[Bug target/21508] New: non-optimal code with __builtin_signbit

["uros at kss-loka dot si" <gcc-bugzilla at gcc dot gnu dot org>]

This testcase:

int test (double a) {
        return __builtin_signbit (a + 1.0);
}

produces quite non-optimal code when compiled with '-O2' (-fomit-frame-pointer):

test:
        subl    $12, %esp
!       xorl    %eax, %eax
!       movl    $1072693248, %edx
!       movl    %eax, (%esp)
!       movl    %edx, 4(%esp)
!       fldl    (%esp)
        faddl   16(%esp)
        fstpl   (%esp)
?       movl    4(%esp), %edx
        addl    $12, %esp
?       movl    %edx, %eax
        andl    $-2147483648, %eax
        ret

Instructions marked with (!) could be replaced with a fld1.
Instructions marked with (?) could be replaced with a "movl 4(%esp), %eax".

Perhaps fxam could be used in this case? Then the code would look something like:

fldl   4(%esp)
fxam
fnstsw %ax
fstp   %st(0)
andl   0x02, %eax
ret

-- 
           Summary: non-optimal code with __builtin_signbit
           Product: gcc
           Version: 4.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: target
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: uros at kss-loka dot si
                CC: gcc-bugs at gcc dot gnu dot org
 GCC build triplet: i686-pc-linux-gnu
  GCC host triplet: i686-pc-linux-gnu
GCC target triplet: i686-pc-linux-gnu


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=21508