https://sourceware.org/bugzilla/show_bug.cgi?id=22871
--- Comment #12 from Linus Torvalds <torva...@linux-foundation.org> ---
(In reply to H.J. Lu from comment #11)
>
> imm8 isn't sign-extended. 8 bits should work.
No, it's not sign-extended to the full size, but it doesn't work because you
change the sign bit in the flags in the _small_ size.
So you can't change
testq $255,%rdx
into
testb $255,%dl
because the flag end result is different.
The first instruction always has a positive result (since the resulting sign
bit in 64 bits is always zero). But the shortened version would be negative if
bit#7 in %dl is set.
Or am I missing something else?
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