[LLVMdev] Making it possible to clear the LLVMContext

[Marcello Maggioni]

Hello,

the need here is to have a single LLVMContext used for multiple
compilations.

You make a good point about that by the way. If there are outstanding users
cleaning the context under their seats might still pose a risk to them, and
in that case deleting + newing a new LLVMContextImpl might actually not be
very different.

Marcello

2014-06-24 17:14 GMT+01:00 David Blaikie <dblaikie at gmail.com>:

> What're the situation in which you need to clear it? If there are
> outstanding users of it (given that you mention clients possibly
> holding references to the pimpl, it sounds like you might have
> outstanding users) then wouldn't they be at risk of breaking if you
> mutate the LLVMContext underneath them?
>
> & if you don't have outstanding users, is there any particular benefit
> to resetting the LLVMContext compared to just making a new one?
>
> On Tue, Jun 24, 2014 at 7:18 AM, Marcello Maggioni <hayarms at gmail.com>
> wrote:
> > Hello,
> >
> > I'm trying to develop a way to reliably clean the LLVMContext in order to
> > make it possible to use it multiple times.
> >
> > LLVMContext itself is an almost empty object delegating almost all its
> > content to LLVMContextImpl.
> > This makes it very clean ideally, because clearing the LLVMContext would
> be
> > as easy as deleting the LLVMContextImpl and creating a new one.
> >
> > The problem is that for some reason which I'm not aware of
> LLVMContextImpl
> > is actually exposed as a public pointer in the LLVMContext
> interface,making
> > it publicly available to objects that use it directly (this seems to
> happen
> > quite a lot in the codebase).
> >
> > In LLVMContext the LLVMContextImpl is contained in a pImpl pointer that
> is
> > const (the pointer itself can't be changed) and I guess this is some
> kind of
> > protection against object replacing the LLVMContextImpl directly, which
> > stops us from just deleting it + getting a new one.
> > In addition to that, being pImpl public, there is no guarantee that
> objects
> > don't rely on pImpl remaining always the same pointer.
> >
> > This makes it more difficult to clear LLVMContext.
> >
> > An approach I thought of could be adding a clear() method to LLVMContext
> > that:
> >  - Calls directly the destructor of LLVMContextImpl on the pImpl object
> >  - Uses a placement new to reinitialize the object.
> >  - Recreates the fixed metadata kinds like the LLVMContext constructor
> does
> >
> > I'm attaching a patch that show this approach in this mail.
> >
> > I would like to know a general idea about what people think about this
> and
> > see what people think would be the best approach would be.
> >
> > Thanks,
> > Marcello
> >
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> >
>
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