[Mesa-dev] [PATCH 4/6] nir: Add an optimization pass to remove trivial continues

[Jason Ekstrand]

On Dec 21, 2016 8:11 PM, "Timothy Arceri" <timothy.arceri at collabora.com>
wrote:

On Mon, 2016-12-19 at 20:11 -0800, Jason Ekstrand wrote:
> ---
>  src/compiler/Makefile.sources                |   1 +
>  src/compiler/nir/nir.h                       |   2 +
>  src/compiler/nir/nir_opt_trivial_continues.c | 141
> +++++++++++++++++++++++++++
>  3 files changed, 144 insertions(+)
>  create mode 100644 src/compiler/nir/nir_opt_trivial_continues.c
>
> diff --git a/src/compiler/Makefile.sources
> b/src/compiler/Makefile.sources
> index ae3e5f0..a0abede 100644
> --- a/src/compiler/Makefile.sources
> +++ b/src/compiler/Makefile.sources
> @@ -242,6 +242,7 @@ NIR_FILES = \
>       nir/nir_opt_loop_unroll.c \
>       nir/nir_opt_peephole_select.c \
>       nir/nir_opt_remove_phis.c \
> +     nir/nir_opt_trivial_continues.c \
>       nir/nir_opt_undef.c \
>       nir/nir_phi_builder.c \
>       nir/nir_phi_builder.h \
> diff --git a/src/compiler/nir/nir.h b/src/compiler/nir/nir.h
> index 89b0c70..a6c8956 100644
> --- a/src/compiler/nir/nir.h
> +++ b/src/compiler/nir/nir.h
> @@ -2563,6 +2563,8 @@ bool nir_opt_peephole_select(nir_shader
> *shader, unsigned limit);
>
>  bool nir_opt_remove_phis(nir_shader *shader);
>
> +bool nir_opt_trivial_continues(nir_shader *shader);
> +
>  bool nir_opt_undef(nir_shader *shader);
>
>  bool nir_opt_conditional_discard(nir_shader *shader);
> diff --git a/src/compiler/nir/nir_opt_trivial_continues.c
> b/src/compiler/nir/nir_opt_trivial_continues.c
> new file mode 100644
> index 0000000..ff3205d
> --- /dev/null
> +++ b/src/compiler/nir/nir_opt_trivial_continues.c
> @@ -0,0 +1,141 @@
> +/*
> + * Copyright © 2016 Intel Corporation
> + *
> + * Permission is hereby granted, free of charge, to any person
> obtaining a
> + * copy of this software and associated documentation files (the
> "Software"),
> + * to deal in the Software without restriction, including without
> limitation
> + * the rights to use, copy, modify, merge, publish, distribute,
> sublicense,
> + * and/or sell copies of the Software, and to permit persons to whom
> the
> + * Software is furnished to do so, subject to the following
> conditions:
> + *
> + * The above copyright notice and this permission notice (including
> the next
> + * paragraph) shall be included in all copies or substantial
> portions of the
> + * Software.
> + *
> + * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
> EXPRESS OR
> + * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
> MERCHANTABILITY,
> + * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.  IN NO
> EVENT SHALL
> + * THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES
> OR OTHER
> + * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE,
> ARISING
> + * FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR
> OTHER DEALINGS
> + * IN THE SOFTWARE.
> + */
> +
> +#include "nir.h"
> +
> +static bool
> +instr_is_continue(nir_instr *instr)
> +{
> +   if (instr->type != nir_instr_type_jump)
> +      return false;
> +
> +   return nir_instr_as_jump(instr)->type == nir_jump_continue;
> +}
> +
> +static bool
> +lower_trivial_continues_block(nir_block *block, nir_loop *loop)
> +{
> +   bool progress = false;
> +   nir_instr *first_instr = nir_block_first_instr(block);
> +   if (!first_instr || instr_is_continue(first_instr)) {
> +      /* The block contains only a continue if anything */
> +      nir_cf_node *prev_node = nir_cf_node_prev(&block->cf_node);
> +      if (prev_node && prev_node->type == nir_cf_node_if) {
> +         nir_if *prev_if = nir_cf_node_as_if(prev_node);
> +         progress |= lower_trivial_continues_block(
> +            nir_if_last_then_block(prev_if), loop);
> +         progress |= lower_trivial_continues_block(
> +            nir_if_last_else_block(prev_if), loop);
> +      }
> +
> +      /* The lower_phis_to_regs helper is smart enough to push phi
