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

Mon Dec 28 10:20:11 PST 2015

On Mon, Dec 28, 2015 at 9:37 AM, Connor Abbott <cwabbott0 at gmail.com> wrote:

> On Mon, Dec 28, 2015 at 10:13 AM, Rob Clark <robdclark at gmail.com> wrote:
> > On Tue, Dec 22, 2015 at 10:11 PM, Connor Abbott <cwabbott0 at gmail.com>
> wrote:
> >> On Tue, Dec 22, 2015 at 9:55 PM, Rob Clark <robdclark at gmail.com> wrote:
> >>> 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..
> >>
> >> Well, if you're not doing any lowering in NIR, then you aren't really
> >> taking any advantage of it. I can't see a plausible scenario where all
> >> the lowering is done in the driver's own IR -- and as soon as you do
> >> anything in NIR, you need the driver-owns-IR semantics.
> >
> > When it comes to shader variants, I have a mix, with some things
> > lowered in nir and others just handled in backend..
> >
> > The re-work / cleanup that I have had on a branch for a while now
> > (since it is currently blocked on refcnt'ing) does a first round of
> > variant-key independent NIR lowering/opt passes.  And then at draw
> > time, if the variant key has anything that is lowered in nir, I do a
> > second round.
>

Just to be clear, your key-dependent lowering happens after all of your
other lowering?  If this is the case, then I guarantee you that you're
unique in this since i965 and vc4 need to at least run out-of-SSA
afterwards.  To be honest, I completely forgot that a driver could use
fully ssa NIR.

> >>>
> >>> 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.
> >>
> >> Still, I can't see how this flexibility is at all useful, and it seems
> >> like overkill since the driver will always want a mutable version of
> >> the IR anyways.
> >
> > Well, due to the structure I mentioned above, at draw time when I need
> > to generate a variant with nothing lowered in NIR, I simply incr the
> > refcnt on the IR which has already gone through first round of NIR
> > passes, and pass that in to my back end compiler.  At the end, once
> > the shader binary is generated, I can unconditionally unref the
> > nir_shader without having to care.
> >
> > Without refcnt'ing I'd either have to generate a pointless clone or
> > keep track that the nir_shader should not actually be free'd.  Not
> > impossible, just a bit more ugly.
>
> Assuming you do all your variant management in your driver's IR, then
> you don't need to do anything. If you do some variant management in
> NIR, then in the function where you do the NIR-based lowering you can
> check if you need to do any lowering based on the shader key, clone
> first, and give the NIR->ir3 function the cloned IR and then free it.
> It might be a "bit more ugly," but it's really not that much different
> from the refcounting, and when the extra shader gets created/freed is
> made explicit.
>
> >
> > (The gallium glsl_to_nir stuff is also currently using refcnt'ing,
> > although at least for freedreno/ir3 it isn't strictly needed.. I could
> > just unconditionally clone in the state tracker.  That said, I'm still
> > of the opinion that refcnt'ing could be useful to some other driver
> > someday)
>
> "It could be useful to some driver someday" isn't a good argument for
> adding stuff today. We've already had enough examples of things in NIR
> that we added because we thought it was useful, but turned out not to
> be.
>
> >
> > BR,
> > -R
> >
> >>>
> >>> 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
>
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