[LLVMdev] Making it possible to clear the LLVMContext

[Eric Christopher]

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
>> >
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>> >
>
>
>
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