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

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

--- Comment #16 from Linus Torvalds <address@hidden> ---
(In reply to H.J. Lu from comment #14)
> We should avoid testb optimization.  This are latencies:
> 
> testb  : 33711871
> testw  : 21204854
> testl  : 18938530
> testq  : 18942712
> 
> on Haswell.  Most of other processors show that testb is the slowest.

Those testb numbers are very interesting and quite unexpected.

Can you share the test program? Are you sure it's not the high byte registers
you tested somehow?

Intriguing.

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