->getLoopLatch()));
+  bool IsTrueBackEdgeVecLoop =
+      UnrolledLoop->contains(*succ_begin(UnrolledLoop->getLoopLatch()));
+
+  if (!IsTrueBackEdgeOrigLoop)
+    std::swap(OrigBackedgeTakenWeight, OrigLoopEntryWeight);
+
+  if (OrigLoopEntryWeight == 0)
+    return;
+
+  // Calculate number of iterations in the original scalar loop.
+  const uint64_t OrigHeaderBlockWeight =
+      OrigBackedgeTakenWeight + OrigLoopEntryWeight;
+  const uint64_t OrigAverageTripCount =
+      OrigHeaderBlockWeight / OrigLoopEntryWeight;
+  // Calculate number of iterations in unrolled loop.
+  uint64_t UrollAverageTripCount = OrigAverageTripCount / UF;
+  // Calculate number of iterations for remainder loop.
+  uint64_t RemainderAverageTripCount = OrigAverageTripCount % UF;
+
+  // Calculate taken and fall through counts for unrolled loop.
+  uint64_t UnrolledLoopBackedgeWeight = 0;
+  uint64_t UnrolledLoopEntryWeight = 0;
+  if (UrollAverageTripCount > 0) {
+    UnrolledLoopBackedgeWeight =
+        (UrollAverageTripCount - 1) * OrigLoopEntryWeight;
+    UnrolledLoopEntryWeight = OrigLoopEntryWeight;
+  }
+
+  // Now calculate counters for remainder loop.
+  uint64_t RemainderLoopEntryWeight = 0;
+  uint64_t RemainderLoopBackedgeWeight = 0;
+  if (RemainderAverageTripCount > 0) {
+    RemainderLoopEntryWeight =
+        (RemainderAverageTripCount - 1) * OrigLoopEntryWeight;
+    RemainderLoopBackedgeWeight = OrigLoopEntryWeight;
+  }
+
+  // Make a swap if back edge is taken when condition "false".
+  if (!IsTrueBackEdgeVecLoop)
+    std::swap(UnrolledLoopBackedgeWeight, UnrolledLoopEntryWeight);
+  // Set new profile metadata.
+  UnrolledBBI->setMetadata(LLVMContext::MD_prof,
+                           MDB.createBranchWeights(UnrolledLoopBackedgeWeight,
+                                                   UnrolledLoopEntryWeight));
+  // Make a swap if back edge is taken when condition "false".
+  if (!IsTrueBackEdgeOrigLoop)
+    std::swap(RemainderLoopEntryWeight, RemainderLoopBackedgeWeight);
+  // Set new profile metadata.
+  OrigLoopLatchBranch->setMetadata(
+      LLVMContext::MD_prof,
+      MDB.createBranchWeights(RemainderLoopEntryWeight,
+                              RemainderLoopBackedgeWeight));
+}

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

 #include "llvm/IR/Function.h"
 #include "llvm/IR/IRBuilder.h"
 #include "llvm/IR/InstrTypes.h"
 #include "llvm/IR/Instruction.h"
 #include "llvm/IR/Instructions.h"
 #include "llvm/IR/IntrinsicInst.h"
 #include "llvm/IR/Intrinsics.h"
 #include "llvm/IR/LLVMContext.h"
+#include "llvm/IR/MDBuilder.h"
 #include "llvm/IR/Metadata.h"
 #include "llvm/IR/Module.h"
 #include "llvm/IR/Operator.h"
 #include "llvm/IR/Type.h"
 #include "llvm/IR/Use.h"
 #include "llvm/IR/User.h"
 #include "llvm/IR/Value.h"
 #include "llvm/IR/ValueHandle.h"
fixupIVUsers(Entry.first, Entry.second,
                  IVEndValues[Entry.first], LoopMiddleBlock);
 
   fixLCSSAPHIs();
   for (Instruction *PI : PredicatedInstructions)
     sinkScalarOperands(&*PI);
 
