diff options
| -rw-r--r-- | lib/Target/README.txt | 10 | ||||
| -rw-r--r-- | lib/Transforms/Scalar/InstructionCombining.cpp | 12 |
2 files changed, 10 insertions, 12 deletions
diff --git a/lib/Target/README.txt b/lib/Target/README.txt index fb6b0cd53d..3c689dbe46 100644 --- a/lib/Target/README.txt +++ b/lib/Target/README.txt @@ -1242,3 +1242,13 @@ Should combine to "20 * (((unsigned)x) & -2)". Currently not optimized with "clang -emit-llvm-bc | opt -std-compile-opts". //===---------------------------------------------------------------------===// + +We would like to do the following transform in the instcombiner: + + -X/C -> X/-C + +However, this isn't valid if (-X) overflows. We can implement this when we +have the concept of a "C signed subtraction" operator that which is undefined +on overflow. + +//===---------------------------------------------------------------------===// diff --git a/lib/Transforms/Scalar/InstructionCombining.cpp b/lib/Transforms/Scalar/InstructionCombining.cpp index 7ad75d24b4..34c5fb3375 100644 --- a/lib/Transforms/Scalar/InstructionCombining.cpp +++ b/lib/Transforms/Scalar/InstructionCombining.cpp @@ -2929,18 +2929,6 @@ Instruction *InstCombiner::visitSDiv(BinaryOperator &I) { // sdiv X, -1 == -X if (RHS->isAllOnesValue()) return BinaryOperator::CreateNeg(Op0); - - // -X/C -> X/-C, if and only if negation doesn't overflow. - if (Value *LHSNeg = dyn_castNegVal(Op0)) { - if (ConstantInt *CI = dyn_cast<ConstantInt>(LHSNeg)) { - Constant *RHSNeg = ConstantExpr::getNeg(RHS); - if (RHS != RHSNeg) { // Check that there is no overflow. - Constant *CINeg = ConstantExpr::getNeg(CI); - if (CI != CINeg) // Check that there is no overflow. - return BinaryOperator::CreateSDiv(LHSNeg, RHSNeg); - } - } - } } // If the sign bits of both operands are zero (i.e. we can prove they are |