Why is the rex.w prefix not generated for certain instructions in inline assembler blocks?

[Joseph Cassman]

On Monday, 9 September 2013 at 20:37:47 UTC, Joseph Cassman wrote:
> So I'm out of ideas on where the error is coming from. Anyone 
> got any ideas on what's going on here?

Still not sure what is going on but I found a work-around that 
somebody else might find useful.

The mov instruction is able to work with 64-bit values 
successfully so the value that is larger than 32 bits can be 
placed in a register and the register used for the operation. 
Here are few examples.

     void main()
     {
         asm
         {
             mov   RAX,0x1ffffffffUL;
             mov   R8,0xFFFFFFFFFFFFFFFFUL;
             and   R8,RAX;
             xor   R8,RAX;
             or    R8,RAX;
             add   R8,RAX;
             sub   R8,RAX;
             cmp   R8,RAX;
         }
     }

And their disassembly (excluding the nonessentials).

     mov RAX,01FFFFFFFFh
     mov R8,0FFFFFFFFFFFFFFFFh
     and R8,RAX
     xor R8,RAX
     or  R8,RAX
     add R8,RAX
     sub R8,RAX
     cmp R8,RAX

The only downside of the work-around is that there is already 
pressure on the limited number of registers in x64. Some 
shuffling might be required.

Joseph