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

[Francisco Jerez]

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."


>> Roland
>> 
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>> https://lists.freedesktop.org/mailman/listinfo/mesa-dev
>
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