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[InstCombine] dropRedundantMaskingOfLeftShiftInput(): some cleanup before upcoming patch

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llvm-svn: 372245
This commit is contained in:
Roman Lebedev 2019-09-18 18:38:40 +00:00
parent cdaf6001ac
commit 7117bf1635
1 changed files with 8 additions and 5 deletions
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13
lib/Transforms/InstCombine/InstCombineShifts.cpp
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@ -164,6 +164,7 @@ dropRedundantMaskingOfLeftShiftInput(BinaryOperator *OuterShift,
// In this pattern SumOfShAmts correlates with the number of low bits that
// shall remain in the root value (OuterShift). If SumOfShAmts is less than
// bitwidth, we'll need to also produce a mask to keep SumOfShAmts low bits.
// So, does *any* channel need a mask?
if (!match(SumOfShAmts, m_SpecificInt_ICMP(ICmpInst::Predicate::ICMP_UGE,
APInt(BitWidth, BitWidth))))
return nullptr; // FIXME.
@ -172,15 +173,17 @@ dropRedundantMaskingOfLeftShiftInput(BinaryOperator *OuterShift,
match(Masked, m_Shr(m_Shl(m_Value(X), m_Value(MaskShAmt)),
m_Deferred(MaskShAmt)))) {
// Can we simplify (ShiftShAmt-MaskShAmt) ?
Value *ShAmtsDiff =
auto *ShAmtsDiff = dyn_cast_or_null<Constant>(
SimplifySubInst(ShiftShAmt, MaskShAmt, /*IsNSW=*/false, /*IsNUW=*/false,
SQ.getWithInstruction(OuterShift));
SQ.getWithInstruction(OuterShift)));
if (!ShAmtsDiff)
return nullptr; // Did not simplify.
// Is the difference non-negative? (is ShiftShAmt u>= MaskShAmt ?)
// FIXME: could also rely on ConstantRange.
// In this pattern ShAmtsDiff correlates with the number of high bits that
// shall be unset in the root value (OuterShift). If ShAmtsDiff is negative,
// we'll need to also produce a mask to unset ShAmtsDiff high bits.
// So, does *any* channel need a mask? (is ShiftShAmt u>= MaskShAmt ?)
if (!match(ShAmtsDiff, m_NonNegative()))
return nullptr;
return nullptr; // FIXME.
// All good, we can do this fold.
} else
return nullptr; // Don't know anything about this pattern.