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[LV] Clamp VF hint when unsafe

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In the following loop the dependence distance is 2 and can only be
vectorized if the vector length is no larger than this.

  void foo(int *a, int *b, int N) {
    #pragma clang loop vectorize(enable) vectorize_width(4)
    for (int i=0; i<N; ++i) {
      a[i + 2] = a[i] + b[i];
    }
  }

However, when specifying a VF of 4 via a loop hint this loop is
vectorized. According to [1][2], loop hints are ignored if the
optimization is not safe to apply.

This patch introduces a check to bail of vectorization if the user
specified VF is greater than the maximum feasible VF, unless explicitly
forced with '-force-vector-width=X'.

[1] https://llvm.org/docs/LangRef.html#llvm-loop-vectorize-and-llvm-loop-interleave
[2] https://clang.llvm.org/docs/LanguageExtensions.html#extensions-for-loop-hint-optimizations

Reviewed By: sdesmalen, fhahn, Meinersbur

Differential Revision: https://reviews.llvm.org/D90687
This commit is contained in:
Cullen Rhodes 2020-11-02 13:02:32 +00:00
parent 32a49915a1
commit c640adbe73
3 changed files with 123 additions and 9 deletions
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43
lib/Transforms/Vectorize/LoopVectorize.cpp
View File

@ -1443,7 +1443,8 @@ private:
/// \return An upper bound for the vectorization factor, a power-of-2 larger
/// than zero. One is returned if vectorization should best be avoided due
/// to cost.
ElementCount computeFeasibleMaxVF(unsigned ConstTripCount);
ElementCount computeFeasibleMaxVF(unsigned ConstTripCount,
ElementCount UserVF);
/// The vectorization cost is a combination of the cost itself and a boolean
/// indicating whether any of the contributing operations will actually
@ -5270,9 +5271,11 @@ LoopVectorizationCostModel::computeMaxVF(ElementCount UserVF, unsigned UserIC) {
return None;
}
ElementCount MaxVF = computeFeasibleMaxVF(TC, UserVF);
switch (ScalarEpilogueStatus) {
case CM_ScalarEpilogueAllowed:
return UserVF ? UserVF : computeFeasibleMaxVF(TC);
return MaxVF;
case CM_ScalarEpilogueNotNeededUsePredicate:
LLVM_DEBUG(
dbgs() << "LV: vector predicate hint/switch found.\n"
@ -5308,7 +5311,6 @@ LoopVectorizationCostModel::computeMaxVF(ElementCount UserVF, unsigned UserIC) {
InterleaveInfo.invalidateGroupsRequiringScalarEpilogue();
}
ElementCount MaxVF = UserVF ? UserVF : computeFeasibleMaxVF(TC);
assert(!MaxVF.isScalable() &&
"Scalable vectors do not yet support tail folding");
assert((UserVF.isNonZero() || isPowerOf2_32(MaxVF.getFixedValue())) &&
@ -5361,7 +5363,9 @@ LoopVectorizationCostModel::computeMaxVF(ElementCount UserVF, unsigned UserIC) {
}
ElementCount
LoopVectorizationCostModel::computeFeasibleMaxVF(unsigned ConstTripCount) {
LoopVectorizationCostModel::computeFeasibleMaxVF(unsigned ConstTripCount,
ElementCount UserVF) {
assert(!UserVF.isScalable() && "scalable vectorization not yet handled");
MinBWs = computeMinimumValueSizes(TheLoop->getBlocks(), *DB, &TTI);
unsigned SmallestType, WidestType;
std::tie(SmallestType, WidestType) = getSmallestAndWidestTypes();
@ -5373,6 +5377,27 @@ LoopVectorizationCostModel::computeFeasibleMaxVF(unsigned ConstTripCount) {
// dependence distance).
unsigned MaxSafeVectorWidthInBits = Legal->getMaxSafeVectorWidthInBits();
if (UserVF.isNonZero()) {
// If legally unsafe, clamp the user vectorization factor to a safe value.
unsigned MaxSafeVF = PowerOf2Floor(MaxSafeVectorWidthInBits / WidestType);
if (UserVF.getFixedValue() <= MaxSafeVF)
return UserVF;
LLVM_DEBUG(dbgs() << "LV: User VF=" << UserVF
<< " is unsafe, clamping to max safe VF=" << MaxSafeVF
<< ".\n");
ORE->emit([&]() {
return OptimizationRemarkAnalysis(DEBUG_TYPE, "VectorizationFactor",
TheLoop->getStartLoc(),
TheLoop->getHeader())
<< "User-specified vectorization factor "
<< ore::NV("UserVectorizationFactor", UserVF)
<< " is unsafe, clamping to maximum safe vectorization factor "
<< ore::NV("VectorizationFactor", MaxSafeVF);
});
return ElementCount::getFixed(MaxSafeVF);
}
WidestRegister = std::min(WidestRegister, MaxSafeVectorWidthInBits);
// Ensure MaxVF is a power of 2; the dependence distance bound may not be.
@ -7031,9 +7056,12 @@ LoopVectorizationPlanner::plan(ElementCount UserVF, unsigned UserIC) {
CM.invalidateCostModelingDecisions();
}
if (!UserVF.isZero()) {
ElementCount MaxVF = MaybeMaxVF.getValue();
assert(MaxVF.isNonZero() && "MaxVF is zero.");
if (!UserVF.isZero() && UserVF.getFixedValue() <= MaxVF.getFixedValue()) {
LLVM_DEBUG(dbgs() << "LV: Using user VF " << UserVF << ".\n");
assert(isPowerOf2_32(UserVF.getKnownMinValue()) &&
assert(isPowerOf2_32(UserVF.getFixedValue()) &&
"VF needs to be a power of two");
// Collect the instructions (and their associated costs) that will be more
// profitable to scalarize.
@ -7044,9 +7072,6 @@ LoopVectorizationPlanner::plan(ElementCount UserVF, unsigned UserIC) {
return {{UserVF, 0}};
}
ElementCount MaxVF = MaybeMaxVF.getValue();
assert(MaxVF.isNonZero() && "MaxVF is zero.");
for (ElementCount VF = ElementCount::getFixed(1);
ElementCount::isKnownLE(VF, MaxVF); VF *= 2) {
// Collect Uniform and Scalar instructions after vectorization with VF.

