libstdc++/8230: Buggy allocator behaviour

[Matt Austern]

On Thursday, November 14, 2002, at 12:39 PM, Gabriel Dos Reis wrote:

> Benjamin Kosnik <bkoz@redhat.com> writes:
>
> | See attached patch for a way to fix this with the pool allocators.
>
> I agree with your patch.
>
> The following comment is for possible improvements.  What about using
>
>    if (__builtin_expect(__ret == 0, 0))
>        __throw_bad_alloc("allocate::allocate");
>
> ?
> That gives a hint to the compiler that the branch is expected to be
> taken very unfrequently, and it should make scheduling accordingly.
>
> In general, I would like to see us starting using use 
> __builtin_expect()
> at places where we do checks for limits cases.  Thoughts?

Two questions.

First, is it known how much of a speedup this gives on typical 
platforms?

Second, if the answer to question 1 is "a lot": is there some sensible
way to use this in the compiler itself?  I suppose it'd have to involve
some ghastly macro hackery because of the bootstrap problem, but
it we're talking big performance wins, it might be worth it.

			--Matt