From 3bdcda82a5e6650f6ba297e9e8bf1b511d79e4fb Mon Sep 17 00:00:00 2001 From: Bill Wendling Date: Tue, 2 Dec 2008 05:12:47 +0000 Subject: [PATCH] - Remove the buggy -X/C -> X/-C transform. This isn't valid when X isn't a constant. If X is a constant, then this is folded elsewhere. - Added a note to Target/README.txt to indicate that we'd like to implement this when we're able. git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@60399 91177308-0d34-0410-b5e6-96231b3b80d8 --- lib/Target/README.txt | 10 ++++++++++ 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 fb6b0cd53d5..3c689dbe46e 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 7ad75d24b48..34c5fb3375d 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(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