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

[Jason Ekstrand]

I had a chat with Caio about this and I'm skeptical.  In general, users of
the CF manipulation code shouldn't be stitching two blocks together where
the first contains a jump and the second is non-empty.  If the caller knows
that this case is ok, then they can check for it and empty out the one
block before stitching.  Also, I'm not really seeing how peel_initial_if
would hit this case from your example.

--Jason

On Fri, Jan 25, 2019 at 11:37 AM 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
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://lists.freedesktop.org/archives/mesa-dev/attachments/20190208/210a083b/attachment.html>