[Mesa-dev] [PATCH 1/2] nir: update nir_lower_returns to only predicate instructions when needed

[Timothy Arceri]

On Fri, 2016-12-09 at 16:49 +1100, Timothy Arceri wrote:
> Unless an if statement contains nested returns we can simply add
> any following instructions to the branch without the return.
> ---
>  src/compiler/nir/nir_lower_returns.c | 36
> ++++++++++++++++++++++++++++++------
>  1 file changed, 30 insertions(+), 6 deletions(-)
> 
> diff --git a/src/compiler/nir/nir_lower_returns.c
> b/src/compiler/nir/nir_lower_returns.c
> index 8dbea6e..73782a1 100644
> --- a/src/compiler/nir/nir_lower_returns.c
> +++ b/src/compiler/nir/nir_lower_returns.c
> @@ -30,6 +30,8 @@ struct lower_returns_state {
>     struct exec_list *cf_list;
>     nir_loop *loop;
>     nir_variable *return_flag;
> +   /* Are there other return statments nested in the current if */
> +   bool if_nested_return;
>  };
>  
>  static bool lower_returns_in_cf_list(struct exec_list *cf_list,
> @@ -82,8 +84,10 @@ lower_returns_in_loop(nir_loop *loop, struct
> lower_returns_state *state)
>      * flag set to true.  We need to predicate everything following
> the loop
>      * on the return flag.
>      */
> -   if (progress)
> +   if (progress) {
>        predicate_following(&loop->cf_node, state);
> +      state->if_nested_return = true;
> +   }
>  
>     return progress;
>  }
> @@ -91,10 +95,12 @@ lower_returns_in_loop(nir_loop *loop, struct
> lower_returns_state *state)
>  static bool
>  lower_returns_in_if(nir_if *if_stmt, struct lower_returns_state
> *state)
>  {
> -   bool progress;
> +   bool progress, then_progress;
>  
> -   progress = lower_returns_in_cf_list(&if_stmt->then_list, state);
> -   progress = lower_returns_in_cf_list(&if_stmt->else_list, state)
> || progress;
> +   state->if_nested_return = false;
> +
> +   then_progress = lower_returns_in_cf_list(&if_stmt->then_list,
> state);
> +   progress = lower_returns_in_cf_list(&if_stmt->else_list, state)
> || then_progress;
>  
>     /* If either of the recursive calls made progress, then there
> were
>      * returns inside of the body of the if.  If we're in a loop,
> then these
> @@ -106,8 +112,26 @@ lower_returns_in_if(nir_if *if_stmt, struct
> lower_returns_state *state)
>      * after a return, we need to predicate everything following on
> the
>      * return flag.
>      */
> -   if (progress && !state->loop)
> -      predicate_following(&if_stmt->cf_node, state);
> +   if (progress && !state->loop) {
> +      if (state->if_nested_return) {
> +         predicate_following(&if_stmt->cf_node, state);
> +      } else {
> +         /* If there are no nested returns we can just add the
> instructions to
> +          * the end of the branch that doesn't have the return.
> +          */
> +         nir_cf_list list;
> +         nir_cf_extract(&list, nir_after_cf_node(&if_stmt->cf_node),
> +                        nir_after_cf_list(state->cf_list));
> +         assert(!exec_list_is_empty(&list.list));

I've removed this assert() locally there is no need to assert if the
list is empty nir_cf_reinsert() will simply return is thats the case.


> +         if (then_progress)
> +            nir_cf_reinsert(&list, nir_after_cf_list(&if_stmt-
> >else_list));
> +         else
> +            nir_cf_reinsert(&list, nir_after_cf_list(&if_stmt-
> >then_list));
> +      }
> +   }
> +
> +   if (progress)
> +      state->if_nested_return = true;
>  
>     return progress;
>  }