Fast sigblock (AKA rtld speedup)

[Alfred Perlstein]

On 1/14/13 11:06 AM, John Baldwin wrote:
> 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).
>
I get frustrated when people ask me "but why are you doing that?", but I 
have to know... why do we/you need fast exception handling?

Are you throwing a high rate of exceptions?  Or is it just that your 
application is that sensitive to exceptions being thrown that a single 
slowish one has an impact?

-Alfred