[Mesa-dev] [PATCH 2/3] i965: Lower min/max after optimization on Gen4/5.

[Matt Turner]

On Sat, Feb 13, 2016 at 4:47 PM, Matt Turner <mattst88 at gmail.com> wrote:
> On Sat, Feb 13, 2016 at 3:30 PM, Francisco Jerez <currojerez at riseup.net> wrote:
>> Matt Turner <mattst88 at gmail.com> writes:
>>
>>> On Sat, Feb 13, 2016 at 2:02 PM, Francisco Jerez <currojerez at riseup.net> wrote:
>>>> Matt Turner <mattst88 at gmail.com> writes:
>>>>
>>>>> On Thu, Feb 11, 2016 at 4:41 PM, Matt Turner <mattst88 at gmail.com> wrote:
>>>>>> Gen4/5's SEL instruction cannot use conditional modifiers, so min/max
>>>>>> are implemented as CMP + SEL. Handling that after optimization lets us
>>>>>> CSE more.
>>>>>>
>>>>>> On Ironlake:
>>>>>>
>>>>>>    total instructions in shared programs: 6426035 -> 6422753 (-0.05%)
>>>>>>    instructions in affected programs: 326604 -> 323322 (-1.00%)
>>>>>>    helped: 1411
>>>>>>
>>>>>>    total cycles in shared programs: 129184700 -> 129101586 (-0.06%)
>>>>>>    cycles in affected programs: 18950290 -> 18867176 (-0.44%)
>>>>>>    helped: 2419
>>>>>>    HURT: 328
>>>>>> ---
>>>>>>  src/mesa/drivers/dri/i965/brw_fs.cpp           | 37 +++++++++++++++++++++++++
>>>>>>  src/mesa/drivers/dri/i965/brw_fs.h             |  1 +
>>>>>>  src/mesa/drivers/dri/i965/brw_fs_builder.h     | 10 ++-----
>>>>>>  src/mesa/drivers/dri/i965/brw_fs_nir.cpp       | 20 +++-----------
>>>>>>  src/mesa/drivers/dri/i965/brw_vec4.cpp         | 38 ++++++++++++++++++++++++++
>>>>>>  src/mesa/drivers/dri/i965/brw_vec4.h           |  2 ++
>>>>>>  src/mesa/drivers/dri/i965/brw_vec4_builder.h   | 10 ++-----
>>>>>>  src/mesa/drivers/dri/i965/brw_vec4_visitor.cpp | 14 ++--------
>>>>>>  8 files changed, 88 insertions(+), 44 deletions(-)
>>>>>>
>>>>>> diff --git a/src/mesa/drivers/dri/i965/brw_fs.cpp b/src/mesa/drivers/dri/i965/brw_fs.cpp
>>>>>> index 0ce7ed1..e83f0ba 100644
>>>>>> --- a/src/mesa/drivers/dri/i965/brw_fs.cpp
>>>>>> +++ b/src/mesa/drivers/dri/i965/brw_fs.cpp
>>>>>> @@ -3475,6 +3475,36 @@ fs_visitor::lower_integer_multiplication()
>>>>>>     return progress;
>>>>>>  }
>>>>>>
>>>>>> +bool
>>>>>> +fs_visitor::lower_minmax()
>>>>>> +{
>>>>>> +   assert(devinfo->gen < 6);
>>>>>> +
>>>>>> +   bool progress = false;
>>>>>> +
>>>>>> +   foreach_block_and_inst_safe(block, fs_inst, inst, cfg) {
>>>>>> +      const fs_builder ibld(this, block, inst);
>>>>>> +
>>>>>> +      if (inst->opcode == BRW_OPCODE_SEL &&
>>>>>> +          inst->predicate == BRW_PREDICATE_NONE) {
>>>>>> +         assert(inst->conditional_mod == BRW_CONDITIONAL_GE ||
>>>>>> +                inst->conditional_mod == BRW_CONDITIONAL_L);
>>>>>
>>>>> Ken asked at the office if this assertion is necessary. I think it is.
>>>>> The PRM doesn't say anything about SEL with conditional modifiers
>>>>> other than .ge or .l.
>>>>
>>>> I'm pretty sure it's not, the SEL instruction works fine with other
>>>> conditional mods, and I've found it moderately useful in the past.  And
>>>> at least the internal hardware docs mention explicitly that conditional
>>>> mods other than .l and .ge follow the cmp rules (rather than the cmpn
>>>> rules), which implies they're allowed...
>>>
>>> Okay, right. The PRM says "and all other conditional modifiers follow
>>> the cmp rules."
>>>
>>> Which ones are be useful? .z/.nz/.o/.u don't make sense.
>>>
>> These are all well-defined.  ISTR having used SEL with .o at some point.
>>
>>> I see that the SEL documentation says
>>>
>>> """
>>> For a sel instruction with a .l or .ge conditional modifier, if one
>>> source is NaN and the other not NaN, the non-NaN source is the result.
>>> If both sources are NaNs, the result is NaN. For all other conditional
>>> modifiers, if either source is NaN then src1 is selected.
>>> """
>>>
>>> So .ge/.l return non-NaN if one source is NaN, while .g/.le propagate NaNs.
>>>
>>> We have mistakenly used the wrong conditional modifier before (see
>>> commit 3b7f683f3).
>>>
>> The old conditional modifiers were only "wrong" because some specific
>> API requires certain NaN propagation behavior for certain built-ins.
>> It's not wrong to use .g/.le internally, the condmod is not required to
>> be .l/ge for the consistency of the IR to be guaranteed or to produce
>> well-formed machine code.  Seems rather mean to me to assert on the
>> condmod being .ge/l.  This is the kind of check that belongs in an
>> API-level integration test (i.e. piglit) rather than in the backend
>> IMHO.
>
> I'll drop the assert.

Do you want to review any of the other 86 lines? :)