[Mesa-dev] [PATCH 1/2] nir/dead_cf: Don't crash on unreachable after-loop blocks

[Connor Abbott]

As-is, this change will make us delete trivially infinite loops (i.e.
loops with no break statement). The most likely scenario where we get
one of those (besides for silly Piglit tests) is a developer with an
accidental bug in their shader. In that case, it seems kinda mean to
then delete the entire loop and pretend like it doesn't exist. Maybe
add something like

if (!after->imm_dom)
   return false;

with a comment explaining what's going on? This would also help us
catch other places where we don't handle infinite loops correctly that
might've been hidden by this.

On Thu, Sep 1, 2016 at 3:11 PM, Jason Ekstrand <jason at jlekstrand.net> wrote:
> Signed-off-by: Jason Ekstrand <jason at jlekstrand.net>
> ---
>  src/compiler/nir/nir_opt_dead_cf.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/src/compiler/nir/nir_opt_dead_cf.c b/src/compiler/nir/nir_opt_dead_cf.c
> index 3551124..1490e68 100644
> --- a/src/compiler/nir/nir_opt_dead_cf.c
> +++ b/src/compiler/nir/nir_opt_dead_cf.c
> @@ -205,7 +205,8 @@ loop_is_dead(nir_loop *loop)
>     nir_metadata_require(impl, nir_metadata_live_ssa_defs |
>                                nir_metadata_dominance);
>
> -   for (nir_block *cur = after->imm_dom; cur != before; cur = cur->imm_dom) {
> +   for (nir_block *cur = after->imm_dom; cur && cur != before;
> +        cur = cur->imm_dom) {
>        nir_foreach_instr(instr, cur) {
>           if (!nir_foreach_ssa_def(instr, def_not_live_out, after))
>              return false;
> --
> 2.5.0.400.gff86faf
>
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