[Mesa-dev] [PATCH 0/9] NIR dead control-flow removal

[Jason Ekstrand]

On Fri, May 8, 2015 at 10:03 PM, Connor Abbott <cwabbott0 at gmail.com> wrote:
> This series implements a bunch of related optimizations that run at
> once as part of the same pass to eliminate control flow that is
> useless. This includes both unreachable code and useless loops, i.e.
> loops that don't compute a result used by the rest of the code. To do
> this, I needed to change some low-level things that change the behavior
> of nir_instr_remove() (see patch 2 for details), but I've tested the
> series on piglit and there are no regressions, and there are no
> shader-db changes either.

I tried to get you shader-db results, but it crashes part-way through.
In particular, it looks like unigen heaven on high will crash it.
Either that or a couple of synmark shaders but I doubt those are the
culprets.

I think it's also worth noting that piglit probably isn't a good test
of whether or not this works.   I doubt there are many piglit shaders
with dead control-flow and I think GLSL IR can get rid of it in some
of the common cases.  If you wanted to "generate NIR" to test with,
you could always hack up my SPIRV -> NIR branch to run an optimization
loop after converting.  Then you can see what happens without GLSL IR
in the way.
--Jason

> This is rebased on Jason's series to use lists for use-def sets; it
> turns out that the changes required were pretty minimal.
>
> The series is also available at:
>
> git://people.freedesktop.org/~cwabbott0/mesa nir-dead-cf-v3
>
> Connor Abbott (9):
>   nir/vars_to_ssa: don't rewrite removed instructions
>   nir: insert ssa_undef instructions when cleanup up defs/uses
>   nir: add an optimization for removing dead control flow
>   nir/validate: validate successors at the end of a loop
>   nir/dead_cf: delete code that's unreachable due to jumps
>   nir: add nir_block_get_following_loop() helper
>   nir: add a helper for iterating over blocks in a cf node
>   nir/dead_cf: add support for removing useless loops
>   i965/fs/nir: enable the dead control flow optimization
>
>  src/glsl/Makefile.sources            |   1 +
>  src/glsl/nir/nir.c                   |  94 ++++++++--
>  src/glsl/nir/nir.h                   |  12 ++
>  src/glsl/nir/nir_lower_vars_to_ssa.c |   3 +-
>  src/glsl/nir/nir_opt_dead_cf.c       | 332 +++++++++++++++++++++++++++++++++++
>  src/glsl/nir/nir_validate.c          |  21 +++
>  src/mesa/drivers/dri/i965/brw_nir.c  |   2 +
>  7 files changed, 452 insertions(+), 13 deletions(-)
>  create mode 100644 src/glsl/nir/nir_opt_dead_cf.c
>
> --
> 2.1.0
>
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