Improve ClampDoubleToUint8 on ia32/x64

src/ia32/macro-assembler-ia32.cc

Index: src/ia32/macro-assembler-ia32.cc
===================================================================
--- src/ia32/macro-assembler-ia32.cc	(revision 12749)
+++ src/ia32/macro-assembler-ia32.cc	(working copy)
@@ -129,14 +129,22 @@
                                         XMMRegister scratch_reg,
                                         Register result_reg) {
   Label done;
-  ExternalReference zero_ref = ExternalReference::address_of_zero();
-  movdbl(scratch_reg, Operand::StaticVariable(zero_ref));
+  Label conv_failure;
+  pxor(scratch_reg, scratch_reg);
+  cvtsd2si(result_reg, input_reg);
+  test(result_reg, Immediate(0xFFFFFF00));
+  j(zero, &done, Label::kNear);
+  cmp(result_reg, Immediate(0x80000000));
+  j(equal, &conv_failure, Label::kNear);
+  mov(result_reg, Immediate(0));
+  setcc(above, result_reg);
+  sub(result_reg, Immediate(1));
+  and_(result_reg, Immediate(255));
+  jmp(&done, Label::kNear);
+  bind(&conv_failure);
   Set(result_reg, Immediate(0));
   ucomisd(input_reg, scratch_reg);
   j(below, &done, Label::kNear);
-  cvtsd2si(result_reg, input_reg);
-  test(result_reg, Immediate(0xFFFFFF00));
-  j(zero, &done, Label::kNear);
   Set(result_reg, Immediate(255));
   bind(&done);
 }