[llvm] r303513 - [InstCombine] Take in account the size in sext->lshr->trunc patterns.
Davide Italiano via llvm-commits
llvm-commits at lists.llvm.org
Sun May 21 13:30:28 PDT 2017
Author: davide
Date: Sun May 21 15:30:27 2017
New Revision: 303513
URL: http://llvm.org/viewvc/llvm-project?rev=303513&view=rev
Log:
[InstCombine] Take in account the size in sext->lshr->trunc patterns.
Otherwise we end up miscompiling, transforming:
define i8 @tinky() {
%sext = sext i1 1 to i16
%hibit = lshr i16 %sext, 15
%tr = trunc i16 %hibit to i8
ret i8 %tr
}
into:
%sext = sext i1 1 to i8
ret i8 %sext
and the first get folded to ret i8 1, while the second gets folded
to ret i8 -1.
Eventually we should get rid of this transform entirely, but for now,
this at least fixes a know correctness bug.
Differential Revision: https://reviews.llvm.org/D33338
Modified:
llvm/trunk/lib/Transforms/InstCombine/InstCombineCasts.cpp
llvm/trunk/test/Transforms/InstCombine/cast.ll
Modified: llvm/trunk/lib/Transforms/InstCombine/InstCombineCasts.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/InstCombine/InstCombineCasts.cpp?rev=303513&r1=303512&r2=303513&view=diff
==============================================================================
--- llvm/trunk/lib/Transforms/InstCombine/InstCombineCasts.cpp (original)
+++ llvm/trunk/lib/Transforms/InstCombine/InstCombineCasts.cpp Sun May 21 15:30:27 2017
@@ -585,6 +585,7 @@ Instruction *InstCombiner::visitTrunc(Tr
return CastInst::CreateIntegerCast(Shift, DestTy, false);
}
+ // FIXME: We should canonicalize to zext/trunc and remove this transform.
// Transform trunc(lshr (sext A), Cst) to ashr A, Cst to eliminate type
// conversion.
// It works because bits coming from sign extension have the same value as
@@ -595,18 +596,24 @@ Instruction *InstCombiner::visitTrunc(Tr
Value *SExt = cast<Instruction>(Src)->getOperand(0);
const unsigned SExtSize = SExt->getType()->getPrimitiveSizeInBits();
const unsigned ASize = A->getType()->getPrimitiveSizeInBits();
+ const unsigned CISize = CI.getType()->getPrimitiveSizeInBits();
+ const unsigned MaxAmt = SExtSize - std::max(CISize, ASize);
unsigned ShiftAmt = Cst->getZExtValue();
+
// This optimization can be only performed when zero bits generated by
// the original lshr aren't pulled into the value after truncation, so we
// can only shift by values no larger than the number of extension bits.
// FIXME: Instead of bailing when the shift is too large, use and to clear
// the extra bits.
- if (SExt->hasOneUse() && ShiftAmt <= SExtSize - ASize) {
- // If shifting by the size of the original value in bits or more, it is
- // being filled with the sign bit, so shift by ASize-1 to avoid ub.
- Value *Shift = Builder->CreateAShr(A, std::min(ShiftAmt, ASize-1));
- Shift->takeName(Src);
- return CastInst::CreateIntegerCast(Shift, CI.getType(), true);
+ if (ShiftAmt <= MaxAmt) {
+ if (CISize == ASize)
+ return BinaryOperator::CreateAShr(A, ConstantInt::get(CI.getType(),
+ std::min(ShiftAmt, ASize - 1)));
+ if (SExt->hasOneUse()) {
+ Value *Shift = Builder->CreateAShr(A, std::min(ShiftAmt, ASize-1));
+ Shift->takeName(Src);
+ return CastInst::CreateIntegerCast(Shift, CI.getType(), true);
+ }
}
}
Modified: llvm/trunk/test/Transforms/InstCombine/cast.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/InstCombine/cast.ll?rev=303513&r1=303512&r2=303513&view=diff
==============================================================================
--- llvm/trunk/test/Transforms/InstCombine/cast.ll (original)
+++ llvm/trunk/test/Transforms/InstCombine/cast.ll Sun May 21 15:30:27 2017
@@ -1513,8 +1513,9 @@ define i4 @pr33078_3(i8 %A) {
define i8 @pr33078_4(i3 %x) {
; Don't turn this in an `ashr`. This was getting miscompiled
; CHECK-LABEL: @pr33078_4(
-; CHECK-NEXT: [[C:%.*]] = ashr i3 %x, 2
-; CHECK-NEXT: [[B:%.*]] = sext i3 [[C]] to i8
+; CHECK-NEXT: [[B:%.*]] = sext i3 %x to i16
+; CHECK-NEXT: [[C:%.*]] = lshr i16 [[B]], 13
+; CHECK-NEXT: [[D:%.*]] = trunc i16 [[C]] to i8
; CHECK-NEXT: ret i8 [[D]]
%B = sext i3 %x to i16
%C = lshr i16 %B, 13
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