[Bug rtl-optimization/60554] New: 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

            Bug ID: 60554
           Summary: redundant instruction is generated for setting the
                    flags on x86
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: minor
          Priority: P3
         Component: rtl-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: l_belev at yahoo dot com

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.