[Mesa-dev] [PATCH 2/2] nir: add opt_if_loop_terminator()

[Timothy Arceri]

On 02/06/18 04:34, Ian Romanick wrote:

> On 05/31/2018 10:37 PM, Timothy Arceri wrote:
>> This pass detects potential loop terminators and moves intructions
>> from the non breaking branch after the if.
>>
>> This enables both the new opt_if_simplification() pass and loop
>> unrolling to potentially progress further.
>>
>> Unexpectedly this change speed up shader-db run times by ~3%
>>
>> Ivy Bridge shader-db results (all changes in dolphin/ubershaders):
> I was going to look at the changes in the generated code, but the
> smallest of these shaders is > 2300 instructions.
>
> Do you have any idea if any piglit or CTS tests hit this optimization path?

Ok I've sent a loop unrolling piglit test for this:

https://patchwork.freedesktop.org/patch/227292/

Which also found a loop unrolling bug which requires the following mesa 
patch to pass:

https://patchwork.freedesktop.org/patch/227299/

>
> One tiny nit below...
>
>> total instructions in shared programs: 9995662 -> 9995338 (-0.00%)
>> instructions in affected programs: 87845 -> 87521 (-0.37%)
>> helped: 27
>> HURT: 0
>>
>> total cycles in shared programs: 230931495 -> 230925015 (-0.00%)
>> cycles in affected programs: 56391385 -> 56384905 (-0.01%)
>> helped: 27
>> HURT: 0
>> ---
>>   src/compiler/nir/nir_opt_if.c | 68 +++++++++++++++++++++++++++++++++++
>>   1 file changed, 68 insertions(+)
>>
>> diff --git a/src/compiler/nir/nir_opt_if.c b/src/compiler/nir/nir_opt_if.c
>> index b03657a4244..1edeefdd5d1 100644
>> --- a/src/compiler/nir/nir_opt_if.c
>> +++ b/src/compiler/nir/nir_opt_if.c
>> @@ -24,6 +24,7 @@
>>   #include "nir.h"
>>   #include "nir/nir_builder.h"
>>   #include "nir_control_flow.h"
>> +#include "nir_loop_analyze.h"
>>   
>>   /**
>>    * This optimization detects if statements at the tops of loops where the
>> @@ -283,6 +284,72 @@ opt_if_simplification(nir_builder *b, nir_if *nif)
>>      return true;
>>   }
>>   
>> +/**
>> + * This optimization simplifies potential loop terminators which then allows
>> + * other passes such as opt_if_simplification() and loop unrolling to progress
>> + * further:
>> + *
>> + *     if (cond) {
>> + *        ... then block instructions ...
>> + *     } else {
>> + *         ...
>> + *        break;
>> + *     }
>> + *
>> + * into:
>> + *
>> + *     if (cond) {
>> + *     } else {
>> + *         ...
>> + *        break;
>> + *     }
>> + *     ... then block instructions ...
>> + */
>> +static bool
>> +opt_if_loop_terminator(nir_if *nif)
>> +{
>> +   nir_block *break_blk = NULL;
>> +   nir_block *continue_from_blk = NULL;
>> +   bool continue_from_then = true;
>> +
>> +   nir_block *last_then = nir_if_last_then_block(nif);
>> +   nir_block *last_else = nir_if_last_else_block(nif);
>> +
>> +   if (nir_block_ends_in_break(last_then)) {
>> +      break_blk = last_then;
>> +      continue_from_blk = last_else;
>> +      continue_from_then = false;
>> +   } else if (nir_block_ends_in_break(last_else)) {
>> +      break_blk = last_else;
>> +      continue_from_blk = last_then;
>> +   }
>> +
>> +   /* Continue if the if contained no jumps at all */
> In prose, I like to use if-statement because I think it reads more
> easily.  "Continue if the if-statement..."
>
>> +   if (!break_blk)
>> +      return false;
>> +
>> +   /* If the continue from block is empty then return as there is nothing to
>> +    * move.
>> +    */
>> +   nir_block *first_continue_from_blk = continue_from_then ?
>> +      nir_if_first_then_block(nif) :
>> +      nir_if_first_else_block(nif);
>> +   if (is_block_empty(first_continue_from_blk))
>> +      return false;
>> +
>> +   if (!nir_is_trivial_loop_if(nif, break_blk))
>> +      return false;
>> +
>> +   /* Finally, move the continue from branch after the if. */
>> +   nir_cf_list tmp;
>> +   nir_cf_extract(&tmp, nir_before_block(first_continue_from_blk),
>> +                        nir_after_block(continue_from_blk));
>> +   nir_cf_reinsert(&tmp, nir_after_cf_node(&nif->cf_node));
>> +   nir_cf_delete(&tmp);
>> +
>> +   return true;
>> +}
>> +
>>   static bool
>>   opt_if_cf_list(nir_builder *b, struct exec_list *cf_list)
>>   {
>> @@ -296,6 +363,7 @@ opt_if_cf_list(nir_builder *b, struct exec_list *cf_list)
>>            nir_if *nif = nir_cf_node_as_if(cf_node);
>>            progress |= opt_if_cf_list(b, &nif->then_list);
>>            progress |= opt_if_cf_list(b, &nif->else_list);
>> +         progress |= opt_if_loop_terminator(nif);
>>            progress |= opt_if_simplification(b, nif);
>>            break;
>>         }
>>