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

[Francisco Jerez]

Roland Scheidegger <sroland at vmware.com> writes:

> 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

The bool constructor *is* subject to floating-point precision
limitations AFAIK, just like anything else dealing with floating-point
numbers.

>> 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.

Yeah, one is allowed to flush denorms to zero on input to any operation,
including bool(), I don't see any reason in the above why bool()
shouldn't be equivalent to "!= 0".

> (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.)
>
The hardware CMP instruction considers NaNs to be different from zero,
and AFAICT the implementation in this patch does the same.

> Roland
>
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> mesa-dev at lists.freedesktop.org
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