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

[Connor Abbott]

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.

>
> (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