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LoopVectorize: MaxVF should not be larger than the loop trip count 

Summary:
Improve how MaxVF is computed while taking into account that MaxVF should not be larger than the loop's trip count.

Other than saving on compile-time by pruning the possible MaxVF candidates, this patch fixes pr34438 which exposed the following flow:
1. Short trip count identified -> Don't bail out, set OptForSize:=True to avoid tail-loop and runtime checks.
2. Compute MaxVF returned 16 on a target supporting AVX512.
3. OptForSize -> choose VF:=MaxVF.
4. Bail out because TripCount = 8, VF = 16, TripCount % VF !=0 means we need a tail loop.

With this patch step 2. will choose MaxVF=8 based on TripCount.

Reviewers: Ayal, dorit, mkuper, hfinkel

Reviewed By: hfinkel

Subscribers: hfinkel, llvm-commits

Differential Revision: https://reviews.llvm.org/D37425

llvm-svn: 312472
This commit is contained in:
Zvi Rackover 2017-09-04 08:35:13 +00:00
parent 7cd826a321
commit 9a087a357a
2 changed files with 43 additions and 4 deletions
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12
llvm/lib/Transforms/Vectorize/LoopVectorize.cpp
View File

@ -1960,7 +1960,7 @@ public:
private: private:
/// \return An upper bound for the vectorization factor, larger than zero. /// \return An upper bound for the vectorization factor, larger than zero.
/// One is returned if vectorization should best be avoided due to cost. /// One is returned if vectorization should best be avoided due to cost.
unsigned computeFeasibleMaxVF(bool OptForSize); unsigned computeFeasibleMaxVF(bool OptForSize, unsigned ConstTripCount = 0);
/// The vectorization cost is a combination of the cost itself and a boolean /// The vectorization cost is a combination of the cost itself and a boolean
/// indicating whether any of the contributing operations will actually /// indicating whether any of the contributing operations will actually
@ -6187,7 +6187,7 @@ Optional<unsigned> LoopVectorizationCostModel::computeMaxVF(bool OptForSize) {
return None; return None;
} }
unsigned MaxVF = computeFeasibleMaxVF(OptForSize); unsigned MaxVF = computeFeasibleMaxVF(OptForSize, TC);
if (TC % MaxVF != 0) { if (TC % MaxVF != 0) {
// If the trip count that we found modulo the vectorization factor is not // If the trip count that we found modulo the vectorization factor is not
@ -6208,7 +6208,9 @@ Optional<unsigned> LoopVectorizationCostModel::computeMaxVF(bool OptForSize) {
return MaxVF; return MaxVF;
} }
unsigned LoopVectorizationCostModel::computeFeasibleMaxVF(bool OptForSize) { unsigned
LoopVectorizationCostModel::computeFeasibleMaxVF(bool OptForSize,
unsigned ConstTripCount) {
MinBWs = computeMinimumValueSizes(TheLoop->getBlocks(), *DB, &TTI); MinBWs = computeMinimumValueSizes(TheLoop->getBlocks(), *DB, &TTI);
unsigned SmallestType, WidestType; unsigned SmallestType, WidestType;
std::tie(SmallestType, WidestType) = getSmallestAndWidestTypes(); std::tie(SmallestType, WidestType) = getSmallestAndWidestTypes();
@ -6237,7 +6239,9 @@ unsigned LoopVectorizationCostModel::computeFeasibleMaxVF(bool OptForSize) {
if (MaxVectorSize == 0) { if (MaxVectorSize == 0) {
DEBUG(dbgs() << "LV: The target has no vector registers.\n"); DEBUG(dbgs() << "LV: The target has no vector registers.\n");
MaxVectorSize = 1; MaxVectorSize = 1;
} } else if (ConstTripCount && ConstTripCount < MaxVectorSize &&
isPowerOf2_32(ConstTripCount))
MaxVectorSize = ConstTripCount;
assert(MaxVectorSize <= 64 && "Did not expect to pack so many elements" assert(MaxVectorSize <= 64 && "Did not expect to pack so many elements"
" into one vector!"); " into one vector!");

35
llvm/test/Transforms/LoopVectorize/X86/pr34438.ll Normal file
View File

@ -0,0 +1,35 @@
; PR34438
; Loop has a short trip count of 8 iterations. It should be vectorized because no runtime checks or tail loop are necessary.
; Two cases tested AVX (MaxVF=8 = TripCount) and AVX512 (MaxVF=16 > TripCount)
; RUN: opt < %s -loop-vectorize -mtriple=x86_64-apple-macosx10.8.0 -mcpu=corei7-avx -S | FileCheck %s
; RUN: opt < %s -loop-vectorize -mtriple=x86_64-apple-macosx10.8.0 -mcpu=skylake-avx512 -S | FileCheck %s
target datalayout = "e-p:64:64:64-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:64:64-f32:32:32-f64:64:64-v64:64:64-v128:128:128-a0:0:64-s0:64:64-f80:128:128-n8:16:32:64-S128"
target triple = "x86_64-apple-macosx10.8.0"
define void @small_tc(float* noalias nocapture %A, float* noalias nocapture readonly %B) {
; CHECK-LABEL: @small_tc
; CHECK: load <8 x float>, <8 x float>*
; CHECK: fadd fast <8 x float>
entry:
br label %for.body
for.body:
%indvars.iv = phi i64 [ 0, %entry ], [ %indvars.iv.next, %for.body ]
%arrayidx = getelementptr inbounds float, float* %B, i64 %indvars.iv
%0 = load float, float* %arrayidx, align 4, !llvm.mem.parallel_loop_access !3
%arrayidx2 = getelementptr inbounds float, float* %A, i64 %indvars.iv
%1 = load float, float* %arrayidx2, align 4, !llvm.mem.parallel_loop_access !3
%add = fadd fast float %0, %1
store float %add, float* %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, 8
br i1 %exitcond, label %for.end, label %for.body, !llvm.loop !4
for.end:
ret void
}
!3 = !{!3}
!4 = !{!4}