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[LV/LAA] Avoid specializing a loop for stride=1 when this predicate implies a

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single-iteration loop

This fixes PR34681. Avoid adding the "Stride == 1" predicate when we know that
Stride >= Trip-Count. Such a predicate will effectively optimize a single
or zero iteration loop, as Trip-Count <= Stride == 1.

Differential Revision: https://reviews.llvm.org/D38785



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@317438 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Dorit Nuzman 2017-11-05 16:53:15 +00:00
parent ee9947c063
commit ee5e318039
3 changed files with 169 additions and 3 deletions
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45
lib/Analysis/LoopAccessAnalysis.cpp
View File

@ -2136,8 +2136,51 @@ void LoopAccessInfo::collectStridedAccess(Value *MemAccess) {
if (!Stride)
return;
DEBUG(dbgs() << "LAA: Found a strided access that we can version");
DEBUG(dbgs() << "LAA: Found a strided access that is a candidate for "
"versioning:");
DEBUG(dbgs() << " Ptr: " << *Ptr << " Stride: " << *Stride << "\n");
// Avoid adding the "Stride == 1" predicate when we know that
// Stride >= Trip-Count. Such a predicate will effectively optimize a single
// or zero iteration loop, as Trip-Count <= Stride == 1.
//
// TODO: We are currently not making a very informed decision on when it is
// beneficial to apply stride versioning. It might make more sense that the
// users of this analysis (such as the vectorizer) will trigger it, based on
// their specific cost considerations; For example, in cases where stride
// versioning does not help resolving memory accesses/dependences, the
// vectorizer should evaluate the cost of the runtime test, and the benefit
// of various possible stride specializations, considering the alternatives
// of using gather/scatters (if available).
const SCEV *StrideExpr = PSE->getSCEV(Stride);
const SCEV *BETakenCount = PSE->getBackedgeTakenCount();
// Match the types so we can compare the stride and the BETakenCount.
// The Stride can be positive/negative, so we sign extend Stride;
// The backdgeTakenCount is non-negative, so we zero extend BETakenCount.
const DataLayout &DL = TheLoop->getHeader()->getModule()->getDataLayout();
uint64_t StrideTypeSize = DL.getTypeAllocSize(StrideExpr->getType());
uint64_t BETypeSize = DL.getTypeAllocSize(BETakenCount->getType());
const SCEV *CastedStride = StrideExpr;
const SCEV *CastedBECount = BETakenCount;
ScalarEvolution *SE = PSE->getSE();
if (BETypeSize >= StrideTypeSize)
CastedStride = SE->getNoopOrSignExtend(StrideExpr, BETakenCount->getType());
else
CastedBECount = SE->getZeroExtendExpr(BETakenCount, StrideExpr->getType());
const SCEV *StrideMinusBETaken = SE->getMinusSCEV(CastedStride, CastedBECount);
// Since TripCount == BackEdgeTakenCount + 1, checking:
// "Stride >= TripCount" is equivalent to checking:
// Stride - BETakenCount > 0
if (SE->isKnownPositive(StrideMinusBETaken)) {
DEBUG(dbgs() << "LAA: Stride>=TripCount; No point in versioning as the "
"Stride==1 predicate will imply that the loop executes "
"at most once.\n");
return;
}
DEBUG(dbgs() << "LAA: Found a strided access that we can version.");
SymbolicStrides[Ptr] = Stride;
StrideSet.insert(Stride);
}