   // Remove redundant induction instructions.
   cse(LoopVectorBody);
+
+  // For cases like foldTailByMasking() and requiresScalarEpiloque() we may
+  // end up getting slightly roughened result but that should be OK since
+  // profile is not inherently precise anyway. Note also possible bypass of
+  // vector code caused by legality checks is not taken into account as unlikely
+  // case.
+  fixProfileInfoAfterUnrolling(LI->getLoopFor(LoopScalarBody),
+                               LI->getLoopFor(LoopVectorBody), VF * UF);
 }
 
 void InnerLoopVectorizer::fixCrossIterationPHIs() {
   // In order to support recurrences we need to be able to vectorize Phi nodes.
   // Phi nodes have cycles, so we need to vectorize them in two stages. This is
   // stage #2: We now need to fix the recurrences by adding incoming edges to
   // the currently empty PHI nodes. At this point every instruction in the
   // original loop is widened to a vector form so we can use them to construct
   // the incoming edges.
   for (PHINode &Phi : OrigLoop->getHeader()->phis()) {
for (unsigned i = 0; i < NumIncomingValues; ++i) {
 
       // Scalar incoming value may need a broadcast
       Value *NewIncV = getOrCreateVectorValue(ScIncV, 0);
       NewPhi->addIncoming(NewIncV, NewPredBB);
     }
   }
 }
 
 void InnerLoopVectorizer::widenPHIInstruction(Instruction *PN, unsigned UF,
                                               unsigned VF) {
   PHINode *P = cast<PHINode>(PN);
   if (EnableVPlanNativePath) {
     // Currently we enter here in the VPlan-native path for non-induction
     // PHIs where all control flow is uniform. We simply widen these PHIs.
     // Create a vector phi with no operands - the vector phi operands will be
     // set at the end of vector code generation.
     Type *VecTy =
         (VF == 1) ? PN->getType() : VectorType::get(PN->getType(), VF);
     Value *VecPhi = Builder.CreatePHI(VecTy, PN->getNumOperands(), "vec.phi");
     VectorLoopValueMap.setVectorValue(P, 0, VecPhi);
     OrigPHIsToFix.push_back(P);
 
     return;
   }
 
   assert(PN->getParent() == OrigLoop->getHeader() &&
          "Non-header phis should have been handled elsewhere");
 
   // In order to support recurrences we need to be able to vectorize Phi nodes.
   // Phi nodes have cycles, so we need to vectorize them in two stages. This is
   // stage #1: We create a new vector PHI node with no incoming edges. We'll use
   // this value when we vectorize all of the instructions that use the PHI.
   if (Legal->isReductionVariable(P) || Legal->isFirstOrderRecurrence(P)) {
     for (unsigned Part = 0; Part < UF; ++Part) {
       // This is phase one of vectorizing PHIs.
       Type *VecTy =
           (VF == 1) ? PN->getType() : VectorType::get(PN->getType(), VF);
       Value *EntryPart = PHINode::Create(
           VecTy, 2, "vec.phi", &*LoopVectorBody->getFirstInsertionPt());
       VectorLoopValueMap.setVectorValue(P, Part, EntryPart);
     }
     return;
   }
 
   setDebugLocFromInst(Builder, P);
 
   // This PHINode must be an induction variable.
   // Make sure that we know about it.
   assert(Legal->getInductionVars()->count(P) && "Not an induction variable");
 
   InductionDescriptor II = Legal->getInductionVars()->lookup(P);
   const DataLayout &DL = OrigLoop->getHeader()->getModule()->getDataLayout();
 
   // FIXME: The newly created binary instructions should contain nsw/nuw flags,
   // which can be found from the original scalar operations.
   switch (II.getKind()) {
   case InductionDescriptor::IK_NoInduction:
     llvm_unreachable("Unknown induction");
   case InductionDescriptor::IK_IntInduction:
   case InductionDescriptor::IK_FpInduction:
     llvm_unreachable("Integer/fp induction is handled elsewhere.");
   case InductionDescriptor::IK_PtrInduction: {
     // Handle the pointer induction variable case.
     assert(P->getType()->isPointerTy() && "Unexpected type.");
     // This is the normalized GEP that starts counting at zero.
     Value *PtrInd = Induction;
     PtrInd = Builder.CreateSExtOrTrunc(PtrInd, II.getStep()->getType());
     // Determine the number of scalars we need to generate for each unroll
     // iteration. If the instruction is uniform, we only need to generate the
     // first lane. Otherwise, we generate all VF values.
     unsigned Lanes = Cost->isUniformAfterVectorization(P, VF) ? 1 : VF;

