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If unsafe FP optimization is not set, don't allow -(A-B) => B-A because 

when A==B, -0.0 != +0.0.

llvm-svn: 63474
This commit is contained in:
Mon P Wang 2009-01-31 06:07:45 +00:00
parent 3b585af0ec
commit cf9ba82324
2 changed files with 14 additions and 1 deletions
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3
llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
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@ -2357,7 +2357,8 @@ SDValue SelectionDAG::getNode(unsigned Opcode, DebugLoc DL,
return Operand.getOperand(0);
break;
case ISD::FNEG:
if (OpOpcode == ISD::FSUB) // -(X-Y) -> (Y-X)
// -(X-Y) -> (Y-X) is unsafe because when X==Y, -0.0 != +0.0
if (UnsafeFPMath && OpOpcode == ISD::FSUB)
return getNode(ISD::FSUB, VT, Operand.getNode()->getOperand(1),
Operand.getNode()->getOperand(0));
if (OpOpcode == ISD::FNEG) // --X -> X

12
llvm/test/CodeGen/X86/neg_fp.ll Normal file
View File

@ -0,0 +1,12 @@
; RUN: llvm-as < %s | llc -march=x86 -mattr=+sse41 -o %t -f
; RUN: grep xorps %t | count 1
; Test that when we don't -enable-unsafe-fp-math, we don't do the optimization
; -0 - (A - B) to (B - A) because A==B, -0 != 0
define float @negfp(float %a, float %b) {
entry:
%sub = sub float %a, %b ; <float> [#uses=1]
%neg = sub float -0.000000e+00, %sub ; <float> [#uses=1]
ret float %neg
}