From c47d1431071f57d9cd7ba1922b29b17b7a313e76 Mon Sep 17 00:00:00 2001 From: Dan Gohman Date: Sun, 15 Feb 2009 23:59:32 +0000 Subject: [PATCH] Don't assume that a left-shift of a value with one bit set will have 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. llvm-svn: 64622 --- lib/CodeGen/SelectionDAG/TargetLowering.cpp | 29 ++++++++++++++++----- 1 file changed, 23 insertions(+), 6 deletions(-) diff --git a/lib/CodeGen/SelectionDAG/TargetLowering.cpp b/lib/CodeGen/SelectionDAG/TargetLowering.cpp index 909789b970b..3f56febafa3 100644 --- a/lib/CodeGen/SelectionDAG/TargetLowering.cpp +++ b/lib/CodeGen/SelectionDAG/TargetLowering.cpp @@ -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(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(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);