[Bug other/26914] New: missed optimization / lack of conditional moves

[pluto at agmk dot net]

flags: -O2 -ffast-math -march=i686 -fomit-frame-pointer

double signum_dbl_gcc( double x )
{
    if ( x < 0.0 )
        return -1.0;
    if ( x > 0.0 )
        return 1.0;
    return 0.0;
}

.LC1:   .long   3212836864

signum_dbl_gcc:
        fldz
        fldl    4(%esp)
        fcomip  %st(1), %st
        jb      .L11
        fld1
        fcmovbe %st(1), %st
        fstp    %st(1)
        ret
.L11:   fstp    %st(0)
        flds    .LC1
        ret

int signum_int_gcc( int x )
{
    if ( x < 0 )
        return -1;
    if ( x > 0 )
        return 1;
    return 0;
}

signum_int_gcc:
        cmpl    $0, 4(%esp)
        movl    $-1, %eax
        jl      .L15
        setne   %al
        movzbl  %al, %eax
.L15:   ret


custom version without branches.

double signum_user_dbl( double x )
{
    asm volatile
    (
        "fld1                                   \n\t"
        "fchs                                   \n\t"
        "fld1                                   \n\t"
        "fldz                                   \n\t"
        "fucomi         %%st(3)                 \n\t"
        "fcmovnbe       %%st(2), %%st(0)        \n\t"
        "fcmovb         %%st(1), %%st(0)        \n\t"
        "fstp           %%st(1)                 \n\t"
        "fstp           %%st(1)                 \n\t"
        "fstp           %%st(1)                 \n\t"
        : "=t" (x)
        : "0" (x)
    );
    return x;
}

int signum_user_int( int x )
{
    int retval;
    asm volatile
    (
        "xor            %%al, %%al      \n\t"
        "mov            $-1, %%cl       \n\t"
        "mov            $1, %%dl        \n\t"
        "cmp            $0, %1          \n\t"
        "cmovl          %%ecx, %%eax    \n\t"
        "cmovg          %%edx, %%eax    \n\t"
        "movsx          %%al, %%eax     \n\t"
        : "=a" (retval)
        : "m" (x)
    );
    return retval;
}


-- 
           Summary: missed optimization / lack of conditional moves
           Product: gcc
           Version: 4.1.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: other
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: pluto at agmk dot net
 GCC build triplet: i686-pld-linux
  GCC host triplet: i686-pld-linux
GCC target triplet: i686-pld-linux


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=26914