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[LV] Clamp the VF to the trip count

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Summary:
When the MaxVectorSize > ConstantTripCount, we should just clamp the
vectorization factor to be the ConstantTripCount.
This vectorizes loops where the TinyTripCountThreshold >= TripCount < MaxVF.

Earlier we were finding the maximum vector width, which could be greater than
the trip count itself. The Loop vectorizer does all the work for generating a
vectorizable loop, but in the end we would always choose the scalar loop (since
the VF > trip count). This allows us to choose the VF keeping in mind the trip
count if available.

This is a fix on top of rL312472.

Reviewers: Ayal, zvi, hfinkel, dneilson

Reviewed by: Ayal

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D37702

llvm-svn: 313046
This commit is contained in:
Anna Thomas 2017-09-12 16:32:45 +00:00
parent 5129faa331
commit c739ce8170
2 changed files with 38 additions and 7 deletions
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19
lib/Transforms/Vectorize/LoopVectorize.cpp
View File

@ -1960,7 +1960,7 @@ public:
private:
/// \return An upper bound for the vectorization factor, larger than zero.
/// One is returned if vectorization should best be avoided due to cost.
unsigned computeFeasibleMaxVF(bool OptForSize, unsigned ConstTripCount = 0);
unsigned computeFeasibleMaxVF(bool OptForSize, unsigned ConstTripCount);
/// The vectorization cost is a combination of the cost itself and a boolean
/// indicating whether any of the contributing operations will actually
@ -6161,8 +6161,9 @@ Optional<unsigned> LoopVectorizationCostModel::computeMaxVF(bool OptForSize) {
return None;
}
unsigned TC = PSE.getSE()->getSmallConstantTripCount(TheLoop);
if (!OptForSize) // Remaining checks deal with scalar loop when OptForSize.
return computeFeasibleMaxVF(OptForSize);
return computeFeasibleMaxVF(OptForSize, TC);
if (Legal->getRuntimePointerChecking()->Need) {
ORE->emit(createMissedAnalysis("CantVersionLoopWithOptForSize")
@ -6175,7 +6176,6 @@ Optional<unsigned> LoopVectorizationCostModel::computeMaxVF(bool OptForSize) {
}
// If we optimize the program for size, avoid creating the tail loop.
unsigned TC = PSE.getSE()->getSmallConstantTripCount(TheLoop);
DEBUG(dbgs() << "LV: Found trip count: " << TC << '\n');
// If we don't know the precise trip count, don't try to vectorize.
@ -6236,15 +6236,20 @@ LoopVectorizationCostModel::computeFeasibleMaxVF(bool OptForSize,
DEBUG(dbgs() << "LV: The Widest register is: " << WidestRegister
<< " bits.\n");
assert(MaxVectorSize <= 64 && "Did not expect to pack so many elements"
" into one vector!");
if (MaxVectorSize == 0) {
DEBUG(dbgs() << "LV: The target has no vector registers.\n");
MaxVectorSize = 1;
} else if (ConstTripCount && ConstTripCount < MaxVectorSize &&
isPowerOf2_32(ConstTripCount))
isPowerOf2_32(ConstTripCount)) {
// We need to clamp the VF to be the ConstTripCount. There is no point in
// choosing a higher viable VF as done in the loop below.
DEBUG(dbgs() << "LV: Clamping the MaxVF to the constant trip count: "
<< ConstTripCount << "\n");
MaxVectorSize = ConstTripCount;
assert(MaxVectorSize <= 64 && "Did not expect to pack so many elements"
" into one vector!");
return MaxVectorSize;
}
unsigned MaxVF = MaxVectorSize;
if (MaximizeBandwidth && !OptForSize) {

26
test/Transforms/LoopVectorize/X86/vector_max_bandwidth.ll
View File

@ -46,3 +46,29 @@ for.body:
%exitcond = icmp eq i64 %indvars.iv.next, 1000
br i1 %exitcond, label %for.cond.cleanup, label %for.body
}
; We should not choose a VF larger than the constant TC.
; VF chosen should be atmost 16 (not the max possible vector width = 32 for AVX2)
define void @not_too_small_tc(i8* noalias nocapture %A, i8* noalias nocapture readonly %B) {
; CHECK-LABEL: not_too_small_tc
; CHECK-AVX2: LV: Selecting VF: 16.
entry:
br label %for.body
for.body:
%indvars.iv = phi i64 [ 0, %entry ], [ %indvars.iv.next, %for.body ]
%arrayidx = getelementptr inbounds i8, i8* %B, i64 %indvars.iv
%l1 = load i8, i8* %arrayidx, align 4, !llvm.mem.parallel_loop_access !3
%arrayidx2 = getelementptr inbounds i8, i8* %A, i64 %indvars.iv
%l2 = load i8, i8* %arrayidx2, align 4, !llvm.mem.parallel_loop_access !3
%add = add i8 %l1, %l2
store i8 %add, i8* %arrayidx2, align 4, !llvm.mem.parallel_loop_access !3
%indvars.iv.next = add nuw nsw i64 %indvars.iv, 1
%exitcond = icmp eq i64 %indvars.iv.next, 16
br i1 %exitcond, label %for.end, label %for.body, !llvm.loop !4
for.end:
ret void
}
!3 = !{!3}
!4 = !{!4}