[Mesa-dev] [RFC] i965: alternative to memctx for cleaning up nir variants

[Rob Clark]

On Tue, Dec 22, 2015 at 9:47 PM, Connor Abbott <cwabbott0 at gmail.com> wrote:
> On Tue, Dec 22, 2015 at 9:02 PM, Rob Clark <robdclark at gmail.com> wrote:
>> On Mon, Dec 21, 2015 at 1:48 PM, Jason Ekstrand <jason at jlekstrand.net> wrote:
>>>
>>> I think two different concepts of ownership are getting conflated here:
>>> Right/responsibility to delete and right to modify.
>>>
>>> The way I understand it, gallium, as it stands, gives neither to the driver.
>>> A back-end using NIR requires the right to modify but who deletes it doesn't
>>> ultimately matter.  I think it's dangerous to pass one of these rights to
>>> the driver and not the other but we need to think about both.
>>
>> yeah, uneasy about driver modifying the IR if the state tracker is
>> still going to potentially spin off variants of the IR.. that sounds
>> like madness.
>>
>> The refcnt'ing I proposed does deal w/ right to modify vs delete via
>> nir_shader(_is)_mutable() which returns something that is guaranteed
>> to be safe to modify (ie. has only a single reference)
>>
>>> What I'm trying to say is that we have two options here:
>>>
>>> 1) gallium passes IR to the back-end that it is free to modify and is
>>> required to delete when it's done.
>>
>> with refcnt'ing, s/delete/unref/
>>
>> The idea is, the st transfers ownership of the reference it passes to
>> the driver.  If the st wants to hang on to a reference itself, it must
>> increment the refcnt before passing to the driver (backend).
>>
>> Without refcnt'ing, I suppose we could (since we don't have to follow
>> TGSI semantics), just decree that the driver always takes ownership of
>> the copy passed in, and if the st wants to hang on to a copy too, then
>> it must clone.  I suppose this would work well enough for
>> freedreno/vc4, which both end up generating variants later.  It does
>> force an extra clone for drivers that immediately translate into their
>> own backend IR and don't need to keep the NIR around, for example.
>> Maybe that is not worth caring about (since at this point it is
>> hypothetical).
>
> While always cloning does have this disadvantage, I don't think it's
> really relevant here. Even if the driver throws away the NIR
> immediately after consuming it, almost invariably it's going to want
> to modify  it. The generic NIR passed in by the state tracker (other
> IR -> NIR + some optimizations) is almost never going to be the same
> as the NIR after going through driver-specific lowering passes, which
> means that drivers are never going to want a read-only version of the
> IR. In light of that, I think making the driver own the IR passed in
> seems like the most sensible thing.

well, unless the driver is already doing it's own lowering in it's own
native IR..

Maybe it is too much of a hypothetical.. I still think refcnt'ing
gives some nice flexibility to deal with various scenarios, and having
to call nir_shader_unref() isn't so much of a burden.

BR,
-R

>>
>> (I guess nouveau is the one driver, that if it ever consumed NIR,
>> would translate immediately into it's own backend IR?)
>>
>>> 2) gallium passes read-only IR to the back-end and it always makes a copy.
>>
>> This is how it is w/ TGSI, but I think with NIR we are free to make a
>> clean break.  And we *definitely* want to avoid
>> the-driver-always-copies semantics..
>>
>>> It sounds like, from what Marek is saying, that gallium is currently doing
>>> (2) and changing it to (1) would be painful.  I think reference counting is
>>> more like an awkward option 1.5 than option 3.  Reference counting would
>>> mean that gallium passes a reference to the driver which it is expected to
>>> unref but may keep a second reference if it wants to keep the driver from
>>> modifying it.  Then the driver may or may not make a copy based on the
>>> number of references.  Why don't we just make it explicit and add a
>>> read-only bit and call it a day.
>>>
>>> One of the reasons I don't like passing a reference is that it effectively
>>> puts allocation and freeing in different components of the driver.
>>
>> With refcnt'ing you should talk in terms of ref/unref rather than
>> allocate/free.. imho.  Although maybe that is what you meant.  (In
>> which case, yes, that was my idea, that passing in to driver transfers
>> ownership of the passed reference.)
>>
>>> This
>>> means that if and driver doesn't care at all about the shader that gets
>>> passed in, it still has to under it to avoid a memory leak.  You can't have
>>> the driver take the reference because then, either it comes in with a
>>> recount of 0 and should have been deleted, or the "can I modify this" check
>>> becomes "recount <= 2" which makes no sense.
>>
>> hmm, no, if ownership of the reference is transferred to the driver,
>> then it becomes "refcount == 1" (and refcount == 0 should be an
>> assert, because something has gone horribly wrong)
>>
>> BR,
>> -R