[Mesa-dev] [PATCH 1/2] i965/fs: fix precision of f2b

[Francisco Jerez]

Ian Romanick <idr at freedesktop.org> writes:

> On 02/25/2016 12:13 PM, Francisco Jerez wrote:
>> Ian Romanick <idr at freedesktop.org> writes:
>> 
>>> On 02/25/2016 08:46 AM, Roland Scheidegger wrote:
>>>> Am 25.02.2016 um 11:15 schrieb Iago Toral Quiroga:
>>>>> From the OpenGL 4.2 spec:
>>>>>
>>>>> "When a constructor is used to convert any integer or floating-point type to a
>>>>>  bool, 0 and 0.0 are converted to false, and non-zero values are converted to
>>>>>  true."
>>>>>
>>>>> Thus, even the smallest non-zero floating value should be translated to true.
>>>>> This behavior has been verified also with proprietary NVIDIA drivers.
>>>>>
>>>>> Currently, we implement this conversion as a cmp.nz operation with floats,
>>>>> subject to floating-point precision limitations, and as a result, relatively
>>>>> small non-zero floating point numbers return false instead of true.
>>>>>
>>>>> This patch fixes the problem by getting rid of the sign bit (to cover the case
>>>>> of -0.0) and testing the result against 0u using an integer comparison instead.
>>>>> ---
>>>>>  src/mesa/drivers/dri/i965/brw_fs_nir.cpp | 15 ++++++++++++---
>>>>>  1 file changed, 12 insertions(+), 3 deletions(-)
>>>>>
>>>>> diff --git a/src/mesa/drivers/dri/i965/brw_fs_nir.cpp b/src/mesa/drivers/dri/i965/brw_fs_nir.cpp
>>>>> index db20c71..7d62d7e 100644
>>>>> --- a/src/mesa/drivers/dri/i965/brw_fs_nir.cpp
>>>>> +++ b/src/mesa/drivers/dri/i965/brw_fs_nir.cpp
>>>>> @@ -913,9 +913,18 @@ fs_visitor::nir_emit_alu(const fs_builder &bld, nir_alu_instr *instr)
>>>>>        bld.MOV(result, negate(op[0]));
>>>>>        break;
>>>>>  
>>>>> -   case nir_op_f2b:
>>>>> -      bld.CMP(result, op[0], brw_imm_f(0.0f), BRW_CONDITIONAL_NZ);
>>>>> -      break;
>>>>> +   case nir_op_f2b: {
>>>>> +      /* Because comparing to 0.0f is subject to precision limitations, do the
>>>>> +       * comparison using integers (we need to get rid of the sign bit for that)
>>>>> +       */
>>>>> +      if (devinfo->gen >= 8)
>>>>> +         op[0] = resolve_source_modifiers(op[0]);
>>>>> +      op[0] = retype(op[0], BRW_REGISTER_TYPE_UD);
>>>>> +      bld.AND(op[0], op[0], brw_imm_ud(0x7FFFFFFFu));
>>>>> +      bld.CMP(result, op[0], brw_imm_ud(0u), BRW_CONDITIONAL_NZ);
>>>>> +       break;
>>>>> +   }
>>>>> +
>>>>>     case nir_op_i2b:
>>>>>        bld.CMP(result, op[0], brw_imm_d(0), BRW_CONDITIONAL_NZ);
>>>>>        break;
>>>>>
>>>>
>>>> Does that fix anything? I don't really see a problem with the existing
>>>> logic. Yes any "non-zero value" should be converted to true. But surely
>>>> that definition cannot include denorms, which you are always allowed to
>>>> flush to zero.
>>>> (Albeit I can't tell what the result would be with NaNs with the float
>>>> compare, nor what the result actually should be in this case since glsl
>>>> doesn't require NaNs neither.)
>>>
>>> Based on this and Jason's comments, I think we need a bunch of new tests.
>>>
>>>  - smallest positive normal number
>>>  - abs of smallest positive normal number
>>>  - neg of     "       "        "      "
>>>  - largest positive subnormal number
>>>  - abs of largest positive subnormal number
>>>  - neg of    "        "        "        "
>>>  - all of the above with negative numbers
>>>  - NaN
>>>  - abs of NaN
>>>  - neg of  "
>>>
>>> Perhaps others? +/-Inf just for kicks?
>> 
>> What's the point?  The result of most of the above (except possibly
>> bool(NaN)) is undefined by the spec:
>> 
>> "Any denormalized value input into a shader or potentially generated by
>>  any operation in a shader can be flushed to 0. [...] NaNs are not
>>  required to be generated. [...] Operations and built-in functions that
>>  operate on a NaN are not required to return a NaN as the result."
>
> Except that apparently one major OpenGL vendor does something well
> defined that's different than what we do.

I'm skeptical that nVidia would treat single-precision denorms
inconsistently between datatype constructors and other floating-point
arithmetic, but assuming that's the case it would be an argument for
proposing the spec change to Khronos rather than introducing a dubiously
compliant change into the back-end.  I think I would argue against
making such a change in the spec in any case, because even though it's
implementation-defined whether denorms are flushed or not, the following
is guaranteed by the spec AFAIUI:

| if (bool(f))
|    random_arithmetic_on(f /* Guaranteed not to be zero here even if
|                              denorms are flushed */);

With this change in, bool(f) would evaluate to true even if f is a
denorm which is flushed to zero for all subsequent arithmetic in the
block.  For that reason this seems more likely to break stuff than to
fix stuff, it's not like applications can expect denorms to be
representable at all regardless of what nVidia does, but they can expect
the above GLSL source to work.

> If we can validate what the
> behavior is across multiple implementations, we may find a set of common
> behavior that differs from ours.  If other people commonly do a thing
> that's different than what we do, there's a very good chance that some
> application depends on that behavior.  This certainly would not be the
> first time.
>
>>>> Roland
>>>>
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>>>
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