[Mesa-dev] [PATCH 2/2] nir: add opt_if_loop_terminator()

[Ian Romanick]

On 05/31/2018 10:37 PM, Timothy Arceri wrote:
> This pass detects potential loop terminators and moves intructions
> from the non breaking branch after the if.
> 
> This enables both the new opt_if_simplification() pass and loop
> unrolling to potentially progress further.
> 
> Unexpectedly this change speed up shader-db run times by ~3%
> 
> Ivy Bridge shader-db results (all changes in dolphin/ubershaders):

I was going to look at the changes in the generated code, but the
smallest of these shaders is > 2300 instructions.

Do you have any idea if any piglit or CTS tests hit this optimization path?

One tiny nit below...

> total instructions in shared programs: 9995662 -> 9995338 (-0.00%)
> instructions in affected programs: 87845 -> 87521 (-0.37%)
> helped: 27
> HURT: 0
> 
> total cycles in shared programs: 230931495 -> 230925015 (-0.00%)
> cycles in affected programs: 56391385 -> 56384905 (-0.01%)
> helped: 27
> HURT: 0
> ---
>  src/compiler/nir/nir_opt_if.c | 68 +++++++++++++++++++++++++++++++++++
>  1 file changed, 68 insertions(+)
> 
> diff --git a/src/compiler/nir/nir_opt_if.c b/src/compiler/nir/nir_opt_if.c
> index b03657a4244..1edeefdd5d1 100644
> --- a/src/compiler/nir/nir_opt_if.c
> +++ b/src/compiler/nir/nir_opt_if.c
> @@ -24,6 +24,7 @@
>  #include "nir.h"
>  #include "nir/nir_builder.h"
>  #include "nir_control_flow.h"
> +#include "nir_loop_analyze.h"
>  
>  /**
>   * This optimization detects if statements at the tops of loops where the
> @@ -283,6 +284,72 @@ opt_if_simplification(nir_builder *b, nir_if *nif)
>     return true;
>  }
>  
> +/**
> + * This optimization simplifies potential loop terminators which then allows
> + * other passes such as opt_if_simplification() and loop unrolling to progress
> + * further:
> + *
> + *     if (cond) {
> + *        ... then block instructions ...
> + *     } else {
> + *         ...
> + *        break;
> + *     }
> + *
> + * into:
> + *
> + *     if (cond) {
> + *     } else {
> + *         ...
> + *        break;
> + *     }
> + *     ... then block instructions ...
> + */
> +static bool
> +opt_if_loop_terminator(nir_if *nif)
> +{
> +   nir_block *break_blk = NULL;
> +   nir_block *continue_from_blk = NULL;
> +   bool continue_from_then = true;
> +
> +   nir_block *last_then = nir_if_last_then_block(nif);
> +   nir_block *last_else = nir_if_last_else_block(nif);
> +
> +   if (nir_block_ends_in_break(last_then)) {
> +      break_blk = last_then;
> +      continue_from_blk = last_else;
> +      continue_from_then = false;
> +   } else if (nir_block_ends_in_break(last_else)) {
> +      break_blk = last_else;
> +      continue_from_blk = last_then;
> +   }
> +
> +   /* Continue if the if contained no jumps at all */

In prose, I like to use if-statement because I think it reads more
easily.  "Continue if the if-statement..."

> +   if (!break_blk)
> +      return false;
> +
> +   /* If the continue from block is empty then return as there is nothing to
> +    * move.
> +    */
> +   nir_block *first_continue_from_blk = continue_from_then ?
> +      nir_if_first_then_block(nif) :
> +      nir_if_first_else_block(nif);
> +   if (is_block_empty(first_continue_from_blk))
> +      return false;
> +
> +   if (!nir_is_trivial_loop_if(nif, break_blk))
> +      return false;
> +
> +   /* Finally, move the continue from branch after the if. */
> +   nir_cf_list tmp;
> +   nir_cf_extract(&tmp, nir_before_block(first_continue_from_blk),
> +                        nir_after_block(continue_from_blk));
> +   nir_cf_reinsert(&tmp, nir_after_cf_node(&nif->cf_node));
> +   nir_cf_delete(&tmp);
> +
> +   return true;
> +}
> +
>  static bool
>  opt_if_cf_list(nir_builder *b, struct exec_list *cf_list)
>  {
> @@ -296,6 +363,7 @@ opt_if_cf_list(nir_builder *b, struct exec_list *cf_list)
>           nir_if *nif = nir_cf_node_as_if(cf_node);
>           progress |= opt_if_cf_list(b, &nif->then_list);
>           progress |= opt_if_cf_list(b, &nif->else_list);
> +         progress |= opt_if_loop_terminator(nif);
>           progress |= opt_if_simplification(b, nif);
>           break;
>        }
>