Re: Bug#198158: architecture i386 isn't i386 anymore

["Martin v. Löwis" <martin@v.loewis.de>]

John Goerzen wrote:

> Nobody has even explained WHY we have this issue.  The summary posted on the
> bug report just said that there is a problem with atomicity.h, not what the
> problem is or why it exists.

Just look at the file for yourself. It is easy enough to see: it uses 
inline assembly that is only available on i486:

static inline _Atomic_word
__attribute__ ((__unused__))
__exchange_and_add (volatile _Atomic_word *__mem, int __val)
{
  register _Atomic_word __result;
  __asm__ __volatile__ ("lock; xaddl %0,%2"
                        : "=r" (__result)
                        : "0" (__val), "m" (*__mem)
                        : "memory");
  return __result;
}

In particular, the lock prefix is not available on i386. Since this is
an inline function, this code is inserted into any C++ binary, so you 
can't change its implementation by replacing the library.

In g++ 3.2, this code was distributed as "i386", and nobody noticed that 
it doesn't work on i386 for quite some time. In gcc 3.3, an 
implementation is provided that works on i386, and this implementation 
here is declared i486. Unfortunately, the two implementations are not 
binary compatible. Debian has to pick one of these, and it needs to pick 
the i486 version for compatibility with other Linux distributions (which 
either ship with gcc 3.2 today, or target i586+ only, anyway).

Regards,
Martin