Change clamping 0..255 instruction sequence for pixel array code

src/x64/ic-x64.cc

 // Copyright 2009 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 473 matching lines @@
   __ JumpIfNotSmi(rax, &slow);
   __ SmiToInteger32(rbx, rbx);
   __ cmpl(rbx, FieldOperand(rcx, PixelArray::kLengthOffset));
   __ j(above_equal, &slow);
   __ movq(rdx, rax);  // Save the value.
   __ SmiToInteger32(rax, rax);
   {  // Clamp the value to [0..255].
     Label done, is_negative;
     __ testl(rax, Immediate(0xFFFFFF00));
     __ j(zero, &done);
-    __ j(negative, &is_negative);
-    __ movl(rax, Immediate(255));
-    __ jmp(&done);
-    __ bind(&is_negative);
-    __ xorl(rax, rax);  // Clear rax.
+    __ setcc(negative, rax);  // 1 if negative, 0 if positive.
+    __ decb(rax);  // 0 if negative, 255 if positive.
     __ bind(&done);
   }
   __ movq(rcx, FieldOperand(rcx, PixelArray::kExternalPointerOffset));
   __ movb(Operand(rcx, rbx, times_1, 0), rax);
   __ movq(rax, rdx);  // Return the original value.
   __ ret(0);
 
   // Extra capacity case: Check if there is extra capacity to
   // perform the store and update the length. Used for adding one
   // element to the array by writing to array[array.length].
@@ skipping 503 matching lines @@
 
   // Cache miss: Jump to runtime.
   Generate(masm, ExternalReference(IC_Utility(kStoreIC_Miss)));
 }
 
 
 #undef __
 
 
 } }  // namespace v8::internal