llvm/test/Transforms/LoopVectorize/check-prof-info.ll

+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt  -passes="print<block-freq>,loop-vectorize" -force-vector-width=4 -force-vector-interleave=1 -S < %s |  FileCheck %s
+; RUN: opt  -passes="print<block-freq>,loop-vectorize" -force-vector-width=4 -force-vector-interleave=4 -S < %s |  FileCheck %s -check-prefix=CHECK-MASKED
+
+target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+@a = dso_local global [1024 x i32] zeroinitializer, align 16
+@b = dso_local global [1024 x i32] zeroinitializer, align 16
+
+; Check correctness of profile info for vectorization without epilog.
+; Function Attrs: nofree norecurse nounwind uwtable
+define dso_local void @_Z3foov() local_unnamed_addr #0 {
+; CHECK-LABEL: @_Z3foov(
+; CHECK:  [[VECTOR_BODY:vector\.body]]:
+; CHECK:    br i1 [[TMP:%.*]], label [[MIDDLE_BLOCK:%.*]], label %[[VECTOR_BODY]], !prof [[LP1_255:\!.*]],
+; CHECK:  [[FOR_BODY:for\.body]]:
+; CHECK:    br i1 [[EXITCOND:%.*]], label [[FOR_END_LOOPEXIT:%.*]], label %[[FOR_BODY]], !prof [[LP0_0:\!.*]],
+; CHECK-MASKED:  [[VECTOR_BODY:vector\.body]]:
+; CHECK-MASKED:    br i1 [[TMP:%.*]], label [[MIDDLE_BLOCK:%.*]], label %[[VECTOR_BODY]], !prof [[LP1_63:\!.*]],
+; CHECK-MASKED:  [[FOR_BODY:for\.body]]:
+; CHECK-MASKED:    br i1 [[EXITCOND:%.*]], label [[FOR_END_LOOPEXIT:%.*]], label %[[FOR_BODY]], !prof [[LP0_0:\!.*]],
+;
+entry:
+  br label %for.body
+
+for.cond.cleanup:                                 ; preds = %for.body
+  ret void
+
+for.body:                                         ; preds = %for.body, %entry
+  %indvars.iv = phi i64 [ 0, %entry ], [ %indvars.iv.next, %for.body ]
+  %arrayidx = getelementptr inbounds [1024 x i32], [1024 x i32]* @b, i64 0, i64 %indvars.iv
+  %0 = load i32, i32* %arrayidx, align 4, !tbaa !2
+  %1 = trunc i64 %indvars.iv to i32
+  %mul = mul nsw i32 %0, %1
+  %arrayidx2 = getelementptr inbounds [1024 x i32], [1024 x i32]* @a, i64 0, i64 %indvars.iv
+  %2 = load i32, i32* %arrayidx2, align 4, !tbaa !2
+  %add = add nsw i32 %2, %mul
+  store i32 %add, i32* %arrayidx2, align 4, !tbaa !2
+  %indvars.iv.next = add nuw nsw i64 %indvars.iv, 1
+  %exitcond = icmp eq i64 %indvars.iv.next, 1024
+  br i1 %exitcond, label %for.cond.cleanup, label %for.body, !prof !6
+}
+
+; Check correctness of profile info for vectorization with epilog.
+; Function Attrs: nofree norecurse nounwind uwtable
+define dso_local void @_Z3foo2v() local_unnamed_addr #0 {
+; CHECK-LABEL: @_Z3foo2v(
+; CHECK:  [[VECTOR_BODY:vector\.body]]:
+; CHECK:    br i1 [[TMP:%.*]], label [[MIDDLE_BLOCK:%.*]], label %[[VECTOR_BODY]], !prof [[LP1_255:\!.*]],
+; CHECK:  [[FOR_BODY:for\.body]]:
+; CHECK:    br i1 [[EXITCOND:%.*]], label [[FOR_END_LOOPEXIT:%.*]], label %[[FOR_BODY]], !prof [[LP1_2:\!.*]],
+; CHECK-MASKED:  [[VECTOR_BODY:vector\.body]]:
+; CHECK-MASKED:    br i1 [[TMP:%.*]], label [[MIDDLE_BLOCK:%.*]], label %[[VECTOR_BODY]], !prof [[LP1_63:\!.*]],
+; CHECK-MASKED:  [[FOR_BODY:for\.body]]:
+; CHECK-MASKED:    br i1 [[EXITCOND:%.*]], label [[FOR_END_LOOPEXIT:%.*]], label %[[FOR_BODY]], !prof [[LP1_2:\!.*]],
+;
+entry:
+  br label %for.body
+
+for.cond.cleanup:                                 ; preds = %for.body
+  ret void
+
+for.body:                                         ; preds = %for.body, %entry
+  %indvars.iv = phi i64 [ 0, %entry ], [ %indvars.iv.next, %for.body ]
+  %arrayidx = getelementptr inbounds [1024 x i32], [1024 x i32]* @b, i64 0, i64 %indvars.iv
+  %0 = load i32, i32* %arrayidx, align 4, !tbaa !2
+  %1 = trunc i64 %indvars.iv to i32
+  %mul = mul nsw i32 %0, %1
+  %arrayidx2 = getelementptr inbounds [1024 x i32], [1024 x i32]* @a, i64 0, i64 %indvars.iv
+  %2 = load i32, i32* %arrayidx2, align 4, !tbaa !2
+  %add = add nsw i32 %2, %mul
+  store i32 %add, i32* %arrayidx2, align 4, !tbaa !2
+  %indvars.iv.next = add nuw nsw i64 %indvars.iv, 1
+  %exitcond = icmp eq i64 %indvars.iv.next, 1027
+  br i1 %exitcond, label %for.cond.cleanup, label %for.body, !prof !7
+}
+
+attributes #0 = { "use-soft-float"="false" }
+
+!llvm.module.flags = !{!0}
+!llvm.ident = !{!1}
+
+; CHECK: [[LP1_255]] = !{!"branch_weights", i32 1, i32 255}
+; CHECK: [[LP0_0]] = !{!"branch_weights", i32 0, i32 0}
+; CHECK-MASKED: [[LP1_63]] = !{!"branch_weights", i32 1, i32 63}
+; CHECK-MASKED: [[LP0_0]] = !{!"branch_weights", i32 0, i32 0}
+; CHECK: [[LP1_2]] = !{!"branch_weights", i32 1, i32 2}
+
+!0 = !{i32 1, !"wchar_size", i32 4}
+!1 = !{!"clang version 10.0.0 (https://github.com/llvm/llvm-project c292b5b5e059e6ce3e6449e6827ef7e1037c21c4)"}
+!2 = !{!3, !3, i64 0}
+!3 = !{!"int", !4, i64 0}
+!4 = !{!"omnipotent char", !5, i64 0}
+!5 = !{!"Simple C++ TBAA"}
+!6 = !{!"branch_weights", i32 1, i32 1023}
+!7 = !{!"branch_weights", i32 1, i32 1026}

