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

[Marek Olšák]

On Mon, Dec 21, 2015 at 11:20 PM, Jason Ekstrand <jason at jlekstrand.net> wrote:
>
> On Dec 21, 2015 1:37 PM, "Marek Olšák" <maraeo at gmail.com> wrote:
>>
>> On Mon, Dec 21, 2015 at 7:48 PM, Jason Ekstrand <jason at jlekstrand.net>
>> wrote:
>> >
>> > On Dec 21, 2015 9:09 AM, "Connor Abbott" <cwabbott0 at gmail.com> wrote:
>> >>
>> >> On Mon, Dec 21, 2015 at 11:45 AM, Marek Olšák <maraeo at gmail.com> wrote:
>> >> > On Mon, Dec 21, 2015 at 4:38 PM, Connor Abbott <cwabbott0 at gmail.com>
>> >> > wrote:
>> >> >> On Mon, Dec 21, 2015 at 6:39 AM, Marek Olšák <maraeo at gmail.com>
>> >> >> wrote:
>> >> >>> On Mon, Dec 21, 2015 at 6:48 AM, Jason Ekstrand
>> >> >>> <jason at jlekstrand.net>
>> >> >>> wrote:
>> >> >>>>
>> >> >>>> On Dec 20, 2015 7:43 PM, "Rob Clark" <robdclark at gmail.com> wrote:
>> >> >>>>>
>> >> >>>>> On Sun, Dec 20, 2015 at 10:29 PM, Connor Abbott
>> >> >>>>> <cwabbott0 at gmail.com>
>> >> >>>>> wrote:
>> >> >>>>> > On Sun, Dec 20, 2015 at 10:04 PM, Rob Clark
>> >> >>>>> > <robdclark at gmail.com>
>> >> >>>>> > wrote:
>> >> >>>>> >> On Sun, Dec 20, 2015 at 9:12 PM, Jason Ekstrand
>> >> >>>>> >> <jason at jlekstrand.net>
>> >> >>>>> >> wrote:
>> >> >>>>> >>>
>> >> >>>>> >>> On Dec 19, 2015 5:55 PM, "Rob Clark" <robdclark at gmail.com>
>> >> >>>>> >>> wrote:
>> >> >>>>> >>>>
>> >> >>>>> >>>> From: Rob Clark <robclark at freedesktop.org>
>> >> >>>>> >>>>
>> >> >>>>> >>>> Jason,
>> >> >>>>> >>>>
>> >> >>>>> >>>> How much do you hate this idea?  Seems like an easy
>> >> >>>>> >>>> alternative
>> >> >>>>> >>>> to
>> >> >>>>> >>>> using ralloc ctx's to clean up nir variants/clones, which
>> >> >>>>> >>>> would
>> >> >>>>> >>>> let
>> >> >>>>> >>>> us drop the parent memctx for nir_shader_create()/clone(),
>> >> >>>>> >>>> making
>> >> >>>>> >>>> it easier to introduce reference counting.
>> >> >>>>> >>>
>> >> >>>>> >>> I think "hate" is a but strong. I don't like it but it works.
>> >> >>>>> >>> If we
>> >> >>>>> >>> really
>> >> >>>>> >>> want nir_shader refcounted, we'll have to do something.
>> >> >>>>> >>
>> >> >>>>> >> I suppose the alternate idea of moving the nir_shader_clone()
>> >> >>>>> >> out
>> >> >>>>> >> of
>> >> >>>>> >> brw_compile_xyz(), and always passing in the clone would be a
>> >> >>>>> >> cleaner
>> >> >>>>> >> way.  It looks like each of the brw_compile_xyz() has exactly
>> >> >>>>> >> one
>> >> >>>>> >> call-site, so doing the nir_shader_clone() inside doesn't
>> >> >>>>> >> really
>> >> >>>>> >> buy
>> >> >>>>> >> anything.
>> >> >>>>
>> >> >>>> Your forgetting that there may be *cough* other users of this
>> >> >>>> API...
>> >> >>>> We can
>> >> >>>> change those too but I would like the needs of the compiler users
>> >> >>>> to
>> >> >>>> drive
>> >> >>>> the API, not the cloning.  I still have some details to work out
>> >> >>>> there. In
>> >> >>>> any case, it doesn't really matter; we can figure something out.
