[Mesa-dev] [PATCH 07/10] nir: add loop unroll support for wrapper loops

[Jason Ekstrand]

On Tue, Jul 10, 2018 at 11:49 PM Timothy Arceri <tarceri at itsqueeze.com>
wrote:

> This adds support for unrolling the classic
>
>     do {
>         // ...
>     } while (false)
>
> that is used to wrap multi-line macros. GLSL IR also wraps switch
> statements in a loop like this.
>
> shader-db results IVB:
>
> total loops in shared programs: 2515 -> 2512 (-0.12%)
> loops in affected programs: 33 -> 30 (-9.09%)
> helped: 3
> HURT: 0
> ---
>  src/compiler/nir/nir_opt_loop_unroll.c | 63 ++++++++++++++++++++++++++
>  1 file changed, 63 insertions(+)
>
> diff --git a/src/compiler/nir/nir_opt_loop_unroll.c
> b/src/compiler/nir/nir_opt_loop_unroll.c
> index 38901470f96..f715fc8ebda 100644
> --- a/src/compiler/nir/nir_opt_loop_unroll.c
> +++ b/src/compiler/nir/nir_opt_loop_unroll.c
> @@ -516,6 +516,69 @@ process_loops(nir_shader *sh, nir_cf_node *cf_node,
> bool *has_nested_loop)
>      */
>     if (!progress) {
>
> +      /* Check for the the classic
> +       *
> +       *    do {
> +       *        // ...
> +       *    } while (false)
> +       *
> +       * that is used to wrap multi-line macros. GLSL IR also wraps switch
> +       * statements in a loop like this.
> +       */
> +      if (loop->info->limiting_terminator == NULL &&
> +          list_empty(&loop->info->loop_terminator_list) &&
> +          !loop->info->complex_loop) {
> +
>

Mind putting all the code below in a "wrapper_unroll" or "trivial_unroll"
helper?


> +         nir_block *blk_after_loop =
> +            nir_cursor_current_block(nir_after_cf_node(&loop->cf_node));
> +
> +         /* There may still be some single src phis following the loop
> that
> +          * have not yet been cleaned up by another pass. Tidy those up
> before
> +          * unrolling the loop.
> +          */
> +         nir_foreach_instr_safe(instr, blk_after_loop) {
> +            if (instr->type != nir_instr_type_phi)
> +               break;
> +
> +            nir_phi_instr *phi = nir_instr_as_phi(instr);
> +            assert(exec_list_length(&phi->srcs) == 1);
> +
> +            nir_phi_src *phi_src =
> +               exec_node_data(nir_phi_src,
> +                              exec_list_get_head(&phi->srcs),
> +                              node);
> +
> +            nir_ssa_def_rewrite_uses(&phi->dest.ssa, phi_src->src);
> +            nir_instr_remove(instr);
> +
> +            progress = true;
> +         }
> +
> +         nir_block *last_loop_blk = nir_loop_last_block(loop);
> +         if (nir_block_ends_in_break(last_loop_blk)) {
>

This confuses me.  Above, you assert that all phis have exactly one source
and here you have an if (nir_block_ends_in_break(last_block)); which one is
it?  Either our loop terminator check above guarantees that we're in the
"do { } while (true)" case where we can assume a single exit at the end of
the loop or it doesn't.  Which one is it?


> +
> +            /* Remove break at end of the loop */
> +            nir_instr *break_instr = nir_block_last_instr(last_loop_blk);
> +            nir_instr_remove(break_instr);
> +
> +            /* Pluck out the loop body. */
> +            nir_cf_list loop_body;
> +            nir_cf_extract(&loop_body,
> +                           nir_before_block(nir_loop_first_block(loop)),
> +                           nir_after_block(nir_loop_last_block(loop)));
> +
> +            /* Reinsert loop body after the loop */
> +            nir_cf_reinsert(&loop_body,
> nir_after_cf_node(&loop->cf_node));
> +
> +            /* The loop has been unrolled so remove it. */
> +            nir_cf_node_remove(&loop->cf_node);
> +
> +            progress = true;
> +         }
> +
> +         goto exit;
> +      }
> +
>        if (*has_nested_loop || loop->info->limiting_terminator == NULL)
>           goto exit;
>
> --
> 2.17.1
>
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