Refine a bit ClampXToUint8

src/x64/macro-assembler-x64.cc

Index: src/x64/macro-assembler-x64.cc
diff --git a/src/x64/macro-assembler-x64.cc b/src/x64/macro-assembler-x64.cc
index b5b7324be0af31dbe9e1a0b0b760d9c17190a282..d62cdceef9fb67a70df1a3fd33ad25c34a8f082d 100644
--- a/src/x64/macro-assembler-x64.cc
+++ b/src/x64/macro-assembler-x64.cc
@@ -3139,8 +3139,8 @@ void MacroAssembler::ClampUint8(Register reg) {
   Label done;
   testl(reg, Immediate(0xFFFFFF00));
   j(zero, &done, Label::kNear);
-  setcc(negative, reg);  // 1 if negative, 0 if positive.
-  decb(reg);  // 0 if negative, 255 if positive.
+  sarl(reg, Immediate(31));
+  notl(reg);  // No zero extension seems to be safe here.
   bind(&done);
 }
 
@@ -3156,16 +3156,14 @@ void MacroAssembler::ClampDoubleToUint8(XMMRegister input_reg,
   j(zero, &done, Label::kNear);
   cmpl(result_reg, Immediate(1));
   j(overflow, &conv_failure, Label::kNear);
-  movl(result_reg, Immediate(0));
-  setcc(sign, result_reg);
-  subl(result_reg, Immediate(1));
-  andl(result_reg, Immediate(255));
+  sarl(result_reg, Immediate(31));
+  notl(result_reg);  // No zero extension seems to be safe here.
   jmp(&done, Label::kNear);
   bind(&conv_failure);
-  Set(result_reg, 0);
+  sarl(result_reg, Immediate(31));
   ucomisd(input_reg, temp_xmm_reg);
-  j(below, &done, Label::kNear);
-  Set(result_reg, 255);
+  j(above_equal, &done, Label::kNear);
+  Set(result_reg, 0);
   bind(&done);
 }