> sources as
> +       * far up if-ladders as it possibly can.  In other words, the
> recursive
> +       * calls above shouldn't be adding instructions to this block
> since it
> +       * follows an if statement.
> +       */
> +      first_instr = nir_block_first_instr(block);
> +      assert(!first_instr || instr_is_continue(first_instr));
> +   }
> +
> +   nir_instr *last_instr = nir_block_last_instr(block);
> +   if (!last_instr || !instr_is_continue(last_instr))
> +      return progress;
> +
> +   /* We're at the end of a block that goes straight through to the
> end of the
> +    * loop and continues to the top.  We can exit normally instead
> of jump.
> +    */
> +   nir_lower_phis_to_regs_block(nir_loop_first_block(loop));
> +   nir_instr_remove(last_instr);
> +   return true;
> +}
> +
> +static bool
> +lower_trivial_continues_list(struct exec_list *cf_list,
> +                             bool list_ends_at_loop_tail,
> +                             nir_loop *loop)
> +{
> +   bool progress = false;
> +   foreach_list_typed(nir_cf_node, cf_node, node, cf_list) {
> +      bool at_loop_tail = list_ends_at_loop_tail &&
> +                          &cf_node->node ==
> exec_list_get_tail(cf_list);
> +      switch (cf_node->type) {
> +      case nir_cf_node_block:
> +         break;
> +
> +      case nir_cf_node_if: {
> +         nir_if *nif = nir_cf_node_as_if(cf_node);
> +         if (lower_trivial_continues_list(&nif->then_list,
> at_loop_tail, loop))
> +            progress = true;
> +         if (lower_trivial_continues_list(&nif->else_list,
> at_loop_tail, loop))
> +            progress = true;
> +         break;
> +      }
> +
> +      case nir_cf_node_loop: {
> +         nir_loop *loop = nir_cf_node_as_loop(cf_node);
> +         if (lower_trivial_continues_list(&loop->body, true, loop))
> +            progress = true;
> +         if
> (lower_trivial_continues_block(nir_loop_last_block(loop), loop))
> +            progress = true;
> +         break;
> +      }
> +
> +      case nir_cf_node_function:
> +         unreachable("Invalid cf type");
> +      }
> +   }
> +
> +   return progress;
> +}
> +
> +/**
> + * This simple pass gets rid of any "trivial" continues, i.e. those
> that are
> + * at the tail of a loop where we can just delete the continue
> instruction and
> + * control-flow will naturally and harmlessly take us back to the
> top of the
> + * loop.
> + */
> +bool
> +nir_opt_trivial_continues(nir_shader *shader)
> +{
> +   bool progress = false;
> +
> +   nir_foreach_function(function, shader) {
> +      if (function->impl == NULL)
> +         continue;
> +
> +      /* First we run the simple pass to get rid of pesky continues
> */
> +      if (lower_trivial_continues_list(&function->impl->body, false,
> NULL)) {
> +         nir_metadata_preserve(function->impl, nir_metadata_none);
> +
> +         /* If that made progress, we're no longer really in SSA
> form.  Go
> +          * back into SSA form and clean up.
> +          */
> +         nir_convert_to_ssa_impl(function->impl);
> +         nir_copy_prop_impl(function->impl);
> +         nir_opt_dce_impl(function->impl);

Just curious why you explicitly call these rather than just let them
get cleaned up normally? Does it cut compile times to get rid of them
straight away?


The nir_opt_if pass I wrote relies on this pass.  Of this pass is run, it
also needs copy_prop and dce.  This potentially allows is to drop an
iteration of the optimization loop. Another option would be to do this in
brw_nir.c:

if (OPT(nir_opt_trivial_continues)) {
   OPT(nir_opt_copy_prop);
   OPT(nir_opt_dce);
}

I don't have a strong preference either way.

Anyway this looks good to me and very easy to follow:

Reviewed-by: Timothy Arceri <timothy.arceri at collabora.com>

> +         progress = true;
> +      }
> +   }
> +
> +   return progress;
> +}
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