[x64] Fixed a rounding error on x64 for the Uint64ToF64 conversion.

src/x64/macro-assembler-x64.cc

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #if V8_TARGET_ARCH_X64
 
 #include "src/base/bits.h"
 #include "src/base/division-by-constant.h"
 #include "src/bootstrapper.h"
 #include "src/codegen.h"
@@ skipping 881 matching lines @@
     CpuFeatureScope scope(this, AVX);
     vxorpd(dst, dst, dst);
     vcvtqsi2sd(dst, dst, src);
   } else {
     xorpd(dst, dst);
     cvtqsi2sd(dst, src);
   }
 }
 
 
-void MacroAssembler::Cvtqui2sd(XMMRegister dst, Register src) {
+void MacroAssembler::Cvtqui2sd(XMMRegister dst, Register src, Register tmp) {
   Label msb_set_src;
   Label jmp_return;
   testq(src, src);
   j(sign, &msb_set_src, Label::kNear);
   Cvtqsi2sd(dst, src);
   jmp(&jmp_return, Label::kNear);
   bind(&msb_set_src);
+  movq(tmp, src);
   shrq(src, Immediate(1));
+  // Recover the least significant bit to avoid rounding errors.
+  andq(tmp, Immediate(1));
+  orq(src, tmp);
   Cvtqsi2sd(dst, src);
   addsd(dst, dst);
   bind(&jmp_return);
 }
 
 
 void MacroAssembler::Cvtsd2si(Register dst, XMMRegister src) {
   if (CpuFeatures::IsSupported(AVX)) {
     CpuFeatureScope scope(this, AVX);
     vcvtsd2si(dst, src);
@@ skipping 4570 matching lines @@
   movl(rax, dividend);
   shrl(rax, Immediate(31));
   addl(rdx, rax);
 }
 
 
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_TARGET_ARCH_X64