[LLVMdev] Making it possible to clear the LLVMContext

[Eric Christopher]

Right. Would make more sense to just start a new context past a
certain limit (assuming that you're done with the existing cached
information from the modules).

-eric

On Tue, Jun 24, 2014 at 11:07 AM, Marcello Maggioni <hayarms at gmail.com> wrote:
> Something like, keeping the compiler alive for a many compilation sessions,
> using the same LLVMContext, but without the problem of accumulating data in
> the LLVMContext that would slowly fill up the memory.
>
> This as much I can enter into details that I can :/
>
> Probably this is also a quite common use case scenario too.
>
> Marcello
>
>
> 2014-06-24 18:52 GMT+01:00 Eric Christopher <echristo at gmail.com>:
>
>> Right, this is likely going to have at least some of the same problems
>> that Bill ran into when he tried to update a TargetMachine by
>> placement newing a new one into it.
>>
>> Can you elaborate a bit more about what you're trying to do? (I
>> realize this may be difficult for reasons.)
>>
>> -eric
>>
>> On Tue, Jun 24, 2014 at 10:44 AM, Marcello Maggioni <hayarms at gmail.com>
>> wrote:
>> > 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
>> >> >
>> >> > _______________________________________________
>> >> > LLVM Developers mailing list
>> >> > LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
>> >> > http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>> >> >
>> >
>> >
>> >
>> > _______________________________________________
>> > LLVM Developers mailing list
>> > LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
>> > http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>> >
>
>