[Bug gas/22871] Encode instructions of 64-bit operand without the REX_W bit

[jbeulich at novell dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=22871

--- Comment #6 from Jan Beulich <jbeulich at novell dot com> ---
(In reply to H.J. Lu from comment #5)
> I updated users/hjl/optimize branch to encode
> 
> testq $imm31, mem
>     
> as
>     
> testl $imm31, mem
> 
> only at -O2.

I was about to suggest that, also because the memory access pattern changes
(the shorter access may not fault when the longer one would, not to think of
side effects when accessing MMIO). I'd even consider moving this higher up, to
-O3.

Another thing to consider here would be to encode e.g. vxorps %zmmM, %zmmM,
%zmmN as vxorps %xmmM, %xmmM, %xmmN for the low 16 registers, as that'll be VEX
encodable, i.e. shorter than the default EVEX variant. Same for vandnps (and of
course all their flavors dealing with different data types).

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