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[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

llvm-svn: 303513
This commit is contained in:
Davide Italiano 2017-05-21 20:30:27 +00:00
parent a3f7546931
commit 21a49dcdf1
2 changed files with 16 additions and 8 deletions
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19
llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
View File

@ -585,6 +585,7 @@ Instruction *InstCombiner::visitTrunc(TruncInst &CI) {
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(TruncInst &CI) {
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);
}
}
}

5
llvm/test/Transforms/InstCombine/cast.ll
View File

@ -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