[Mesa-dev] [PATCH] nir: Do not use progress for unreachable code in return lowering.

[Bas Nieuwenhuizen]

On Mon, Apr 23, 2018 at 2:20 AM, Timothy Arceri <tarceri at itsqueeze.com> wrote:
>
>
> On 23/04/18 03:26, Bas Nieuwenhuizen wrote:
>>
>> We seems to use progress for two cases:
>
>
> seems -> seem
>
>> 1) When we lowered some returns.
>> 2) When we remove unreachable code.
>>
>> If just case 2 happens we assert as state->return_flag has not
>> been allocated yet, but we are still trying to do insert all
>> predicates based on it.
>>
>> This splits the concerns. We only use progress internally for case 1
>> and then keep track of 2 in a separate variable to indicate progress
>> in the return value of the pass.
>>
>> This is slightly better than transforming the assert into
>> if (!state->return_flag) return, as it avoids inserting predicates
>
>
> "as it avoids" -> "as that would avoid" ?

No, the solution I am proposing in this patch avoids, not the
alternative solution of this paragraph. I think your proposed change
would point to the latter. I'll clarify it to clearly point to the
former though.

>
>> even if some other part of the code might need them.
>>
>> Fixes: 6e22ad6edc "nir: return early when lowering a return at the end of
>> a function"
>> CC: 18.1 <mesa-stable at lists.freedesktop.org>
>
>
> Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=106174
>
> Reviewed-by: Timothy Arceri <tarceri at itsqueeze.com>
>
> Thanks for fixing!
>
>
>> ---
>>   src/compiler/nir/nir_lower_returns.c | 7 ++++++-
>>   1 file changed, 6 insertions(+), 1 deletion(-)
>>
>> diff --git a/src/compiler/nir/nir_lower_returns.c
>> b/src/compiler/nir/nir_lower_returns.c
>> index 3ea69e2520..9c4881112e 100644
>> --- a/src/compiler/nir/nir_lower_returns.c
>> +++ b/src/compiler/nir/nir_lower_returns.c
>> @@ -37,6 +37,8 @@ struct lower_returns_state {
>>       * needs to be predicated on the return flag variable.
>>       */
>>      bool has_predicated_return;
>> +
>> +   bool removed_unreachable_code;
>>   };
>>     static bool lower_returns_in_cf_list(struct exec_list *cf_list,
>> @@ -162,8 +164,9 @@ lower_returns_in_block(nir_block *block, struct
>> lower_returns_state *state)
>>             */
>>            return false;
>>         } else {
>> +         state->removed_unreachable_code = true;
>>            nir_cf_delete(&list);
>> -         return true;
>> +         return false;
>>         }
>>      }
>>   @@ -262,9 +265,11 @@ nir_lower_returns_impl(nir_function_impl *impl)
>>      state.loop = NULL;
>>      state.return_flag = NULL;
>>      state.has_predicated_return = false;
>> +   state.removed_unreachable_code = false;
>>      nir_builder_init(&state.builder, impl);
>>        bool progress = lower_returns_in_cf_list(&impl->body, &state);
>> +   progress = progress || state.removed_unreachable_code;
>>        if (progress) {
>>         nir_metadata_preserve(impl, nir_metadata_none);
>>
>