[Mesa-dev] [PATCH 3/4] nir: support to flatten_all in peephole-select

[Rob Clark]

On Wed, Apr 1, 2015 at 6:56 PM, Jason Ekstrand <jason at jlekstrand.net> wrote:
> On Tue, Mar 31, 2015 at 3:57 PM, Rob Clark <robdclark at gmail.com> wrote:
>> From: Rob Clark <robclark at freedesktop.org>
>>
>> Freedreno and vc4 want this behavior for the time being (until we have
>> real flow control).  Even after that, we probably want to turn this into
>> some sort of driver tunable threshold, since for at least some hardware,
>> reasonably large if/else is best flattend rather than having divergent
>> flow control.
>>
>> NOTE: wasn't sure about some of the additional restrictions in
>> block_check_for_allowed_instrs()..  are there some other cases where
>> I might need to fix things up in order to be guaranteed to be able to
>> flatten?
>>
>> NOTE: drop vc4 hunk if this is merged first, ofc
>>
>> Signed-off-by: Rob Clark <robclark at freedesktop.org>
>> ---
>>  src/gallium/drivers/vc4/vc4_program.c    |  2 +-
>>  src/glsl/nir/nir.h                       |  2 +-
>>  src/glsl/nir/nir_opt_peephole_select.c   | 21 +++++++++++++++------
>>  src/mesa/drivers/dri/i965/brw_fs_nir.cpp |  2 +-
>>  4 files changed, 18 insertions(+), 9 deletions(-)
>>
>> diff --git a/src/gallium/drivers/vc4/vc4_program.c b/src/gallium/drivers/vc4/vc4_program.c
>> index db51665..09896ce 100644
>> --- a/src/gallium/drivers/vc4/vc4_program.c
>> +++ b/src/gallium/drivers/vc4/vc4_program.c
>> @@ -1673,7 +1673,7 @@ vc4_optimize_nir(struct nir_shader *s)
>>                  progress = nir_copy_prop(s) || progress;
>>                  progress = nir_opt_dce(s) || progress;
>>                  progress = nir_opt_cse(s) || progress;
>> -                progress = nir_opt_peephole_select(s) || progress;
>> +                progress = nir_opt_peephole_select(s, true) || progress;
>>                  progress = nir_opt_algebraic(s) || progress;
>>                  progress = nir_opt_constant_folding(s) || progress;
>>          } while (progress);
>> diff --git a/src/glsl/nir/nir.h b/src/glsl/nir/nir.h
>> index 74927e5..cd03d6b 100644
>> --- a/src/glsl/nir/nir.h
>> +++ b/src/glsl/nir/nir.h
>> @@ -1631,7 +1631,7 @@ bool nir_opt_dce(nir_shader *shader);
>>
>>  void nir_opt_gcm(nir_shader *shader);
>>
>> -bool nir_opt_peephole_select(nir_shader *shader);
>> +bool nir_opt_peephole_select(nir_shader *shader, bool flatten_all);
>>  bool nir_opt_peephole_ffma(nir_shader *shader);
>>
>>  bool nir_opt_remove_phis(nir_shader *shader);
>> diff --git a/src/glsl/nir/nir_opt_peephole_select.c b/src/glsl/nir/nir_opt_peephole_select.c
>> index b89451b..e48fc7a 100644
>> --- a/src/glsl/nir/nir_opt_peephole_select.c
>> +++ b/src/glsl/nir/nir_opt_peephole_select.c
>> @@ -44,10 +44,16 @@
>>   * whose only use is one of the following phi nodes.  This happens all the
>>   * time when the SSA form comes from a conditional assignment with a
>>   * swizzle.
>> + *
>> + * If 'flatten_all' is true, then flatten *all* if/else into one block
>> + * and use select to pick the winners.  This is useful for drivers that
>> + * do not (yet) have proper flow control.  Eventually we probably want
>> + * to make this a more clever driver tunable threshold.
>>   */
>>
>>  struct peephole_select_state {
>>     void *mem_ctx;
>> +   bool flatten_all;
>>     bool progress;
>>  };
>>
>> @@ -150,9 +156,10 @@ nir_opt_peephole_select_block(nir_block *block, void *void_state)
>>     nir_block *else_block = nir_cf_node_as_block(else_node);
>>
>>     /* ... and those blocks must only contain "allowed" instructions. */
>> -   if (!block_check_for_allowed_instrs(then_block) ||
>> -       !block_check_for_allowed_instrs(else_block))
>> -      return true;
>> +   if (!state->flatten_all)
>> +      if (!block_check_for_allowed_instrs(then_block) ||
>> +          !block_check_for_allowed_instrs(else_block))
>> +         return true;
>
> This isn't valid.  We can't blindly flatten if either list of blocks
> contains an instruction that has side-effects.  In order to do that,
> we would have to add predication back into NIR.
>

