[Mesa-dev] [PATCH v2 2/3] nir: Add a discard optimization pass

[Jason Ekstrand]

On Wed, Jul 4, 2018 at 10:00 AM, Matt Turner <mattst88 at gmail.com> wrote:

> On Wed, Jul 4, 2018 at 9:59 AM Jason Ekstrand <jason at jlekstrand.net>
> wrote:
> >
> > Many fragment shaders do a discard using relatively little information
> > but still put the discard fairly far down in the shader for no good
> > reason.  If the discard is moved higher up, we can possibly avoid doing
> > some or almost all of the work in the shader.  When this lets us skip
> > texturing operations, it's an especially high win.
> >
> > One of the biggest offenders here is DXVK.  The D3D APIs have different
> > rules for discards than OpenGL and Vulkan.  One effective way (which is
> > what DXVK uses) to implement DX behavior on top of GL or Vulkan is to
> > wait until the very end of the shader to discard.  This ends up in the
> > pessimal case where we always do all of the work before discarding.
> > This pass helps some DXVK shaders significantly.
> >
> > v2 (Jason Ekstrand):
> >  - Fix a couple of typos (Grazvydas, Ian)
> >  - Use the new nir_instr_move helper
> >  - Find all movable discards before moving anything so we don't
> >    accidentally re-order anything and break dependencies
> > ---
> >  src/compiler/Makefile.sources      |   1 +
> >  src/compiler/nir/meson.build       |   1 +
> >  src/compiler/nir/nir.h             |  10 +
> >  src/compiler/nir/nir_opt_discard.c | 396 +++++++++++++++++++++++++++++
> >  4 files changed, 408 insertions(+)
> >  create mode 100644 src/compiler/nir/nir_opt_discard.c
> >
> > diff --git a/src/compiler/Makefile.sources b/src/compiler/Makefile.
> sources
> > index 9e3fbdc2612..8600ce81281 100644
> > --- a/src/compiler/Makefile.sources
> > +++ b/src/compiler/Makefile.sources
> > @@ -271,6 +271,7 @@ NIR_FILES = \
> >         nir/nir_opt_cse.c \
> >         nir/nir_opt_dce.c \
> >         nir/nir_opt_dead_cf.c \
> > +       nir/nir_opt_discard.c \
> >         nir/nir_opt_gcm.c \
> >         nir/nir_opt_global_to_local.c \
> >         nir/nir_opt_if.c \
> > diff --git a/src/compiler/nir/meson.build b/src/compiler/nir/meson.build
> > index 28aa8de7014..e339258bb94 100644
> > --- a/src/compiler/nir/meson.build
> > +++ b/src/compiler/nir/meson.build
> > @@ -156,6 +156,7 @@ files_libnir = files(
> >    'nir_opt_cse.c',
> >    'nir_opt_dce.c',
> >    'nir_opt_dead_cf.c',
> > +  'nir_opt_discard.c',
> >    'nir_opt_gcm.c',
> >    'nir_opt_global_to_local.c',
> >    'nir_opt_if.c',
> > diff --git a/src/compiler/nir/nir.h b/src/compiler/nir/nir.h
> > index c40a88c8ccc..dac019c17e8 100644
> > --- a/src/compiler/nir/nir.h
> > +++ b/src/compiler/nir/nir.h
> > @@ -2022,6 +2022,13 @@ typedef struct nir_shader_compiler_options {
> >      */
> >     bool vs_inputs_dual_locations;
> >
> > +   /**
> > +    * Whether or not derivatives are still a safe operation after a
> discard
> > +    * has occurred.  Optimization passes may be able to be a bit more
> > +    * agressive if this is true.
>
> s/agressive/aggressive/
>

fixed locally.
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