[Bug c/56767] New: gcc does not generate correct code with -O2

[jupitercuso4 at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56767

             Bug #: 56767
           Summary: gcc does not generate correct code with -O2
    Classification: Unclassified
           Product: gcc
           Version: 4.7.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: jupitercuso4@gmail.com


== How to repeat (on a 32-bit Linux machine) ==

With following code:

// begin of code
#include <stdio.h>

int round2(int n) {
    int ret;
    if (n == 0) { return 0; }
    __asm__ __volatile__ ("bsr %0, %1\n\t"
                          :"=r"(ret): "r"(n));
    ret = 1 << ret;
}

int main() {
    int a;
    scanf("%d", &a);
    printf("%d\n", round2(a));
    return 0;
}
// end of code

With -O0, we get correct result, but with -O2, the generated assembly is:
main:
.LFB12:
    .cfi_startproc
    pushl   %ebp
    .cfi_def_cfa_offset 8
    .cfi_offset 5, -8
    movl    %esp, %ebp
    .cfi_def_cfa_register 5
    andl    $-16, %esp
    subl    $32, %esp
    leal    28(%esp), %eax
    movl    %eax, 4(%esp)
    movl    $.LC0, (%esp)
    call    __isoc99_scanf
    movl    28(%esp), %eax
    testl   %eax, %eax
    je  .L6
#APP
# 6 "bit.c" 1
    bsr %eax, %eax

# 0 "" 2
#NO_APP
.L6:
    movl    $0, 4(%esp) // ALWAYS output 0
    movl    $.LC1, (%esp)
    call    printf
    xorl    %eax, %eax
    leave