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[InstCombine] restrict shift-trunc-shift fold to opposite direction shifts 

This is NFCI because the pattern with 2 left-shifts should get
folded independently by smaller folds.

The motivation is to refine this block to avoid infinite loops
seen with D110170.
This commit is contained in:
Sanjay Patel 2021-09-30 14:47:02 -04:00
parent 66c069d7d6
commit 3fcb00df5d
1 changed files with 3 additions and 5 deletions
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8
llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp
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@ -853,15 +853,13 @@ Instruction *InstCombinerImpl::visitShl(BinaryOperator &I) {
return BinaryOperator::CreateShl(X, ConstantInt::get(Ty, AmtSum));
}
// Fold shl(trunc(shift1(x,c1)), c2) -> trunc(shift2(shift1(x,c1),c2))
// If 'shift2' is an ashr, we would have to get the sign bit into a funny
// place. Don't try to do this transformation in this case. Also, we
// require that the input operand is a non-poison shift-by-constant so that
// Fold shl(trunc(shr(x,c1)),c2) -> trunc(and(shl(shr(x,c1),c2),c2'))
// Require that the input operand is a non-poison shift-by-constant so that
// we have confidence that the shifts will get folded together.
Instruction *TrOp;
const APInt *TrShiftAmt;
if (match(Op0, m_OneUse(m_Trunc(m_Instruction(TrOp)))) &&
match(TrOp, m_OneUse(m_Shift(m_Value(), m_APInt(TrShiftAmt)))) &&
match(TrOp, m_OneUse(m_Shr(m_Value(), m_APInt(TrShiftAmt)))) &&
TrShiftAmt->ult(TrOp->getType()->getScalarSizeInBits())) {
Type *SrcTy = TrOp->getType();