llvm/test/Transforms/LoopVectorize/tripcount.ll

for.end:                                          ; preds = %for.body
   ret i32 0
 }
 
 define i32 @foo_low_trip_count3(i1 %cond, i32 %bound) !prof !0 {
 ; The loop has low invocation count compare to the function invocation count,
 ; but has a high trip count per invocation. Vectorize it.
 
 ; CHECK-LABEL: @foo_low_trip_count3(
-; CHECK: vector.body:
-
+; CHECK:  [[VECTOR_BODY:vector\.body]]:
+; CHECK:    br i1 [[TMP9:%.*]], label [[MIDDLE_BLOCK:%.*]], label %[[VECTOR_BODY]], !prof [[LP3:\!.*]],
+; CHECK:  [[FOR_BODY:for\.body]]:
+; CHECK:    br i1 [[EXITCOND:%.*]], label [[FOR_END_LOOPEXIT:%.*]], label %[[FOR_BODY]], !prof [[LP6:\!.*]],
 entry:
   br i1 %cond, label %for.preheader, label %for.end, !prof !2
 
 for.preheader:
   br label %for.body
 
 for.body:                                         ; preds = %for.body, %entry
   %i.08 = phi i32 [ 0, %for.preheader ], [ %inc, %for.body ]
for.body:                                         ; preds = %for.body, %entry
   %inc = add nsw i32 %i.08, 1
   %exitcond = icmp slt i32 %i.08, 1000
   br i1 %exitcond, label %for.body, label %for.end, !prof !1
 
 for.end:                                          ; preds = %for.body
   ret i32 0
 }
 
+; CHECK: [[LP3]] = !{!"branch_weights", i32 10, i32 2490}
+; CHECK: [[LP6]] = !{!"branch_weights", i32 10, i32 0}
+
 !0 = !{!"function_entry_count", i64 100}
 !1 = !{!"branch_weights", i32 100, i32 0}
 !2 = !{!"branch_weights", i32 10, i32 90}
 !3 = !{!"branch_weights", i32 10, i32 10000}