[Mesa-dev] [PATCH] nir: remove simple dead if detection from nir_opt_dead_cf()

[Connor Abbott]

Reviewed-by: Connor Abbott <cwabbott0 at gmail.com>

I agree that if we ever need to bring this back, we should just check for
both branches empty and no phis afterwards.

On Thu, Feb 14, 2019 at 2:38 AM Timothy Arceri <tarceri at itsqueeze.com>
wrote:

> This was probably useful when it was first written, however it
> looks to be no longer necessary.
>
> As far as I can tell these days dce is smart enough to remove useless
> instructions from if branches. Once this is done
> nir_opt_peephole_select() will end up removing the empty if.
>
> Removing this support reduces the dolphin uber shader compilation
> time by around 60%. Compile time is reduced due to no longer having
> to compute the live ssa defs metadata so much.
>
> No shader-db changes on i965 or radeonsi.
> ---
>  src/compiler/nir/nir_opt_dead_cf.c | 9 ++-------
>  1 file changed, 2 insertions(+), 7 deletions(-)
>
> diff --git a/src/compiler/nir/nir_opt_dead_cf.c
> b/src/compiler/nir/nir_opt_dead_cf.c
> index 14986732096..053c5743527 100644
> --- a/src/compiler/nir/nir_opt_dead_cf.c
> +++ b/src/compiler/nir/nir_opt_dead_cf.c
> @@ -180,7 +180,7 @@ def_not_live_out(nir_ssa_def *def, void *state)
>  }
>
>  /*
> - * Test if a loop node or if node is dead. Such nodes are dead if:
> + * Test if a loop node is dead. Such nodes are dead if:
>   *
>   * 1) It has no side effects (i.e. intrinsics which could possibly affect
> the
>   * state of the program aside from producing an SSA value, indicated by a
> lack
> @@ -198,7 +198,7 @@ def_not_live_out(nir_ssa_def *def, void *state)
>  static bool
>  node_is_dead(nir_cf_node *node)
>  {
> -   assert(node->type == nir_cf_node_loop || node->type == nir_cf_node_if);
> +   assert(node->type == nir_cf_node_loop);
>
>     nir_block *before = nir_cf_node_as_block(nir_cf_node_prev(node));
>     nir_block *after = nir_cf_node_as_block(nir_cf_node_next(node));
> @@ -230,11 +230,6 @@ dead_cf_block(nir_block *block)
>  {
>     nir_if *following_if = nir_block_get_following_if(block);
>     if (following_if) {
> -      if (node_is_dead(&following_if->cf_node)) {
> -         nir_cf_node_remove(&following_if->cf_node);
> -         return true;
> -      }
> -
>        if (!nir_src_is_const(following_if->condition))
>           return false;
>
> --
> 2.20.1
>
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