https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96846
Jakub Jelinek <jakub at gcc dot gnu.org> changed:
What |Removed |Added
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CC| |jakub at gcc dot gnu.org
--- Comment #1 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
This is something GCC is well aware of and attempts to optimize, it has several
peephole2s, starting with the
;; Convert setcc + movzbl to xor + setcc if operands don't overlap.
comment in i386.md.
The reason why a xor isn't added in your first function is that there is no
extension in that case, usually callers will just use the 8-bit register and
then the xor would be a waste of time.
The second case isn't handled because there is no place to insert the xor to,
xor modifies flags, so it can't go after the comparison that sets the flags,
and in this case can't go before either, because the register is live there (it
is an operand of the comparison). So, I guess the only way around would be to
look if there is some currently unused register that could be used instead of
the one chosen by the register allocation, but that is not always the case.
