[Bug gas/25438] x86 MOVZ* anomalies for unusual/wrong operand combinations

[hjl.tools at gmail dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=25438

--- Comment #7 from H.J. Lu <hjl.tools at gmail dot com> ---
(In reply to Jan Beulich from comment #6)
> (In reply to H.J. Lu from comment #5)
> > Also
> > 
> > [hjl@gnu-cfl-1 pr25438]$ cat y.s
> > movzwl      %ax, %rcx
> > [hjl@gnu-cfl-1 pr25438]$ gcc -c y.s
> > [hjl@gnu-cfl-1 pr25438]$ objdump -dw y.o
> > 
> > y.o:     file format elf64-x86-64
> > 
> > 
> > Disassembly of section .text:
> > 
> > 0000000000000000 <.text>:
> >    0:       48 0f b7 c8             movzwq %ax,%rcx
> 
> 
> And? How is this related here? This is an appropriate encoding for the insn.
> When optimzing, dropping the REX prefix would seem desirable. In fact this
> is already on my todo list.

It is OK to encode "movzwq %ax,%rcx" as "movzwl %ax,%ecx".  But assembler
shouldn't accept "movzwl %ax,%rcx".

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