[Bug tree-optimization/31272] New: gcc is being too clever

[matt at 3am-software dot com]

[This problem occurs in 4.1.2 and 4.1.3 as well]
Given the following source:

int
in_canforward(unsigned int in)
{
        unsigned int net;

        if ((in & ~0xffffff0f) == 0xf0 || (in & ~0xffffff0f) == 0xe0)
                return 0;
        if ((in & 0x80) == 0) {
                net = in & 0xff;
                if (net == 0 || net == 0x7f)
                        return 0;
        }
        return 1;

"cc1 -O2 -quiet" emit:  

#NO_APP
        .file   "inc.c"
        .text
        .align 1
.globl in_canforward
        .type   in_canforward, @function
in_canforward:
        .word 0x0
        subl2 $4,%sp
        movl 4(%ap),%r1
        bicl3 $-241,%r1,%r0
        cmpl %r0,$240
        jeql .L2
        cmpl %r0,$224
        jeql .L2
        bicb3 $127,%r1,%r0
        jneq .L12
        bicl2 $-256,%r1
        jneq .L13
.L2:
        clrl %r0
        ret
.L12:
        movl $1,%r0
        ret
.L13:
        cmpl %r1,$127
        jeql .L14
        xorb2 $1,%r0
        movzbl %r0,%r0
        ret
.L14:
        movb $1,%r0
        xorb2 $1,%r0
        movzbl %r0,%r0
        ret
        .size   in_canforward, .-in_canforward
        .ident  "GCC: (GNU) 4.3.0 20070319 (experimental)"

Notice the code at .L14?  Why go through all extra that effort?  Why not just
do clrl %r0 and ret or preferrably branch to .L2 which already does that?  The
same argument goes for the xorb2/movzbl right  before .L14.   movl $1,%r0 and
ret or preferably branch to .L12 which already does that.

Now if you move .L13 before .L12 (so it can all fall through) and take the
mentioned branches, the almost-optimal version becomes:

in_canforward:
        .word 0x0
        subl2 $4,%sp
        movl 4(%ap),%r1
        bicl3 $-241,%r1,%r0
        cmpl %r0,$240
        jeql .L2
        cmpl %r0,$224
        jeql .L2
        bicb3 $127,%r1,%r0
        jneq .L12
        bicl2 $-256,%r1
        jneq .L13
.L2:
        clrl %r0
        ret
.L13:
        cmpl %r1,$127
        jeql .L2
.L12:
        movl $1,%r0
        ret


-- 
           Summary: gcc is being too clever
           Product: gcc
           Version: 4.3.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: matt at 3am-software dot com
 GCC build triplet: x86_64--netbsd
  GCC host triplet: x86_64--netbsd
GCC target triplet: vax--netbsdelf


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=31272