43
test/Transforms/LoopVectorize/AArch64/unsafe-vf-hint-remark.ll Normal file
View File

@ -0,0 +1,43 @@
; REQUIRES: asserts
; RUN: opt -loop-vectorize -mtriple=arm64-apple-iphoneos -pass-remarks-analysis=loop-vectorize -debug-only=loop-vectorize -S < %s 2>&1 | FileCheck %s
; Specify a large unsafe vectorization factor of 32 that gets clamped to 16,
; then test an even smaller VF of 2 is selected based on the cost-model.
; CHECK: LV: User VF=32 is unsafe, clamping to max safe VF=16.
; CHECK: remark: <unknown>:0:0: User-specified vectorization factor 32 is unsafe, clamping to maximum safe vectorization factor 16
; CHECK: LV: Selecting VF: 2.
; CHECK-LABEL: @test
; CHECK: <2 x i64>
define void @test(i64* nocapture %a, i64* nocapture readonly %b) {
entry:
br label %loop.header
loop.header:
%iv = phi i64 [ 0, %entry ], [ %iv.next, %latch ]
%arrayidx = getelementptr inbounds i64, i64* %a, i64 %iv
%0 = load i64, i64* %arrayidx, align 4
%arrayidx2 = getelementptr inbounds i64, i64* %b, i64 %iv
%1 = load i64, i64* %arrayidx2, align 4
%add = add nsw i64 %1, %0
%2 = add nuw nsw i64 %iv, 16
%arrayidx5 = getelementptr inbounds i64, i64* %a, i64 %2
%c = icmp eq i64 %1, 120
br i1 %c, label %then, label %latch
then:
store i64 %add, i64* %arrayidx5, align 4
br label %latch
latch:
%iv.next = add nuw nsw i64 %iv, 1
%exitcond.not = icmp eq i64 %iv.next, 1024
br i1 %exitcond.not, label %exit, label %loop.header, !llvm.loop !0
exit:
ret void
}
!0 = !{!0, !1, !2}
!1 = !{!"llvm.loop.vectorize.width", i64 32}
!2 = !{!"llvm.loop.vectorize.enable", i1 true}

46
test/Transforms/LoopVectorize/unsafe-vf-hint-remark.ll Normal file
View File

@ -0,0 +1,46 @@
; REQUIRES: asserts
; RUN: opt -loop-vectorize -pass-remarks-analysis=loop-vectorize -debug-only=loop-vectorize -S < %s 2>&1 | FileCheck %s
; Make sure the unsafe user specified vectorization factor is clamped.
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"
; void foo(int *a, int *b) {
; #pragma clang loop vectorize(enable) vectorize_width(4)
; for (int i=0; i < 1024; ++i) {
; a[i + 2] = a[i] + b[i];
; }
; }
; CHECK: LV: User VF=4 is unsafe, clamping to max safe VF=2.
; CHECK: remark: <unknown>:0:0: User-specified vectorization factor 4 is unsafe, clamping to maximum safe vectorization factor 2
; CHECK-LABEL: @foo
; CHECK: <2 x i32>
define void @foo(i32* %a, i32* %b) {
entry:
br label %loop.ph
loop.ph:
br label %loop
loop:
%iv = phi i64 [ 0, %loop.ph ], [ %iv.next, %loop ]
%arrayidx = getelementptr inbounds i32, i32* %a, i64 %iv
%0 = load i32, i32* %arrayidx, align 4
%arrayidx2 = getelementptr inbounds i32, i32* %b, i64 %iv
%1 = load i32, i32* %arrayidx2, align 4
%add = add nsw i32 %1, %0
%2 = add nuw nsw i64 %iv, 2
%arrayidx5 = getelementptr inbounds i32, i32* %a, i64 %2
store i32 %add, i32* %arrayidx5, align 4
%iv.next = add nuw nsw i64 %iv, 1
%exitcond.not = icmp eq i64 %iv.next, 1024
br i1 %exitcond.not, label %exit, label %loop, !llvm.loop !0
exit:
ret void
}
!0 = !{!0, !1, !2}
!1 = !{!"llvm.loop.vectorize.width", i32 4}
!2 = !{!"llvm.loop.vectorize.enable", i1 true}