>> >> >>>>
>> >> >>>>> >>> About refcounting... The more I think about it the more I'm
>> >> >>>>> >>> not
>> >> >>>>> >>> convinced
>> >> >>>>> >>> it's useful.  As it stands, we have no use for it an I'm not
>> >> >>>>> >>> convinced
>> >> >>>>> >>> you
>> >> >>>>> >>> do either.  We'll see if I can convince you. :-)
>> >> >>>>> >>>
>> >> >>>>> >>> In the history of i965 using NIR, we've had about three
>> >> >>>>> >>> different ways
>> >> >>>>> >>> of
>> >> >>>>> >>> doing things:
>> >> >>>>> >>>
>> >> >>>>> >>> 1) GLSL is the gold copy and we run glsl_to_nir for every
>> >> >>>>> >>> shader/variant
>> >> >>>>> >>> compile.  This is what we did when we first stated using NIR
>> >> >>>>> >>> because
>> >> >>>>> >>> it was
>> >> >>>>> >>> easy and didn't involve reworking any plumbing.
>> >> >>>>> >>>
>> >> >>>>> >>> 2) Lowered NIR is the gold copy; variants are done entirely
>> >> >>>>> >>> in
>> >> >>>>> >>> the
>> >> >>>>> >>> back-end
>> >> >>>>> >>> IR.  This is what we did up until about a month ago.  Because
>> >> >>>>> >>> variants
>> >> >>>>> >>> are
>> >> >>>>> >>> done in the back-end, we can run gksl_to_nir and do all of
>> >> >>>>> >>> our
>> >> >>>>> >>> optimizing
>> >> >>>>> >>> and lowering at link time.  Going from NIR to the final
>> >> >>>>> >>> shader
>> >> >>>>> >>> binary
>> >> >>>>> >>> is
>> >> >>>>> >>> then a read-only operation as far as NIR is concerned.
>> >> >>>>> >>>
>> >> >>>>> >>> 3) Optimized but not lowered NIR is the gold copy; variants
>> >> >>>>> >>> are
>> >> >>>>> >>> sometimes
>> >> >>>>> >>> done in NIR.  This is the scheme we use now.  We call
>> >> >>>>> >>> glsl_to_nir and
>> >> >>>>> >>> do
>> >> >>>>> >>> some of the optimization and lowering at link time but leave
>> >> >>>>> >>> it
>> >> >>>>> >>> in SSA
>> >> >>>>> >>> form.
>> >> >>>>> >>> When we go to compile the final shader, we make a copy, apply
>> >> >>>>> >>> variants, do
>> >> >>>>> >>> the final lowering and then go into the back-end IR.
>> >> >>>>> >>>
>> >> >>>>> >>> In each of these cases, we know exactly where we need to make
>> >> >>>>> >>> a
>> >> >>>>> >>> copy
>> >> >>>>> >>> without
>> >> >>>>> >>> the help of reference counting.  In the first, we get a fresh
>> >> >>>>> >>> copy
>> >> >>>>> >>> each time
>> >> >>>>> >>> so we are free to destroy the copy.  In the second, we never
>> >> >>>>> >>> have to
>> >> >>>>> >>> modify
>> >> >>>>> >>> the NIR so no copy.  In the third scheme, we always have to
>> >> >>>>> >>> make
>> >> >>>>> >>> a
>> >> >>>>> >>> copy
>> >> >>>>> >>> because, even if variants are a no-op, we still have to go
>> >> >>>>> >>> out
>> >> >>>>> >>> of SSA
>> >> >>>>> >>> form
>> >> >>>>> >>> and do final lowering.  You could say that we could avoid
>> >> >>>>> >>> making
>> >> >>>>> >>> that
>> >> >>>>> >>> copy.
>> >> >>>>> >>> However, the work to determine when we don't need variants
>> >> >>>>> >>> and
>> >> >>>>> >>> can do
>> >> >>>>> >>> all
>> >> >>>>> >>> our lowering up-front is far more than the work saved by
>> >> >>>>> >>> reference
>> >> >>>>> >>> counting.
>> >> >>>>> >>>
>> >> >>>>> >>> How about gallium?  Here's how I imagine it would work
>> >> >>>>> >>> (please
>> >> >>>>> >>> correct
>> >> >>>>> >>> me of
>> >> >>>>> >>> I'm wrong):
>> >> >>>>> >>>
>> >> >>>>> >>> 1) In the TGSI case, tgsi_to_nir gets called for each compile
>> >> >>>>> >>> so
>> >> >>>>> >>> you
>> >> >>>>> >>> get a
>> >> >>>>> >>> fresh mutable shader each time.  In this case, you are free
>> >> >>>>> >>> to
>> >> >>>>> >>> do
>> >> >>>>> >>> whatever
>> >> >>>>> >>> you want with the shader without making a copy.
>> >> >>>>> >>>
>> >> >>>>> >>> 2) In the GLSL case, you run glsl_to_nir and do some basic
>> >> >>>>> >>> optimizations at
>> >> >>>>> >>> link time and hold onto the NIR shader. (Hold a reference of
>> >> >>>>> >>> you'd
>> >> >>>>> >>> like.)
