[Mesa-dev] [PATCH 2/4] i965/fs: Wire up control flow correctly in predicated break pass.

[Matt Turner]

On Thu, Nov 6, 2014 at 10:07 AM, Jason Ekstrand <jason at jlekstrand.net> wrote:
>
>
> On Wed, Nov 5, 2014 at 4:13 PM, Matt Turner <mattst88 at gmail.com> wrote:
>>
>> When the earlier block ended with control flow, we'd mistakenly remove
>> some of its links to its children. The same happened with the later
>> block.
>> ---
>>  src/mesa/drivers/dri/i965/brw_fs_peephole_predicated_break.cpp | 10
>> +++++++---
>>  1 file changed, 7 insertions(+), 3 deletions(-)
>>
>> diff --git
>> a/src/mesa/drivers/dri/i965/brw_fs_peephole_predicated_break.cpp
>> b/src/mesa/drivers/dri/i965/brw_fs_peephole_predicated_break.cpp
>> index b7a1d7e..047c2c0 100644
>> --- a/src/mesa/drivers/dri/i965/brw_fs_peephole_predicated_break.cpp
>> +++ b/src/mesa/drivers/dri/i965/brw_fs_peephole_predicated_break.cpp
>> @@ -107,10 +107,14 @@ fs_visitor::opt_peephole_predicated_break()
>>        }
>>        endif_inst->remove(endif_block);
>>
>> -      earlier_block->children.make_empty();
>> -      later_block->parents.make_empty();
>> +      if (!earlier_block->ends_with_control_flow()) {
>> +         earlier_block->children.make_empty();
>> +         earlier_block->add_successor(cfg->mem_ctx, jump_block);
>> +      }
>>
>> -      earlier_block->add_successor(cfg->mem_ctx, jump_block);
>> +      if (!later_block->starts_with_control_flow()) {
>> +         later_block->parents.make_empty();
>> +      }
>
>
> I *think* this is correct.  I didn't really understand it before and I don't
> 100% now.

Basically, the earlier code incorrectly assumed that the blocks were
were attempting to merge did not start/end in control flow and
therefore had only a single successor. In that case it was safe to
erase all of the successor/predecessors and recreate them.

When the blocks actually start/end with control flow, they have
multiple successors/predecessors and that assumption is false. In that
case, we're not going to be able to merge blocks and we shouldn't
destroy their successor/predecessor information.