Fast sigblock (AKA rtld speedup)

[John Baldwin]

On Saturday, January 12, 2013 11:25:47 am Jilles Tjoelker wrote:
> With that, I think fast sigblock is too much code and complication for a
> niche case.

It does seem a bit complicated to me as well.

> Most of the extra atomics in multi-threaded applications are conditional
> on __isthreaded (or can be made so); therefore, performance loss from
> linking in libthr should be negligible in most cases.

Sadly, this is not true.  libstdc++ turns on locking if you merely link
against libthr, not based on testing __isthreaded.  (It does this by testing
to see if pthread_once() works during startup, and we have to intentionally
sabotage the pthread_once() in libc to fail for this to work which annoys me
for an entirely different set of reasons.)

At work we go to great lengths to avoid linking in libthr for exactly this
reason (e.g. we have a custom port of boost that builds a separate set of
boost libraries that are explicitly not linked against libthr), and we also
care about exception performance (one of my co-workers submitted the PR about
exception performance).

-- 
John Baldwin