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

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 3460da46e8eed70dc4f34bf7a89913d525302998..1a936afc98c514be89427b82104925b7174dd794 100644
--- a/src/x64/macro-assembler-x64.cc
+++ b/src/x64/macro-assembler-x64.cc
@@ -899,7 +899,7 @@ void MacroAssembler::Cvtqsi2sd(XMMRegister dst, const Operand& 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);
@@ -907,7 +907,11 @@ void MacroAssembler::Cvtqui2sd(XMMRegister dst, Register src) {
   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);