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| author | Bill Wendling <isanbard@gmail.com> | 2008-11-30 05:01:05 +0000 |
|---|---|---|
| committer | Bill Wendling <isanbard@gmail.com> | 2008-11-30 05:01:05 +0000 |
| commit | 524d61aab17ce8b6734f09f2dd15b2dfb2e300f7 (patch) | |
| tree | d0701269fd0842f8d8836012164ab7b7fb8a05f2 /lib/Transforms | |
| parent | 156a66847d6988b5947b5d62e7e65648e7aa5c6f (diff) | |
| download | external_llvm-524d61aab17ce8b6734f09f2dd15b2dfb2e300f7.zip external_llvm-524d61aab17ce8b6734f09f2dd15b2dfb2e300f7.tar.gz external_llvm-524d61aab17ce8b6734f09f2dd15b2dfb2e300f7.tar.bz2 | |
From Hacker's Delight:
"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.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@60278 91177308-0d34-0410-b5e6-96231b3b80d8
Diffstat (limited to 'lib/Transforms')
| -rw-r--r-- | lib/Transforms/Scalar/InstructionCombining.cpp | 18 |
1 files changed, 10 insertions, 8 deletions
diff --git a/lib/Transforms/Scalar/InstructionCombining.cpp b/lib/Transforms/Scalar/InstructionCombining.cpp index 8461aa7..fce2373 100644 --- a/lib/Transforms/Scalar/InstructionCombining.cpp +++ b/lib/Transforms/Scalar/InstructionCombining.cpp @@ -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)); } |