[Mesa-dev] [PATCH v2 2/3] i965/fs: Change SEL and MOV types as needed to propagate source modifiers

[Jason Ekstrand]

On Mon, Apr 6, 2015 at 4:55 PM, Matt Turner <mattst88 at gmail.com> wrote:
> On Fri, Apr 3, 2015 at 2:06 PM, Jason Ekstrand <jason at jlekstrand.net> wrote:
>> SEL and MOV instructions, as long as they don't have source modifiers, are
>> just copying bits around.  This commit adds support to copy propagation to
>> switch the type of a SEL or MOV instruction as needed so that it can
>> propagate source modifiers.  This is needed because NIR generates integer
>> SEL and MOV instructions whenver it doesn't know what else to generate.
>>
>> shader-db results with NIR:
>> total FS instructions in shared programs: 4360910 -> 4360186 (-0.02%)
>> FS instructions in affected programs:     59094 -> 58370 (-1.23%)
>> helped:                                   341
>> HURT:                                     0
>> GAINED:                                   2
>> LOST:                                     0
>> ---
>>  .../drivers/dri/i965/brw_fs_copy_propagation.cpp   | 33 +++++++++++++++++++---
>>  1 file changed, 29 insertions(+), 4 deletions(-)
>>
>> diff --git a/src/mesa/drivers/dri/i965/brw_fs_copy_propagation.cpp b/src/mesa/drivers/dri/i965/brw_fs_copy_propagation.cpp
>> index e8d092c..d321509 100644
>> --- a/src/mesa/drivers/dri/i965/brw_fs_copy_propagation.cpp
>> +++ b/src/mesa/drivers/dri/i965/brw_fs_copy_propagation.cpp
>> @@ -275,6 +275,16 @@ is_logic_op(enum opcode opcode)
>>             opcode == BRW_OPCODE_NOT);
>>  }
>>
>> +static bool
>> +can_change_source_types(fs_inst *inst)
>> +{
>> +   return !inst->src[0].abs && !inst->src[0].negate &&
>> +          (inst->opcode == BRW_OPCODE_MOV ||
>> +           (inst->opcode == BRW_OPCODE_SEL &&
>> +            inst->conditional_mod == BRW_CONDITIONAL_NONE &&
>
> I understand what this means, but I'd change this line to
> inst->predicate (!= BRW_PREDICATE_NONE).

Seems reasonable.  I'll do that.

>> +            !inst->src[1].abs && !inst->src[1].negate));
>> +}
>> +
>>  bool
>>  fs_visitor::try_copy_propagate(fs_inst *inst, int arg, acp_entry *entry)
>>  {
>> @@ -346,7 +356,9 @@ fs_visitor::try_copy_propagate(fs_inst *inst, int arg, acp_entry *entry)
>>          type_sz(inst->src[arg].type)) % type_sz(entry->src.type) != 0)
>>        return false;
>>
>> -   if (has_source_modifiers && entry->dst.type != inst->src[arg].type)
>> +   if (has_source_modifiers &&
>> +       entry->dst.type != inst->src[arg].type &&
>> +       !can_change_source_types(inst))
>>        return false;
>>
>>     if (brw->gen >= 8 && (entry->src.negate || entry->src.abs) &&
>> @@ -412,9 +424,22 @@ fs_visitor::try_copy_propagate(fs_inst *inst, int arg, acp_entry *entry)
>>        break;
>>     }
>>
>> -   if (!inst->src[arg].abs) {
>> -      inst->src[arg].abs = entry->src.abs;
>> -      inst->src[arg].negate ^= entry->src.negate;
>> +   if (has_source_modifiers) {
>> +      if (entry->dst.type != inst->src[arg].type) {
>> +         /* We are propagating source modifiers from a MOV with a different
>
> Like Anuj mentioned, MOV or SEL.

You can only propagate *from* a MOV

>> +          * type.  If we got here, then we can just change the source and
>> +          * destination types of the instruction and keep going.
>> +          */
>> +         assert(can_change_source_types(inst));
>> +         for (int i = 0; i < inst->sources; i++)
>> +            inst->src[i].type = entry->dst.type;
>
> Probably put braces araound this.

Sure.

>> +         inst->dst.type = entry->dst.type;
>> +      }
>> +
>> +      if (!inst->src[arg].abs) {
>> +         inst->src[arg].abs = entry->src.abs;
>> +         inst->src[arg].negate ^= entry->src.negate;
>> +      }
>>     }
>>
>>     return true;
>> --
>> 2.3.4
>
> Seems totally believable. I'm a little surprised the shader-db results
> aren't bigger. I'd like to play with it and confirm it handles what I
> was seeing with SEL instructions.

Ok.  Let me know when you get done playing with it.
--Jason