oh, right.. non-predicated kill would have to get converted into a
predicated kill.

Beyond that, do I just need to check for
NIR_INTRINSIC_CAN_ELIMINATE/REORDER?  Or is there a better way to
identify instructions with side effects?


>>     /* At this point, we know that the previous CFG node is an if-then
>>      * statement containing only moves to phi nodes in this block.  We can
>> @@ -217,11 +224,12 @@ nir_opt_peephole_select_block(nir_block *block, void *void_state)
>>  }
>>
>>  static bool
>> -nir_opt_peephole_select_impl(nir_function_impl *impl)
>> +nir_opt_peephole_select_impl(nir_function_impl *impl, bool flatten_all)
>>  {
>>     struct peephole_select_state state;
>>
>>     state.mem_ctx = ralloc_parent(impl);
>> +   state.flatten_all = flatten_all;
>>     state.progress = false;
>>
>>     nir_foreach_block(impl, nir_opt_peephole_select_block, &state);
>> @@ -233,13 +241,14 @@ nir_opt_peephole_select_impl(nir_function_impl *impl)
>>  }
>>
>>  bool
>> -nir_opt_peephole_select(nir_shader *shader)
>> +nir_opt_peephole_select(nir_shader *shader, bool flatten_all)
>
> As matt said, boolean arguments aren't great.  I also don't like
> arbitrary bitfields.  We do, however, have a shader params struct.
> You could add a "flatten" flag to it I guess.  However, doing so kind
> of implies that we actually flatten everything which (as stated above)
> we can't here.

sure, I'll add it to the params struct.. I guess for the most part
things that cannot be flattened are not something to care about for
backends that can't (yet) do proper flow control (other than the
kill->kill_if), so best-effort flatten_everything is fine..

BR,
-R

> --Jason
>
>>  {
>>     bool progress = false;
>>
>>     nir_foreach_overload(shader, overload) {
>>        if (overload->impl)
>> -         progress |= nir_opt_peephole_select_impl(overload->impl);
>> +         progress |= nir_opt_peephole_select_impl(overload->impl,
>> +                                                  flatten_all);
>>     }
>>
>>     return progress;
>> diff --git a/src/mesa/drivers/dri/i965/brw_fs_nir.cpp b/src/mesa/drivers/dri/i965/brw_fs_nir.cpp
>> index 0b8ed1a..7bd7cd3 100644
>> --- a/src/mesa/drivers/dri/i965/brw_fs_nir.cpp
>> +++ b/src/mesa/drivers/dri/i965/brw_fs_nir.cpp
>> @@ -47,7 +47,7 @@ nir_optimize(nir_shader *nir)
>>        nir_validate_shader(nir);
>>        progress |= nir_opt_cse(nir);
>>        nir_validate_shader(nir);
>> -      progress |= nir_opt_peephole_select(nir);
>> +      progress |= nir_opt_peephole_select(nir, false);
>>        nir_validate_shader(nir);
>>        progress |= nir_opt_algebraic(nir);
>>        nir_validate_shader(nir);
>> --
>> 2.1.0
>>