>> >> >>>>> >>> When you go to compile it in the back-end, it needs to do
>> >> >>>>> >>> it's
>> >> >>>>> >>> own
>> >> >>>>> >>> lowering
>> >> >>>>> >>> so it takes a reference and ends up making a copy.
>> >> >>>>> >>>
>> >> >>>>> >>> If this description is anywhere close to correct, then I
>> >> >>>>> >>> don't
>> >> >>>>> >>> think
>> >> >>>>> >>> you
>> >> >>>>> >>> really need it either.  Determining whether or not you need
>> >> >>>>> >>> to
>> >> >>>>> >>> copy is
>> >> >>>>> >>> simply "if (comes_from_tgsi)”.  Maybe there's something
>> >> >>>>> >>> subtle
>> >> >>>>> >>> about
>> >> >>>>> >>> the
>> >> >>>>> >>> gallium layer that I don't know that makes refcounting the
>> >> >>>>> >>> best
>> >> >>>>> >>> solution.
>> >> >>>>> >>> Please enlighten me of there is.
>> >> >>>>> >>
>> >> >>>>> >> This issue is that we *potentially* have both the state
>> >> >>>>> >> tracker
>> >> >>>>> >> and
>> >> >>>>> >> the driver both doing some of there own variant management.
>> >> >>>>> >> (Which
>> >> >>>>> >> tbh, isn't awesome, it would have been nice if someone
>> >> >>>>> >> realized
>> >> >>>>> >> earlier on that nearly every driver is going to have to do
>> >> >>>>> >> some
>> >> >>>>> >> sort
>> >> >>>>> >> of variant mgmt and figured out a way just to push it all down
>> >> >>>>> >> to
>> >> >>>>> >> the
>> >> >>>>> >> driver.. but I can't see a good way to get there from here.)
>> >> >>>>> >>
>> >> >>>>> >> With TGSI as the IR, driver just unconditionally does
>> >> >>>>> >> tgsi_dup_tokens().. because of the whole thing where st does
>> >> >>>>> >> variants
>> >> >>>>> >> in some cases, things are defined that driver doesn't own the
>> >> >>>>> >> copy of
>> >> >>>>> >> the TGSI IR passed in after the fxn call to driver returns.
>> >> >>>>> >>
>> >> >>>>> >> With NIR I was hoping to fix this, esp. since
>> >> >>>>> >> nir_shader_clone()
>> >> >>>>> >> is
>> >> >>>>> >> more heavyweight than tgsi_dup_tokens() (memcpy()).
>> >> >>>>> >>
>> >> >>>>> >> Refcnt'ing is a nice solution so that we can pass the driver a
>> >> >>>>> >> reference that it owns.  In cases where state tracker isn't
>> >> >>>>> >> doing
>> >> >>>>> >> variant mgmt, we pass it the one-and-only ref (avoiding
>> >> >>>>> >> clone).
>> >> >>>>> >>
>> >> >>>>> >> I'd suggested that in cases where st does variant mgmt, that
>> >> >>>>> >> st
>> >> >>>>> >> should
>> >> >>>>> >> do the clone/dup.  But that was shot down:
>> >> >>>>> >>
>> >> >>>>> >>
>> >> >>>>> >>
>> >> >>>>> >> http://lists.freedesktop.org/archives/mesa-dev/2015-October/097748.html
>> >> >>>>
>> >> >>>> It sounds like Marek's argument there is more about lifetime
>> >> >>>> management than
>> >> >>>> anything.  He wants gallium modules to be able to create IR, call
>> >> >>>> into the
>> >> >>>> driver, and then throw it away.  In particular, he doesn't want
>> >> >>>> them
>> >> >>>> to have
>> >> >>>> to think about cloning. In a lot of ways it sounds a lot like what
>> >> >>>> i965
>> >> >>>> wants too.  I really like having brw_compile_foo take a const
>> >> >>>> nir_shader.
>> >> >>>> The difference is that i965 basically always wants to clone
>> >> >>>> whereas a
>> >> >>>> gallium driver may not have to if gallium doesn't care what
>> >> >>>> happens
>> >> >>>> to the
>> >> >>>> shader when it's done.  How common is this case? How important is
>> >> >>>> it
>> >> >>>> to
>> >> >>>> optimize for?  I don't know.
