[LLVMdev] Use of statics and ManagedStatics in LLVM

[David Blaikie]

On Mon, Jun 9, 2014 at 11:44 AM, Zachary Turner <zturner at google.com> wrote:
> Based on a recent discussion[1], I started trying to remove the functions
> llvm_start_multithreaded() and llvm_stop_multithreaded() from the codebase.
> It turns out this is a little bit tricky.  Consider the following scenario:
>
> During program initialization, a global static object's constructor
> dereferences a ManagedStatic.  During dereferencing of the ManagedStatic, it
> needs to know whether or not to acquire the global lock in order to allocate
> the ManagedStatic
>
> There are 3 possible types for the global_lock, and none of them solve the
> problem.
>
> 1) If global_lock is simply another global static, it may not have been
> constructed yet.

I sort of assumed there was a constexpr (or otherwise
link-time-initialized sort of thing) mutex? Though I recall someone
complaining that this wasn't available on some platform (ARM?
Windows?) or another.

> 2) If global_lock is a raw pointer to a mutex, it would have to be
> explicitly allocated, and we can't guarantee this during static
> initialization.

Can't guarantee that the memory allocation will succeed? But it could
be done without a memory allocation - by placement new-ing into
existing memory.

Though, alternatively, something I've done in the past to avoid the
need for mutexes to initialized static state is to use something like
ManagedStatic, except with a global ctor - so it's lazy, initializes
on first use (thwarting global ctor ordering, unless there's a true
circular dependency), but with the added protection that it will be
initialized (one way or another) by a static ctor. This way, if we can
safely assume that global ctors run single threaded (?), we don't have
to worry about racing access to these things later on.

This doesn't help with resurrection that you mention, since after a
shutdown you'd be left in the unitialized state, so if you went right
back into LLVM you would be getting possibly racy initializations -
and then you'd want that "InitLLVM" function you were alluding to.

All that said, I know a bunch of people want to get rid of static
ctors in LLVM anyway - so if there's a path forward that takes us
closer to that goal, I expect it'll get some support.

> 3) If global_lock is a ManagedStatic<Mutex>, then it will get into an
> infinite recursion here when trying to allocate this ManagedStatic.
>
> I actually started to feel this way since the first time I started looking
> at LLVM, but even moreso increasingly I feel that the solution is that
> ManagedStatics should not be allowed to be accessed until after main begins.
> llvm_shutdown() gives deterministic order of destroying managed statics, but
> we don't have deterministic order of *creation* of those ManagedStatics.  I
> have a patch up[2] (still awaiting review) that shows a possible solution to
> these problems and how we might migrate the existing cases where this
> happens over to a ManagedStatic free static initialization.
>
> There is only one requirement: You must be able to insert a call very early
> in main() that will do the ManagedStatic initialization.  However, we can
> catch this with an assertion and all anyone would have to do is add one line
> to their main function.  As in [2], all this early main() code would do is
> copy fields over from one structure to another.
>
> As a side benefit, this provides the ability to "resurrect" after an
> llvm_shutdown(), shoudl that be desired, because the initial static state of
> the program always remains in-tact once main() is entered.
>
> Thoughts?  Better ideas?
>
>
> [1] - http://lists.cs.uiuc.edu/pipermail/llvmdev/2014-June/073543.html
> [2] -
> http://lists.cs.uiuc.edu/pipermail/llvm-commits/Week-of-Mon-20140602/219684.html
>
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