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

Marek Olšák maraeo at gmail.com
Mon Dec 21 03:39:04 PST 2015

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.


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