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Don't assume that a left-shift of a value with one bit set will have

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one bit set, because the bit may be shifted off the end. Instead,
just check for a constant 1 being shifted. This is still sufficient
to handle all the cases in test/CodeGen/X86/bt.ll. This fixes PR3583.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@64622 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Dan Gohman 2009-02-15 23:59:32 +00:00
parent 35188eb54d
commit 97d116346c
1 changed files with 23 additions and 6 deletions
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29
lib/CodeGen/SelectionDAG/TargetLowering.cpp
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@ -1346,14 +1346,31 @@ unsigned TargetLowering::ComputeNumSignBitsForTargetNode(SDValue Op,
return 1;
}
/// ValueHasExactlyOneBitSet - Test if the given value is known to have exactly
/// one bit set. This differs from ComputeMaskedBits in that it doesn't need to
/// determine which bit is set.
///
static bool ValueHasExactlyOneBitSet(SDValue Val, const SelectionDAG &DAG) {
// Logical shift right or left won't ever introduce new set bits.
// We check for this case because we don't care which bits are
// set, but ComputeMaskedBits won't know anything unless it can
// determine which specific bits may be set.
if (Val.getOpcode() == ISD::SHL || Val.getOpcode() == ISD::SRL)
return ValueHasExactlyOneBitSet(Val.getOperand(0), DAG);
// A left-shift of a constant one will have exactly one bit set, because
// shifting the bit off the end is undefined.
if (Val.getOpcode() == ISD::SHL)
if (ConstantSDNode *C =
dyn_cast<ConstantSDNode>(Val.getNode()->getOperand(0)))
if (C->getAPIntValue() == 1)
return true;
// Similarly, a right-shift of a constant sign-bit will have exactly
// one bit set.
if (Val.getOpcode() == ISD::SRL)
if (ConstantSDNode *C =
dyn_cast<ConstantSDNode>(Val.getNode()->getOperand(0)))
if (C->getAPIntValue().isSignBit())
return true;
// More could be done here, though the above checks are enough
// to handle some common cases.
// Fall back to ComputeMaskedBits to catch other known cases.
MVT OpVT = Val.getValueType();
unsigned BitWidth = OpVT.getSizeInBits();
APInt Mask = APInt::getAllOnesValue(BitWidth);