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From Hacker's Delight:

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"For signed integers, the determination of overflow of x*y is not so simple. If
x and y have the same sign, then overflow occurs iff xy > 2**31 - 1. If they
have opposite signs, then overflow occurs iff xy < -2**31."

In this case, x == -1.

llvm-svn: 60278
This commit is contained in:
Bill Wendling 2008-11-30 05:01:05 +00:00
parent 0ef5e1dc82
commit 4e018f4c22
1 changed files with 10 additions and 8 deletions
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18
lib/Transforms/Scalar/InstructionCombining.cpp
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@ -2957,20 +2957,22 @@ Instruction *InstCombiner::visitSDiv(BinaryOperator &I) {
return BinaryOperator::CreateNeg(Op0);
ConstantInt *RHSNeg = cast<ConstantInt>(ConstantExpr::getNeg(RHS));
APInt RHSNegAPI(RHSNeg->getBitWidth(), RHSNeg->getSExtValue(), true);
APInt NegOne = -APInt(RHSNeg->getBitWidth(), 1, true);
APInt TwoToExp(RHSNeg->getBitWidth(), 1 << (RHSNeg->getBitWidth() - 1),
true);
// -X/C -> X/-C, if and only if negation doesn't overflow.
if ((RHS->getSExtValue() < 0 &&
RHS->getSExtValue() < RHSNeg->getSExtValue()) ||
(RHS->getSExtValue() > 0 &&
RHS->getSExtValue() > RHSNeg->getSExtValue())) {
if ((RHS->getSExtValue() < 0 && RHSNegAPI.slt(TwoToExp - 1)) ||
(RHS->getSExtValue() > 0 && RHSNegAPI.sgt(TwoToExp * NegOne))) {
if (Value *LHSNeg = dyn_castNegVal(Op0)) {
if (ConstantInt *CI = dyn_cast<ConstantInt>(LHSNeg)) {
ConstantInt *CINeg = cast<ConstantInt>(ConstantExpr::getNeg(CI));
APInt CINegAPI(CINeg->getBitWidth(), CINeg->getSExtValue(), true);
if ((CI->getSExtValue() < 0 &&
CI->getSExtValue() < CINeg->getSExtValue()) ||
(CI->getSExtValue() > 0 &&
CI->getSExtValue() > CINeg->getSExtValue()))
if ((CI->getSExtValue() < 0 && CINegAPI.slt(TwoToExp - 1)) ||
(CI->getSExtValue() > 0 && CINegAPI.sgt(TwoToExp * NegOne)))
return BinaryOperator::CreateSDiv(LHSNeg,
ConstantExpr::getNeg(RHS));
}