[PATCH] use -0.0 when creating an fneg instruction

[Sanjay Patel]

Hi hfinkel, resistor,

Simple (I hope) bug fix when generating an fneg operation in InstCombine. Backends recognize (-0.0 - X) as the canonical form for fneg and produce better code. Eg, ppc64 with 0.0:
   lis r2, ha16(LCPI0_0)
   lfs f0, lo16(LCPI0_0)(r2)
   fsubs f1, f0, f1
   blr

Vs. -0.0:
   fneg f1, f1
   blr

http://reviews.llvm.org/D6723

Files:
  lib/Transforms/InstCombine/InstCombineAddSub.cpp
  test/Transforms/InstCombine/fast-math.ll

Index: lib/Transforms/InstCombine/InstCombineAddSub.cpp
===================================================================
--- lib/Transforms/InstCombine/InstCombineAddSub.cpp
+++ lib/Transforms/InstCombine/InstCombineAddSub.cpp
@@ -760,7 +760,7 @@
 }
 
 Value *FAddCombine::createFNeg(Value *V) {
-  Value *Zero = cast<Value>(ConstantFP::get(V->getType(), 0.0));
+  Value *Zero = cast<Value>(ConstantFP::getZeroValueForNegation(V->getType()));
   Value *NewV = createFSub(Zero, V);
   if (Instruction *I = dyn_cast<Instruction>(NewV))
     createInstPostProc(I, true); // fneg's don't receive instruction numbers.
Index: test/Transforms/InstCombine/fast-math.ll
===================================================================
--- test/Transforms/InstCombine/fast-math.ll
+++ test/Transforms/InstCombine/fast-math.ll
@@ -93,7 +93,7 @@
   ret float %t3
 
 ; CHECK-LABEL: @fold9(
-; CHECK: fsub fast float 0.000000e+00, %f2
+; CHECK: fsub fast float -0.000000e+00, %f2
 }
 
 ; Let C3 = C1 + C2. (f1 + C1) + (f2 + C2) => (f1 + f2) + C3 instead of

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