[Bug c/29964] New: volatile problem in optimized functions

["sb at anoto dot com" <gcc-bugzilla at gcc dot gnu dot org>]

Using -Os and volatile results in functions, sometimes the optimizer 'forgets'
the volatile property of the function and makes it pure. This has a major
impact in some cases. Test case 1 gives buggy code where test case 2 & 3 works.
The command line to compile this was arm-elf-gcc -c test.c -Os

/* Test case 1 */

struct test_a { volatile int a; };

int func_a(void) __attribute__ ((noinline));
int func_a(void)
{
        return ((struct test_a *)0)->a;
}

void a(void)
{
        while(func_a() == 0);
}

/* Test case 2 */

struct test_b { volatile int a;};

int func_b(void) __attribute__ ((noinline));
int func_b(void)
{
        volatile struct test_b *t= 0;
        return t->a;
}

void b(void)
{
        while(func_b() == 0);
}

/* Test case 3 */

struct test_c { volatile int a; };

int func_c(void) __attribute__ ((noinline));
int func_c(void)
{
        return ((volatile struct test_c *)0)->a;
}

void c(void)
{
        while(func_c() == 0);
}


The result from this is:


00000000 <func_a>:
   0:   e3a03000        mov     r3, #0  ; 0x0
   4:   e5930000        ldr     r0, [r3]
   8:   e12fff1e        bx      lr

0000000c <a>:
   c:   e52de004        str     lr, [sp, #-4]!
  10:   ebfffffe        bl      0 <func_a>
  14:   e3500000        cmp     r0, #0  ; 0x0
  18:   0a000003        beq     14 <a+0x8>      <--- looping back to the cmp!?!
  1c:   e49df004        ldr     pc, [sp], #4

00000020 <func_b>:
  20:   e3a03000        mov     r3, #0  ; 0x0
  24:   e5930000        ldr     r0, [r3]
  28:   e12fff1e        bx      lr

0000002c <b>:
  2c:   e52de004        str     lr, [sp, #-4]!
  30:   ebfffffe        bl      20 <func_b>
  34:   e3500000        cmp     r0, #0  ; 0x0
  38:   0a00000a        beq     30 <b+0x4> <--- looping back to the functon
call
  3c:   e49df004        ldr     pc, [sp], #4

00000040 <func_c>:
  40:   e3a03000        mov     r3, #0  ; 0x0
  44:   e5930000        ldr     r0, [r3]
  48:   e12fff1e        bx      lr

0000004c <c>:
  4c:   e52de004        str     lr, [sp, #-4]!
  50:   ebfffffe        bl      40 <func_c>
  54:   e3500000        cmp     r0, #0  ; 0x0
  58:   0a000012        beq     50 <c+0x4> <--- looping back to the functon
call
  5c:   e49df004        ldr     pc, [sp], #4


-- 
           Summary: volatile problem in optimized functions
           Product: gcc
           Version: 4.1.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: sb at anoto dot com
 GCC build triplet: x86-pc-cygwin
  GCC host triplet: x86-pc-cygwin
GCC target triplet: arm-elf-gcc


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=29964