[Mesa-dev] [PATCH 1/1] nir: add a pass that removes continue blocks

[Timothy Arceri]

On 20/3/19 9:41 pm, Samuel Pitoiset wrote:
> 28717 shaders in 14931 tests
> Totals:
> SGPRS: 1267317 -> 1267549 (0.02 %)
> VGPRS: 896876 -> 895920 (-0.11 %)
> Spilled SGPRs: 24701 -> 26367 (6.74 %)
> Code Size: 48379452 -> 48507880 (0.27 %) bytes
> Max Waves: 241159 -> 241190 (0.01 %)
> 
> Totals from affected shaders:
> SGPRS: 23584 -> 23816 (0.98 %)
> VGPRS: 25908 -> 24952 (-3.69 %)
> Spilled SGPRs: 503 -> 2169 (331.21 %)
> Code Size: 2471392 -> 2599820 (5.20 %) bytes
> Max Waves: 586 -> 617 (5.29 %)
> 
> The codesize increases is related to Wolfenstein II, maybe it has
> too many continue blocks and we should add an arbitrary limit.
> 
> This gives +10% FPS with Doom on my Vega56.

Nice find. However as discussed on IRC this is just an unrestricted 
version of opt_if_loop_last_continue(), we should merge them but I would 
also like time to test this again with shader-db. The reason I 
restricted opt_if_loop_last_continue() was because it was causing extra 
register pressure in shaders. Its late so I wont be able to test until 
tomorrow.

> 
> Signed-off-by: Samuel Pitoiset <samuel.pitoiset at gmail.com>
> ---
>   src/compiler/nir/nir_opt_if.c | 87 +++++++++++++++++++++++++++++++++++
>   1 file changed, 87 insertions(+)
> 
> diff --git a/src/compiler/nir/nir_opt_if.c b/src/compiler/nir/nir_opt_if.c
> index bc128f79f3c..47a8a65aad3 100644
> --- a/src/compiler/nir/nir_opt_if.c
> +++ b/src/compiler/nir/nir_opt_if.c
> @@ -890,6 +890,92 @@ opt_if_loop_last_continue(nir_loop *loop)
>      return true;
>   }
>   
> +/**
> + * This optimization allows to remove continue blocks by moving the rest of the
> + * loop to the other side. This is only applied if continue blocks are in the
> + * top-level control flow of the loop. This turns:
> + *
> + *    if (cond) {
> + *       ... code ...
> + *       continue
> + *    } else {
> + *    }
> + *    ... code ...
> + *
> + * into:
> + *
> + *    if (cond) {
> + *       ... code ...
> + *       continue
> + *    } else {
> + *       ... code ...
> + *    }
> + */
> +static bool
> +opt_if_loop_remove_continue(nir_loop *loop)
> +{
> +   nir_block *header_block = nir_loop_first_block(loop);
> +   nir_block *bottom_block = nir_loop_last_block(loop);
> +   nir_block *prev_block =
> +      nir_cf_node_as_block(nir_cf_node_prev(&loop->cf_node));
> +
> +   /* It would be insane if this were not true */
> +   assert(_mesa_set_search(header_block->predecessors, prev_block));
> +
> +   /* The loop must have, at least, two continue blocks (including the normal
> +    * continue at the end of loop).
> +    */
> +   if (header_block->predecessors->entries < 3)
> +      return false;
> +
> +   /* Scan the control flow of the loop from the last to the first node, and
> +    * optimize when an if block contains a continue.
> +    */
> +   nir_cf_node *last_node = &bottom_block->cf_node;
> +   nir_cf_node *first_node = &header_block->cf_node;
> +
> +   while (last_node != first_node) {
> +      last_node = nir_cf_node_prev(last_node);
> +      if (last_node->type != nir_cf_node_if)
> +         continue;
> +
> +      nir_if *nif = nir_cf_node_as_if(last_node);
> +      nir_block *then_block = nir_if_last_then_block(nif);
> +
> +      /* Nothing to do if the block isn't a continue block. */
> +      nir_instr *instr = nir_block_last_instr(then_block);
> +      if (!instr ||
> +          instr->type != nir_instr_type_jump ||
> +          nir_instr_as_jump(instr)->type != nir_jump_continue)
> +         continue;
> +
> +      /* Get the block immediately following the if statement. */
> +      nir_block *after_if_block =
> +         nir_cf_node_as_block(nir_cf_node_next(&nif->cf_node));
> +
> +      /* Nothing to do if the block following the if statement is empty. */
> +      if (is_block_empty(after_if_block))
> +         continue;
> +
> +      nir_cf_node *if_first_node = &after_if_block->cf_node;
> +      nir_cf_node *if_last_node = &bottom_block->cf_node;
> +      nir_block *else_block = nir_if_last_else_block(nif);
> +
> +      /* Move all nodes following the if statement to the last else block, in
> +       * case there is complicated control flow.
> +       */
> +      nir_cf_list tmp;
> +      nir_cf_extract(&tmp, nir_before_cf_node(if_first_node),
> +                           nir_after_cf_node(if_last_node));
> +      nir_cf_reinsert(&tmp, nir_after_block(else_block));
> +      nir_cf_delete(&tmp);
> +
> +      return true;
> +   }
> +
> +   return false;
> +}
> +
>   /* Walk all the phis in the block immediately following the if statement and
>    * swap the blocks.
>    */
> @@ -1355,6 +1441,7 @@ opt_if_cf_list(nir_builder *b, struct exec_list *cf_list)
>            progress |= opt_simplify_bcsel_of_phi(b, loop);
>            progress |= opt_peel_loop_initial_if(loop);
>            progress |= opt_if_loop_last_continue(loop);
> +         progress |= opt_if_loop_remove_continue(loop);
>            break;
>         }
>   
>