[PATCH] [ms-cxxabi] Implement guard variables for static initialization

[John McCall]

On Aug 15, 2013, at 4:59 PM, Reid Kleckner <rnk at google.com> wrote:
> Static locals requiring initialization are not thread safe on Windows.
> Unfortunately, it's possible to create static locals that are actually
> externally visible with inline functions and templates.  As a result, we
> have to implement an initialization guard scheme that is compatible with
> TUs built by MSVC, which makes thread safety prohibitively difficult.
> 
> MSVC's scheme is that every function that requires a guard gets an i32
> bitfield.  Each static local is assigned a bit that indicates if it has
> been initialized, up to 32 bits, at which point a new bitfield is
> created.  MSVC rejects inline functions with more than 32 static locals,
> and the externally visible mangling (?_B) only allows for one guard
> variable per function.

Hilarious.

Itanium has a similar concept of function-specific mangling state tracked
with startNewFunction() on the mangle context; you should try to unify
the two.

You should also figure out whether the same things that cause a new
context to be suppressed should apply here.

It would be nice to let the FunctionState be lazily initialized.

Otherwise looks good.

John.