[Bug regression/38674] New: optimization bug.

["gilles dot chanteperdrix at xenomai dot org" <gcc-bugzilla at gcc dot gnu dot org>]

When compiling the following code with -O2:

#include <stdio.h>

static unsigned long long __attribute__((noinline)) f(void *ptr, unsigned long
long ull)
{
        register unsigned long __r2 __asm__ ("r2") = (unsigned long) (&ull);
        __asm__ __volatile__ ("" : "+r" (__r2));
        return * (unsigned long long *) __r2;
}

int main(void)
{
        fprintf(stderr, "f(NULL, 0): %Lu\n", f(NULL, 0));
        return 0;
}

we get the following warning:
/tmp/ccJNzMaH.s: Assembler messages:
/tmp/ccJNzMaH.s:21: Warning: source register same as write-back base

And when running the program on target, the printed return value of f is not 0.

A disassembly of f:
000083f4 <f>:
    83f4:       e24dd008        sub     sp, sp, #8      ; 0x8
    83f8:       e28d2008        add     r2, sp, #8      ; 0x8
    83fc:       e16220f8        strd    r2, [r2, #-8]!
    8400:       e8920003        ldm     r2, {r0, r1}
    8404:       e28dd008        add     sp, sp, #8      ; 0x8
    8408:       e12fff1e        bx      lr

shows that r2 seems to be clobbered before its value is saved on stack.


-- 
           Summary: optimization bug.
           Product: gcc
           Version: 4.3.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: regression
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: gilles dot chanteperdrix at xenomai dot org
GCC target triplet: armeb-unknown-linux-gnueabi


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38674