https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114490
--- Comment #3 from Kang-Che Sung <Explorer09 at gmail dot com> ---
(In reply to Andrew Pinski from comment #2)
>
> Basically if you are shifting by 0, then the CF bit never changes from the
> previous and this can't be done.
>
> So the comment in the backend is correct and there is no way to remove the
> test (unless you know ch can't be 0).
What if I had the CF cleared to 0 already before the shift?
Even when the flags don't change when the value in the CL register is zero, if
I had the flags in a known state, the optimization can still be done.
And my suggested 'my_isxdigit_2' code worked before I know the caveat of the
shift instructions... ("xorl %eax, %eax" -> CF = SF = 0)
