https://gcc.gnu.org/bugzilla/show_bug.cgi?id=60554
--- Comment #3 from Uroš Bizjak <ubizjak at gmail dot com> ---
(In reply to Luchezar Belev from comment #0)
> consider this simple function:
> int is_float_negative(int x) { return (int)(x ^ 0x80000000) > 0; }
>
> for x86, with options "-O3 -march=core2", GCC 4.8.2 generates the following
> code:
> ....
> _is_float_negative:
> movl 4(%esp), %eax
> addl $-2147483648, %eax
> testl %eax, %eax
> setg %al
> movzbl %al, %eax
> ret
> ....
>
> apparently (x^0x80000000) is replaced with (x+0x80000000), that's ok.
> the problem is the testl instruction - why the compiler decided to emit it?
> the addl instruction itself sets the needed flags (Z and N) just right.
Nope, setg means "Set byte if greater (ZF=0 and SF=OF)". The problem is when
%eax is -1. addl sets OF flag and clears SF, so setg doesn't set %al.
