Hello,
The following transcript indicates a problem with the stack growth
direction check, present at functions.m4:328 in autoconf 2.68:
wingo@badger:/tmp$ cat foo.c
int
find_stack_direction ()
{
static char *addr = 0;
auto char dummy;
if (addr == 0)
{
addr = &dummy;
return find_stack_direction ();
}
else
return (&dummy > addr) ? 1 : -1;
}
int
main ()
{
return find_stack_direction () < 0;
}
wingo@badger:/tmp$ gcc -O1 -o test foo.c
wingo@badger:/tmp$ ./test; echo $?
1
wingo@badger:/tmp$ gcc -O3 -o test foo.c
wingo@badger:/tmp$ ./test; echo $?
0
$ gcc --version
gcc (Debian 4.6.0-13) 4.6.1 20110611 (prerelease)
Copyright (C) 2011 Free Software Foundation, Inc.
This is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
As you see, the check (on an x86-64 system) gives the correct result at
-O1 but not at -O3.
(I don't actually use this check from _AC_LIBOBJ_ALLOCA, but Guile has
the exact same test to check for stack growth direction, so it's the
same issue. Reported by Marco Maggi.)
Looking a bit more closely at this, I find the assembly to be a bit odd
here:
00000000004004a0 <find_stack_direction>:
4004a0: mov 0x2003f9(%rip),%rax # 6008a0 <addr.1586>
4004a7: test %rax,%rax
4004aa: je 4004c0 <find_stack_direction+0x20>
4004ac: lea -0x2(%rsp),%rdx
4004b1: cmp %rdx,%rax
4004b4: sbb %eax,%eax
4004b6: and $0x2,%eax
4004b9: sub $0x1,%eax
4004bc: retq
4004bd: nopl (%rax)
4004c0: lea -0x2(%rsp),%rax
4004c5: lea -0x1(%rsp),%rdx
4004ca: cmp %rdx,%rax
4004cd: mov %rax,0x2003cc(%rip) # 6008a0 <addr.1586>
4004d4: sbb %eax,%eax
4004d6: and $0x2,%eax
4004d9: sub $0x1,%eax
4004dc: retq
4004dd: nop
4004de: nop
4004df: nop
As you can see there is no call. The test will always be true, thus the
second branch is always taken. I don't know what allows GCC to do this
inlining. Could it be a GCC bug? Every time I think I have a GCC bug
I'm wrong, though :)
I tried changing the test to the following:
int
find_stack_direction (char *addr)
{
char dummy;
if (addr == 0)
return find_stack_direction (&dummy);
else
return (&dummy > addr) ? 1 : -1;
}
int
main ()
{
return find_stack_direction (0) < 0;
}
But I get the same behavior.
Andy
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