https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103090
Petr Skocik <pskocik at gmail dot com> changed:
What |Removed |Added
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CC| |pskocik at gmail dot com
--- Comment #3 from Petr Skocik <pskocik at gmail dot com> ---
Tangentially related: Gcc and clang both kinda do badly with inline assembly
equivalents of these (locked or unlocked) too.
If you "output" a particular flag and test that, the codegen is great but if
you try to say "flags are set according to the result" (rather a common thing
in x86(-64)) with something like
asm("lock incl %0" : "+m"(*X), "=@ccz"(zf), "=@ccs"(sf));
if ( zf!=(*X==0) ){ __builtin_unreachable(); }
if ( sf!=(*X<0) ){ __builtin_unreachable(); }
and then immediately afterwards try to do with-0 comparisons against *X, then a
reload and a test/0-compare will be generated even though a smarter compiler
could just pick up on the flag assertions and avoid the reload and
test/0-compare: https://godbolt.org/z/7e8PKhTjY
