[Bug rtl-optimization/60554] redundant instruction is generated for setting the flags on x86

[Bug rtl-optimization/60554] redundant instruction is generated for setting the flags on x86

[l_belev at yahoo dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=60554

--- Comment #1 from Luchezar Belev <l_belev at yahoo dot com> ---
since all (x^0x80000000), (x+0x80000000) and (x-0x80000000) are equivalend in
this case, if GCC had used the subtraction (instead of the addition) it could
optimize the code even further by considering the fact that intermediate result
is not needed and use cmpl instead of subl:

_is_float_negative:
   cmpl $0x80000000, 4(%esp)
   setg %al
   movzbl %al, %eax
   ret

which is the same as if the function was written as
 int is_float_negative(int x) { return (unsigned)x > 0x80000000; }

[Bug rtl-optimization/60554] redundant instruction is generated for setting the flags on x86

[l_belev at yahoo dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=60554

--- Comment #2 from Luchezar Belev <l_belev at yahoo dot com> ---
sorry, the above code to be correct "setg %al" should be "seta %al" instead.

also in the compiler-generated code, the testl instruction is still redundant,
but when it is removed the following "setg %al" should be replaced with "seta
%al".
Maybe this is too complicated for a compiler to figure out though.