PPC: Match -0 - x with sign bit flip.

Optimiation similar to what is done for x86

R=mbrandy@us.ibm.com

BUG=

Review URL: https://codereview.chromium.org/1072173002

Cr-Commit-Position: refs/heads/master@{#27718}

src/compiler/ppc/instruction-selector-ppc.cc

@@ -909,6 +909,13 @@ void InstructionSelector::VisitFloat64Add(Node* node) {
 
 
 void InstructionSelector::VisitFloat32Sub(Node* node) {
+  PPCOperandGenerator g(this);
+  Float32BinopMatcher m(node);
+  if (m.left().IsMinusZero()) {
+    Emit(kPPC_NegDouble, g.DefineAsRegister(node),
+         g.UseRegister(m.right().node()));
+    return;
+  }
   VisitRRR(this, kPPC_SubDouble, node);
 }
 
@@ -917,18 +924,23 @@ void InstructionSelector::VisitFloat64Sub(Node* node) {
   // TODO(mbrandy): detect multiply-subtract
   PPCOperandGenerator g(this);
   Float64BinopMatcher m(node);
-  if (m.left().IsMinusZero() && m.right().IsFloat64RoundDown() &&
-      CanCover(m.node(), m.right().node())) {
-    if (m.right().InputAt(0)->opcode() == IrOpcode::kFloat64Sub &&
-        CanCover(m.right().node(), m.right().InputAt(0))) {
-      Float64BinopMatcher mright0(m.right().InputAt(0));
-      if (mright0.left().IsMinusZero()) {
-        // -floor(-x) = ceil(x)
-        Emit(kPPC_CeilDouble, g.DefineAsRegister(node),
-             g.UseRegister(mright0.right().node()));
-        return;
+  if (m.left().IsMinusZero()) {
+    if (m.right().IsFloat64RoundDown() &&
+        CanCover(m.node(), m.right().node())) {
+      if (m.right().InputAt(0)->opcode() == IrOpcode::kFloat64Sub &&
+          CanCover(m.right().node(), m.right().InputAt(0))) {
+        Float64BinopMatcher mright0(m.right().InputAt(0));
+        if (mright0.left().IsMinusZero()) {
+          // -floor(-x) = ceil(x)
+          Emit(kPPC_CeilDouble, g.DefineAsRegister(node),
+               g.UseRegister(mright0.right().node()));
+          return;
+        }
       }
     }
+    Emit(kPPC_NegDouble, g.DefineAsRegister(node),
+         g.UseRegister(m.right().node()));
+    return;
   }
   VisitRRR(this, kPPC_SubDouble, node);
 }