Re: Definition of BUG on x86

Ingo Molnar
2009-02-19T20:27:17Z

* H. Peter Anvin <hpa@zytor.com> wrote:

> Jeremy Fitzhardinge wrote:
>> Ingo Molnar wrote:
>>> Well, the important question is thatGCC will optimize out whatever
>>> comes after the __builtin_trap(), right? To guarantee an assert we
>>> can do something like:
>>>
>>> __builtin_trap();
>>> panic("should never get here");
>>>
>>> to guarantee a message. (But realistically GCC will at most generate
>>> a build error.)
>>>
>>
>> Ah, right, I remember the problem. There's no guaranteed way of
>> getting the address of the ud2a instruction __builtin_trap generates to
>> put it into the bug table.
>>
>
> Did we actually run into any instance where that failed?
>
> It's true that it's not guaranteed, but it seems highly
> unlikely that it would happen in real life. We *could* do a
> forward search at that point, that should catch the vast
> majority of the failing cases, again, but once again there are
> no guarantees.
>
> I guess I should ask the gcc people...

The whole thing is borderline anyway (the win is small), and the
combination of relying on __builtin_trap() [which is documented
as a non-stable interface], and the reliance on basic block
non-ordering.

Another complication is that this is _debug_ code - i.e. if
there's a rare bug here we'll only see it if a bug triggers
there - which is very rare in itself.

So i'm rather uneasy to rely on GCC to this level. They should
allow to pass __noreturn to asm()s - that's a far cleaner
approach.

Ingo