[LLVMdev] Making it possible to clear the LLVMContext

David Blaikie dblaikie at gmail.com
Tue Jun 24 10:51:12 PDT 2014


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.

What I'm trying to understand is why is that a need - is that not
equivalent to just building a new LLVMContext for each compilation? Is
there a performance concern (with data to back it up?) about doing
that?

What is it that goes wrong if you just start the next compilation
without clearing the context in any way? (what's the persistent state
that interferes with the next run?)

- David

>
> 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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>> > LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
>> > http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>> >
>
>



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