Fix rounding in Uint8ClampedArray setter.

src/x64/macro-assembler-x64.cc

 // Copyright 2012 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 2828 matching lines @@
 
 void MacroAssembler::ClampDoubleToUint8(XMMRegister input_reg,
                                         XMMRegister temp_xmm_reg,
                                         Register result_reg,
                                         Register temp_reg) {
   Label done;
   Set(result_reg, 0);
   xorps(temp_xmm_reg, temp_xmm_reg);
   ucomisd(input_reg, temp_xmm_reg);
   j(below, &done, Label::kNear);
-  uint64_t one_half = BitCast<uint64_t, double>(0.5);
-  Set(temp_reg, one_half);
-  movq(temp_xmm_reg, temp_reg);
-  addsd(temp_xmm_reg, input_reg);
-  cvttsd2si(result_reg, temp_xmm_reg);
+  cvtsd2si(result_reg, input_reg);
   testl(result_reg, Immediate(0xFFFFFF00));
   j(zero, &done, Label::kNear);
   Set(result_reg, 255);
   bind(&done);
 }
 
 
 void MacroAssembler::LoadInstanceDescriptors(Register map,
                                              Register descriptors) {
   Register temp = descriptors;
@@ skipping 1641 matching lines @@
   bind(&check_prototype);
   movq(rcx, FieldOperand(rbx, Map::kPrototypeOffset));
   cmpq(rcx, null_value);
   j(not_equal, &next);
 }
 
 
 } }  // namespace v8::internal
 
 #endif  // V8_TARGET_ARCH_X64