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

[Francisco Jerez]

Matt Turner <mattst88 at gmail.com> writes:

> 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? :)

Sure, but aren't you planning to resend?
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