[Bug c/31575] New: Extra push+pop generated on x86

["dann at godzilla dot ics dot uci dot edu" <gcc-bugzilla at gcc dot gnu dot org>]

For this code: 

struct data {
    unsigned data[4][1];
    unsigned char valid[4];
    unsigned char flags[4];
};

void
def(struct data *info, int index, unsigned *v)
{
    if (info->flags[index]) {
        info->valid[index] = 1;
        info->data[index][0] = v[0];
    }
}
SVN HEAD generates an extra push+pop compared to 4.1.1 when compiling with 
-O2 -march=pentium4

4.1.1 generates this code (3.4.3 generates identical code):   
def:
        pushl   %ebp
        movl    %esp, %ebp
        movl    8(%ebp), %ecx
        movl    12(%ebp), %edx
        cmpb    $0, 20(%edx,%ecx)
        je      .L4
        movb    $1, 16(%edx,%ecx)
        movl    16(%ebp), %eax
        movl    (%eax), %eax
        movl    %eax, (%ecx,%edx,4)
.L4:
        popl    %ebp
        ret

SVN HEAD:
def:
        pushl   %ebp
        movl    %esp, %ebp
        pushl   %ebx  <--- extra instruction
        movl    8(%ebp), %ecx
        movl    12(%ebp), %edx
        cmpb    $0, 20(%ecx,%edx)
        je      .L4
        movb    $1, 16(%ecx,%edx)
        movl    16(%ebp), %ebx
        movl    (%ebx), %eax
        movl    %eax, (%ecx,%edx,4)
.L4:
        popl    %ebx  <--- extra instruction
        popl    %ebp
        ret


This is a regression from (at least) 3.4.3 and 4.1.1


-- 
           Summary: Extra push+pop generated on x86
           Product: gcc
           Version: 4.3.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: dann at godzilla dot ics dot uci dot edu
GCC target triplet: i686-pc-linux-gnu


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=31575