Re: bug in check for stack growth direction in _AC_LIBOBJ_ALLOCA

[Andrew W. Nosenko]

On Fri, Jun 17, 2011 at 13:21, Andy Wingo <address@hidden> wrote:
> Hello,
>
> The following transcript indicates a problem with the stack growth
> direction check, present at functions.m4:328 in autoconf 2.68:
>
> address@hidden:/tmp$ cat foo.c
>  int
>  find_stack_direction ()
>  {
>    static char *addr = 0;

Try to rewrite this line as
        volatile static char *addr = 0;
It should help.

>    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;
>  }


> address@hidden:/tmp$ gcc -O1 -o test foo.c
> address@hidden:/tmp$ ./test; echo $?
>  1
> address@hidden:/tmp$ gcc -O3 -o test foo.c
> address@hidden:/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 :)

No.  It indeed traceable.  Just start from main() and you will have
full trace even with pen and paper.

>
> 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.

Just because it still traceable :-)  And modern Gcc is smart enough
for do it :-)

-- 
Andrew W. Nosenko <address@hidden>