[Bug c/44474] New: GCC inserts redundant "test" instruction due to incorrect clobber

["darkshikari at gmail dot com" <gcc-bugzilla at gcc dot gnu dot org>]

Take the following test case:

int foo();

int test( int *b )
{
    (*b)--;
    if( *b == 0 )
        return foo();
    return 0;
}

On x86_64, with "-fomit-frame-pointer -O3 -c", gcc 4.6 compiles this to:

   0:   8b 07                   mov    eax, [rdi]
   2:   83 e8 01                sub    eax, 0x1
   5:   85 c0                   test   eax, eax
   7:   89 07                   mov    [rdi], eax
   9:   74 05                   je     10 <test+0x10>
   b:   31 c0                   xor    eax, eax
   d:   c3                      ret
 10:    e9 00 00 00 00          jmp    15 <test+0x15> //foo

As can be seen, gcc inserts a redundant "test" instruction.  This problem is
present in all versions of gcc I tested (3.4, 4.3.4, 4.4.1, and 4.6 SVN
r160330).

According to Alexander Strange on IRC:

<astrange> the problem is that subl $1, %eax does (clobber (reg:CC 17 flags))
instead of (set (reg:CCZ 17 flags) (compare:CCZ ....
<astrange> which i don't think is fixable in a real way, but i didn't write it


-- 
           Summary: GCC inserts redundant "test" instruction due to
                    incorrect clobber
           Product: gcc
           Version: 4.6.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: darkshikari at gmail dot com


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=44474