[Bug libc/16291] feature request: provide simpler ways to compute stack and tls boundaries

["konstantin.s.serebryany at gmail dot com" <sourceware-bugzilla at sourceware dot org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=16291

--- Comment #4 from Kostya Serebryany <konstantin.s.serebryany at gmail dot com> ---
> Can you clarify why you need to know the max size the stack might grow to,
> rather than the currently valid stack range? The latter would be much easier
> to obtain, and the former could (I think) become wrong if the application
> uses setrlimit between the time the stack size is obtained and the time your
> code needs to know it.

Our code computes the stack bounds at thread initialization and never computes
them again for performance reasons: the bounds are needed in the 
frame-pointer-based unwinder which needs to know the stack bounds,
and we call the unwinder on every malloc/free, so this is hot.
And we don't want to have special code like "if (in_main_thread)" in hot
places.
We need the stack bounds in a few other less performance-critical places too.

Anyway, the moving end of the main thread's stack is the least of our concerns,
tls size and non-main-thread stack bounds are more painful.

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