micompilation of autoconf stack growth test at -O > 0 on ARM

[Adam Conrad]

This autoconf test works on x86, but appears to fail on ARM when built with optimisation. It worked fine with gcc-4.6, and fails with 4.7. Versions here were all from current saucy:

gcc-4.6: 4.6.4-3ubuntu1
gcc-4.7: 4.7.3-6ubuntu1
gcc-4.8: 4.8.1-7ubuntu1

(saucy-kde)adconrad@shiva:~/librep-0.90.2$ gcc-4.6 -o conftest -O2 stack.c
(saucy-kde)adconrad@shiva:~/librep-0.90.2$ ./conftest ; echo $?
0
(saucy-kde)adconrad@shiva:~/librep-0.90.2$ gcc-4.7 -o conftest -O2 stack.c
(saucy-kde)adconrad@shiva:~/librep-0.90.2$ ./conftest ; echo $?
1
(saucy-kde)adconrad@shiva:~/librep-0.90.2$ gcc-4.8 -o conftest -O2 stack.c
(saucy-kde)adconrad@shiva:~/librep-0.90.2$ ./conftest ; echo $?
1
(saucy-kde)adconrad@shiva:~/librep-0.90.2$ gcc-4.8 -o conftest -O1 stack.c
(saucy-kde)adconrad@shiva:~/librep-0.90.2$ ./conftest ; echo $?
1
(saucy-kde)adconrad@shiva:~/librep-0.90.2$ gcc-4.8 -o conftest -O0 stack.c
(saucy-kde)adconrad@shiva:~/librep-0.90.2$ ./conftest ; echo $?
0
(saucy-kde)adconrad@shiva:~/librep-0.90.2$ cat stack.c
#include <stdlib.h>

void inner (char *foo) {
  char bar;
  exit (!(foo >= &bar));
}

void main () {
  char foo;
  inner (&foo);
}

[Doko-v (doko-v)]

seen on arm-linux-gnueabihf, as part of an autoconf test in librep:

$ cat stack.c
#include <stdlib.h>

void inner (char *foo) {
  char bar;
  exit (!(foo >= &bar));
}

void main () {
  char foo;
  inner (&foo);
}

$ gcc-4.6 -o conftest -O2 stack.c && ./conftest ; echo $?
0
$ gcc-4.7 -o conftest -O2 stack.c && ./conftest ; echo $?
1
$ gcc-4.8 -o conftest -O2 stack.c && ./conftest ; echo $?
1
$ gcc-4.8 -o conftest -O0 stack.c && ./conftest ; echo $?
0

[Rguenth (rguenth)]

Well, it's clearly a bogus test - you are comparing addresses of two distinct objects which invokes undefined behavior.

[Andreas Schwab (schwab-linux-m68k)]

Casting to uintptr_t should probably make this work as intented.

[Rguenth (rguenth)]

Well, until the compiler comes along and inlines 'inner'.

This is really a GIGO test.

[Mattia Rizzolo (mapreri)]

librep workarounded it in 0.90.2-1.4ubuntu1.