122
test/Transforms/LoopVectorize/pr34681.ll Normal file
View File

@ -0,0 +1,122 @@
; RUN: opt -S -loop-vectorize -force-vector-width=4 -force-vector-interleave=1 < %s | FileCheck %s
target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
; Check the scenario where we have an unknown Stride, which happens to also be
; the loop iteration count, so if we specialize the loop for the Stride==1 case,
; this also implies that the loop will iterate no more than a single iteration,
; as in the following example:
;
; unsigned int N;
; int tmp = 0;
; for(unsigned int k=0;k<N;k++) {
; tmp+=(int)B[k*N+j];
; }
;
; We check here that the following runtime scev guard for Stride==1 is NOT generated:
; vector.scevcheck:
; %ident.check = icmp ne i32 %N, 1
; %0 = or i1 false, %ident.check
; br i1 %0, label %scalar.ph, label %vector.ph
; Instead the loop is vectorized with an unknown stride.
; CHECK-LABEL: @foo1
; CHECK: for.body.lr.ph
; CHECK-NOT: %ident.check = icmp ne i32 %N, 1
; CHECK-NOT: %[[TEST:[0-9]+]] = or i1 false, %ident.check
; CHECK-NOT: br i1 %[[TEST]], label %scalar.ph, label %vector.ph
; CHECK: vector.ph
; CHECK: vector.body
; CHECK: <4 x i32>
; CHECK: middle.block
; CHECK: scalar.ph
define i32 @foo1(i32 %N, i16* nocapture readnone %A, i16* nocapture readonly %B, i32 %i, i32 %j) {
entry:
%cmp8 = icmp eq i32 %N, 0
br i1 %cmp8, label %for.end, label %for.body.lr.ph
for.body.lr.ph:
br label %for.body
for.body:
%tmp.010 = phi i32 [ 0, %for.body.lr.ph ], [ %add1, %for.body ]
%k.09 = phi i32 [ 0, %for.body.lr.ph ], [ %inc, %for.body ]
%mul = mul i32 %k.09, %N
%add = add i32 %mul, %j
%arrayidx = getelementptr inbounds i16, i16* %B, i32 %add
%0 = load i16, i16* %arrayidx, align 2
%conv = sext i16 %0 to i32
%add1 = add nsw i32 %tmp.010, %conv
%inc = add nuw i32 %k.09, 1
%exitcond = icmp eq i32 %inc, %N
br i1 %exitcond, label %for.end.loopexit, label %for.body
for.end.loopexit:
%add1.lcssa = phi i32 [ %add1, %for.body ]
br label %for.end
for.end:
%tmp.0.lcssa = phi i32 [ 0, %entry ], [ %add1.lcssa, %for.end.loopexit ]
ret i32 %tmp.0.lcssa
}
; Check the same, but also where the Stride and the loop iteration count
; are not of the same data type.
;
; unsigned short N;
; int tmp = 0;
; for(unsigned int k=0;k<N;k++) {
; tmp+=(int)B[k*N+j];
; }
;
; We check here that the following runtime scev guard for Stride==1 is NOT generated:
; vector.scevcheck:
; %ident.check = icmp ne i16 %N, 1
; %0 = or i1 false, %ident.check
; br i1 %0, label %scalar.ph, label %vector.ph
; CHECK-LABEL: @foo2
; CHECK: for.body.lr.ph
; CHECK-NOT: %ident.check = icmp ne i16 %N, 1
; CHECK-NOT: %[[TEST:[0-9]+]] = or i1 false, %ident.check
; CHECK-NOT: br i1 %[[TEST]], label %scalar.ph, label %vector.ph
; CHECK: vector.ph
; CHECK: vector.body
; CHECK: <4 x i32>
; CHECK: middle.block
; CHECK: scalar.ph
define i32 @foo2(i16 zeroext %N, i16* nocapture readnone %A, i16* nocapture readonly %B, i32 %i, i32 %j) {
entry:
%conv = zext i16 %N to i32
%cmp11 = icmp eq i16 %N, 0
br i1 %cmp11, label %for.end, label %for.body.lr.ph
for.body.lr.ph:
br label %for.body
for.body:
%tmp.013 = phi i32 [ 0, %for.body.lr.ph ], [ %add4, %for.body ]
%k.012 = phi i32 [ 0, %for.body.lr.ph ], [ %inc, %for.body ]
%mul = mul nuw i32 %k.012, %conv
%add = add i32 %mul, %j
%arrayidx = getelementptr inbounds i16, i16* %B, i32 %add
%0 = load i16, i16* %arrayidx, align 2
%conv3 = sext i16 %0 to i32
%add4 = add nsw i32 %tmp.013, %conv3
%inc = add nuw nsw i32 %k.012, 1
%exitcond = icmp eq i32 %inc, %conv
br i1 %exitcond, label %for.end.loopexit, label %for.body
for.end.loopexit:
%add4.lcssa = phi i32 [ %add4, %for.body ]
br label %for.end
for.end:
%tmp.0.lcssa = phi i32 [ 0, %entry ], [ %add4.lcssa, %for.end.loopexit ]
ret i32 %tmp.0.lcssa
}

5
test/Transforms/LoopVectorize/version-mem-access.ll
View File

@ -65,7 +65,8 @@ for.end:
define void @fn1(double* noalias %x, double* noalias %c, double %a) {
entry:
%conv = fptosi double %a to i32
%cmp8 = icmp sgt i32 %conv, 0
%conv2 = add i32 %conv, 4
%cmp8 = icmp sgt i32 %conv2, 0
br i1 %cmp8, label %for.body.preheader, label %for.end
for.body.preheader:
@ -82,7 +83,7 @@ for.body:
store double %1, double* %arrayidx3, align 8
%indvars.iv.next = add nuw nsw i64 %indvars.iv, 1
%lftr.wideiv = trunc i64 %indvars.iv.next to i32
%exitcond = icmp eq i32 %lftr.wideiv, %conv
%exitcond = icmp eq i32 %lftr.wideiv, %conv2
br i1 %exitcond, label %for.end.loopexit, label %for.body
for.end.loopexit: