Thread safety of alloca

[dsimcha]

== Quote from Denis Koroskin (2korden at gmail.com)'s article
> On Sun, 25 Oct 2009 03:34:24 +0300, dsimcha <dsimcha at yahoo.com> wrote:
> > Does anyone know if alloca() is by some chance not thread safe?  I'm
> > working
> > on improving my parallelization library and I keep running into all
> > kinds of
> > erratic behavior (but not stack overflows) when I use alloca() instead
> > of heap
> > as an optimization, but when I disable this optimization, everything
> > seems to
> > work.  I do know that several threads are likely calling alloca() at the
> > same
> > time and with requests large enough that their stacks might need to be
> > expanded.
> >
> > At first, I thought this was just a symptom of a subtle race condition,
> > but I
> > tried inserting sleep statements and synchronized blocks in various
> > places
> > around the alloca() calls to change the timing of things and the
> > conclusion is
> > the same:  malloc() works, alloca() doesn't.
> You are probably misusing alloca (e.g. returning it from a function) or
> overwriting the stack space in some other way. This is strange otherwise.
> I don't think it is thread-unsafe since it doesn't access any global state.
> Is the problematic source code available for inspection online?
> btw, alloca is implemented in druntime\src\compiler\dmd\alloca.d

Yep, you're right.  There was a very subtle and interesting bug in my code.  It's
too complicated to explain here, but suffice to say I was escaping stuff in a
pretty subtle way.