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[LoopIdiom] check for safety while expanding 

Loop Idiom recognition was generating memset in a case that
would result generating a division operation to an unsafe location.

Differential Revision: https://reviews.llvm.org/D32674

llvm-svn: 302238
This commit is contained in:
Aditya Kumar 2017-05-05 14:49:45 +00:00
parent e608f6a632
commit 1c42d135e1
2 changed files with 65 additions and 0 deletions
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10
llvm/lib/Transforms/Scalar/LoopIdiomRecognize.cpp
View File

@ -783,6 +783,11 @@ bool LoopIdiomRecognize::processLoopStridedStore(
if (NegStride) if (NegStride)
Start = getStartForNegStride(Start, BECount, IntPtr, StoreSize, SE); Start = getStartForNegStride(Start, BECount, IntPtr, StoreSize, SE);
// TODO: ideally we should still be able to generate memset if SCEV expander
// is taught to generate the dependencies at the latest point.
if (!isSafeToExpand(Start, *SE))
return false;
// Okay, we have a strided store "p[i]" of a splattable value. We can turn // Okay, we have a strided store "p[i]" of a splattable value. We can turn
// this into a memset in the loop preheader now if we want. However, this // this into a memset in the loop preheader now if we want. However, this
// would be unsafe to do if there is anything else in the loop that may read // would be unsafe to do if there is anything else in the loop that may read
@ -814,6 +819,11 @@ bool LoopIdiomRecognize::processLoopStridedStore(
SCEV::FlagNUW); SCEV::FlagNUW);
} }
// TODO: ideally we should still be able to generate memset if SCEV expander
// is taught to generate the dependencies at the latest point.
if (!isSafeToExpand(NumBytesS, *SE))
return false;
Value *NumBytes = Value *NumBytes =
Expander.expandCodeFor(NumBytesS, IntPtr, Preheader->getTerminator()); Expander.expandCodeFor(NumBytesS, IntPtr, Preheader->getTerminator());

55
llvm/test/Transforms/LoopIdiom/unsafe.ll Normal file
View File

@ -0,0 +1,55 @@
; RUN: opt -S < %s -loop-idiom | FileCheck %s
; CHECK-NOT: memset
; check that memset is not generated (for stores) because that will result
; in udiv hoisted out of the loop by the SCEV Expander
; TODO: ideally we should be able to generate memset
; if SCEV expander is taught to generate the dependencies
; at the right point.
@a = global i32 0, align 4
@b = global i32 0, align 4
@c = external local_unnamed_addr global [1 x i8], align 1
define void @e() local_unnamed_addr {
entry:
%d0 = load i32, i32* @a, align 4
%d1 = load i32, i32* @b, align 4
br label %for.cond1thread-pre-split
for.cond1thread-pre-split: ; preds = %for.body5, %entry
%div = udiv i32 %d0, %d1
br label %for.body5
for.body5: ; preds = %for.body5, %for.cond1thread-pre-split
%indvars.iv = phi i64 [ 0, %for.cond1thread-pre-split ], [ %indvars.iv.next, %for.body5 ]
%divx = sext i32 %div to i64
%0 = add nsw i64 %divx, %indvars.iv
%arrayidx = getelementptr inbounds [1 x i8], [1 x i8]* @c, i64 0, i64 %0
store i8 0, i8* %arrayidx, align 1
%indvars.iv.next = add nsw i64 %indvars.iv, 1
%1 = trunc i64 %indvars.iv.next to i32
%tobool4 = icmp eq i32 %1, 0
br i1 %tobool4, label %for.cond1thread-pre-split, label %for.body5
}
; The loop's trip count is depending on an unsafe operation
; udiv. SCEV expander hoists it out of the loop, so loop-idiom
; should check that the memset is not generated in this case.
define void @f(i32 %a, i32 %b, i8* nocapture %x) local_unnamed_addr {
entry:
br label %for.body
for.body: ; preds = %for.body6, %entry
%div = udiv i32 %a, %b
%conv = zext i32 %div to i64
br label %for.body6
for.body6: ; preds = %for.body6, %for.body
%i.09 = phi i64 [ %inc, %for.body6 ], [ 0, %for.body ]
%arrayidx = getelementptr inbounds i8, i8* %x, i64 %i.09
store i8 0, i8* %arrayidx, align 1
%inc = add nuw nsw i64 %i.09, 1
%cmp3 = icmp slt i64 %inc, %conv
br i1 %cmp3, label %for.body6, label %for.body
}