>> >> >>>>
>> >> >>>> One other thing that bothers me a bit:  From Marek's comment, it
>> >> >>>> sounds like
>> >> >>>> the components want to just pass in IR and be agnostic about
>> >> >>>> whether
>> >> >>>> the
>> >> >>>> driver wants its own copy or wants to change it or whatever.  This
>> >> >>>> seems
>> >> >>>> like an argument for always cloning to me.  From the perspective
>> >> >>>> of a
>> >> >>>> gallium module, "I want to hang in to this, I'll keep a reference"
>> >> >>>> seems
>> >> >>>> exactly the same as "I want to hang onto this, I'll give the
>> >> >>>> driver a
>> >> >>>> copy".
>> >> >>>> How are they actually different given that the driver basically
>> >> >>>> has
>> >> >>>> to
>> >> >>>> modify what you give it in order to do lowering?
>> >> >>>>
>> >> >>>>> > Ugh... I didn't read this at the time, but I don't like Marek's
>> >> >>>>> > response. My understanding of the situation, based on this
>> >> >>>>> > thread,
>> >> >>>>> > is
>> >> >>>>> > that there are some cases where the st knows that there's only
>> >> >>>>> > going
>> >> >>>>> > to be one variant and can throw away the (NIR or TGSI) shader
>> >> >>>>> > after it
>> >> >>>>> > hands it to the driver, while at other times it has to hold
>> >> >>>>> > onto
>> >> >>>>> > all
>> >> >>>>> > the variants and only give the driver a read-only copy (or
>> >> >>>>> > duplicate
>> >> >>>>
>> >> >>>> As per above, my interpretation of Marek's comment is that he
>> >> >>>> doesn't
>> >> >>>> want
>> >> >>>> the st to have to think about cloning ever.  He wants it to assume
>> >> >>>> that
>> >> >>>> compilation never modifies the IR so the driver should always
>> >> >>>> clone.
>> >> >>>> You
>> >> >>>> have to keep in mind that Marek is most likely thinking about
>> >> >>>> caching
>> >> >>>> the
>> >> >>>> TGSI rather than doing in-place lowering in it.
>> >> >>>>
>> >> >>>> If I'm understanding Marek correctly, then it sounds like shader
>> >> >>>> compilation
>> >> >>>> should never touch the IR that's passed in.  If this is the case,
>> >> >>>> it
>> >> >>>> sounds
>> >> >>>> like always cloning is the way to go.  At least its not *that*
>> >> >>>> expensive.
>> >> >>>
>> >> >>> Note that st/mesa needs to keep the FS IR because of glDrawPixels
>> >> >>> and
>> >> >>> glBitmap, and the VS IR because of edge flags, glRasterPos
>> >> >>> evaluation,
>> >> >>> selection and feedback modes. The last three are done with
>> >> >>> Draw/LLVM
>> >> >>> and only support TGSI.
>> >> >>>
>> >> >>> Therefore, st/mesa always hangs onto the IR and drivers can't
>> >> >>> modify
>> >> >>> it. It also needs VS in TGSI to be able to do everything correctly.
>> >> >>>
>> >> >>> What other Gallium modules want or not want is not that important,
>> >> >>> but
>> >> >>> changing the current semantics will require fixing a lot of places.
>> >> >>> (state trackers - mesa, nine, xa; modules - blitter, draw, hud,
>> >> >>> postprocess, tests, vl)
>> >> >>>
>> >> >>> You really better think about whether changing all those and the
>> >> >>> risk
>> >> >>> of breaking them is worth it.
>> >> >>>
>> >> >>> Marek
>> >> >>
>> >> >> Well, we're talking about passing NIR here, not TGSI, so none of
>> >> >> those
>> >> >> places will need to be updated. NIR is a much more heavyweight IR,
>> >> >> and
>> >> >> copying is much more expensive, so it makes more sense there for the
>> >> >> st to duplicate it and let the driver own the IR it passes in, to
>> >> >> reduce copying when there's no variant management necessary.
>> >> >
>> >> > My main concern was about TGSI, not so much about NIR.
>> >>
>> >> Yes, exactly -- we don't plan on changing the semantics of passing in
>> >> TGSI, so this only matters when the user passes in a NIR shader.
>> >
>> > 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.
>> >
>> > 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.
>> >
>> > 2) gallium passes read-only IR to the back-end and it always makes a
>> > copy.
>>
>> Not always. The copy is optional. Drivers are encouraged not to make a
>> copy if they don't need it. Or they can keep a copy in a different IR,
>> which is the same scenario for the original IR.
>
> Let's suppose that the driver always has to modify the IR before it can be
> used. Does it have to make a copy then?  I know that may not be the case for
> TGSI but it is currently the case for NIR.

If the driver wants to modify TGSI, it does have to make a copy
(tgsi_transform_shader automatically makes a copy). Regarding NIR,
define and use whatever makes sense for it.

Marek