[Mesa-dev] [RFC] i965: alternative to memctx for cleaning up nir variants
Rob Clark
robdclark at gmail.com
Sun Dec 20 19:04:00 PST 2015
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.
> 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 seems like in the end, since memctx isn't really buying us that
much, that refcnt'ing is the more useful thing.
BR,
-R
> --Jason
>
>> ---
>> src/mesa/drivers/dri/i965/brw_compiler.h | 22
>> ++++++++++++++++++++++
>> src/mesa/drivers/dri/i965/brw_fs.cpp | 6 ++++--
>> src/mesa/drivers/dri/i965/brw_vec4.cpp | 3 ++-
>> src/mesa/drivers/dri/i965/brw_vec4_gs_visitor.cpp | 3 ++-
>> 4 files changed, 30 insertions(+), 4 deletions(-)
>>
>> diff --git a/src/mesa/drivers/dri/i965/brw_compiler.h
>> b/src/mesa/drivers/dri/i965/brw_compiler.h
>> index c9e0317..eb1087a 100644
>> --- a/src/mesa/drivers/dri/i965/brw_compiler.h
>> +++ b/src/mesa/drivers/dri/i965/brw_compiler.h
>> @@ -26,6 +26,7 @@
>> #include <stdio.h>
>> #include "brw_device_info.h"
>> #include "main/mtypes.h"
>> +#include "nir.h"
>>
>> #ifdef __cplusplus
>> extern "C" {
>> @@ -651,6 +652,27 @@ struct brw_gs_prog_data
>>
>> /** @} */
>>
>> +#ifdef __cplusplus
>> +/* A common pattern is for brw_compile_xyz to create a clone of the
>> original
>> + * shader, modify it (various lowering passes, etc), and then throw it
>> away.
>> + * The scoped_clone helper class is just a way to use RAII to clean up
>> the
>> + * shader when it goes out of scope.
>> + */
>> +class scoped_clone
>> +{
>> +public:
>> + nir_shader *shader;
>> + scoped_clone(const nir_shader *s)
>> + {
>> + shader = nir_shader_clone(NULL, s);
>> + }
>> + ~scoped_clone()
>> + {
>> + ralloc_free(shader);
>> + }
>> +};
>> +#endif
>> +
>> /**
>> * Compile a vertex shader.
>> *
>> diff --git a/src/mesa/drivers/dri/i965/brw_fs.cpp
>> b/src/mesa/drivers/dri/i965/brw_fs.cpp
>> index c833ef0..44ea156 100644
>> --- a/src/mesa/drivers/dri/i965/brw_fs.cpp
>> +++ b/src/mesa/drivers/dri/i965/brw_fs.cpp
>> @@ -5490,7 +5490,8 @@ brw_compile_fs(const struct brw_compiler *compiler,
>> void *log_data,
>> unsigned *final_assembly_size,
>> char **error_str)
>> {
>> - nir_shader *shader = nir_shader_clone(mem_ctx, src_shader);
>> + scoped_clone sc(src_shader);
>> + nir_shader *shader = sc.shader;
>> shader = brw_nir_apply_sampler_key(shader, compiler->devinfo,
>> &key->tex,
>> true);
>> shader = brw_postprocess_nir(shader, compiler->devinfo, true);
>> @@ -5616,7 +5617,8 @@ brw_compile_cs(const struct brw_compiler *compiler,
>> void *log_data,
>> unsigned *final_assembly_size,
>> char **error_str)
>> {
>> - nir_shader *shader = nir_shader_clone(mem_ctx, src_shader);
>> + scoped_clone sc(src_shader);
>> + nir_shader *shader = sc.shader;
>> shader = brw_nir_apply_sampler_key(shader, compiler->devinfo,
>> &key->tex,
>> true);
>> shader = brw_postprocess_nir(shader, compiler->devinfo, true);
>> diff --git a/src/mesa/drivers/dri/i965/brw_vec4.cpp
>> b/src/mesa/drivers/dri/i965/brw_vec4.cpp
>> index a697bdf..a89e884 100644
>> --- a/src/mesa/drivers/dri/i965/brw_vec4.cpp
>> +++ b/src/mesa/drivers/dri/i965/brw_vec4.cpp
>> @@ -1957,7 +1957,8 @@ brw_compile_vs(const struct brw_compiler *compiler,
>> void *log_data,
>> unsigned *final_assembly_size,
>> char **error_str)
>> {
>> - nir_shader *shader = nir_shader_clone(mem_ctx, src_shader);
>> + scoped_clone sc(src_shader);
>> + nir_shader *shader = sc.shader;
>> shader = brw_nir_apply_sampler_key(shader, compiler->devinfo,
>> &key->tex,
>>
>> compiler->scalar_stage[MESA_SHADER_VERTEX]);
>> shader = brw_postprocess_nir(shader, compiler->devinfo,
>> diff --git a/src/mesa/drivers/dri/i965/brw_vec4_gs_visitor.cpp
>> b/src/mesa/drivers/dri/i965/brw_vec4_gs_visitor.cpp
>> index b13d36e..9ed7e0d 100644
>> --- a/src/mesa/drivers/dri/i965/brw_vec4_gs_visitor.cpp
>> +++ b/src/mesa/drivers/dri/i965/brw_vec4_gs_visitor.cpp
>> @@ -617,7 +617,8 @@ brw_compile_gs(const struct brw_compiler *compiler,
>> void *log_data,
>> memset(&c, 0, sizeof(c));
>> c.key = *key;
>>
>> - nir_shader *shader = nir_shader_clone(mem_ctx, src_shader);
>> + scoped_clone sc(src_shader);
>> + nir_shader *shader = sc.shader;
>> shader = brw_nir_apply_sampler_key(shader, compiler->devinfo,
>> &key->tex,
>>
>> compiler->scalar_stage[MESA_SHADER_GEOMETRY]);
>> shader = brw_postprocess_nir(shader, compiler->devinfo,
>> --
>> 2.5.0
>>
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