[Mesa-dev] [PATCH] nir: allow stitching of non-empty block

[Jason Ekstrand]

This makes me a bit nervous. I'll have to look at it in more detail.

On January 25, 2019 09:37:52 "Juan A. Suarez Romero" <jasuarez at igalia.com> 
wrote:

> When stitching two blocks A and B, where A's last instruction is a jump,
> it is not required that B is empty; it can be plainly removed.
>
> This can happen in a situation like this:
>
> vec1 1 ssa_1 = load_const (true)
> vec1 1 ssa_2 = load_const (false)
> block block_1:
> [...]
> loop {
>  vec1 ssa_3 = phi block_1: ssa_2, block_4: ssa_1
>  if ssa_3 {
>    block block_2:
>    [...]
>    break
>  } else {
>    block block_3:
>  }
>  vec1 ssa_4 = <alu operation>
>  if ssa_4 {
>    block block_4:
>    continue
>  } else {
>    block block_5:
>  }
>  block block_6:
>  [...]
> }
>
> And opt_peel_loop_initial_if is applied. In this case, we would be
> ending up stitching block_2 (which finalizes with a jump) with
> block_4, which is not empty.
>
> CC: Jason Ekstrand <jason at jlekstrand.net>
> ---
> src/compiler/nir/nir_control_flow.c | 1 -
> 1 file changed, 1 deletion(-)
>
> diff --git a/src/compiler/nir/nir_control_flow.c 
> b/src/compiler/nir/nir_control_flow.c
> index ddba2e55b45..27508f230d6 100644
> --- a/src/compiler/nir/nir_control_flow.c
> +++ b/src/compiler/nir/nir_control_flow.c
> @@ -550,7 +550,6 @@ stitch_blocks(nir_block *before, nir_block *after)
>     */
>
>    if (nir_block_ends_in_jump(before)) {
> -      assert(exec_list_is_empty(&after->instr_list));
>       if (after->successors[0])
>          remove_phi_src(after->successors[0], after);
>       if (after->successors[1])
> --
> 2.20.1