[Mesa-dev] [PATCH 02/10] gallivm: implement the correct version of LRP

[Jose Fonseca]

Roland was on PTO.

IMO, the change makes sense from a numeric accuracy POV.

I fear this might cause some slowdown with llvmpipe (two muls intead of 
one), but hopefully it won't be significant.  The accuracy issue could 
cause glitches to llvmpipe too.

Jose

On 15/10/15 15:44, Marek Olšák wrote:
> Any comment or is this okay with people? Given, "(1-t)*a + t*b", the
> original code didn't return b for t=1 because it's "floating-point".
>
> Marek
>
> On Sun, Oct 11, 2015 at 3:29 AM, Marek Olšák <maraeo at gmail.com> wrote:
>> From: Marek Olšák <marek.olsak at amd.com>
>>
>> The previous version has precision issues. This can be a problem
>> with tessellation. Sadly, I can't find the article where I read it
>> anymore. I'm not sure if the unsafe-fp-math flag would be enough to revert
>> this.
>> ---
>>   src/gallium/auxiliary/gallivm/lp_bld_tgsi_action.c | 13 +++++++------
>>   1 file changed, 7 insertions(+), 6 deletions(-)
>>
>> diff --git a/src/gallium/auxiliary/gallivm/lp_bld_tgsi_action.c b/src/gallium/auxiliary/gallivm/lp_bld_tgsi_action.c
>> index 0ad78b0..512558b 100644
>> --- a/src/gallium/auxiliary/gallivm/lp_bld_tgsi_action.c
>> +++ b/src/gallium/auxiliary/gallivm/lp_bld_tgsi_action.c
>> @@ -538,12 +538,13 @@ lrp_emit(
>>      struct lp_build_tgsi_context * bld_base,
>>      struct lp_build_emit_data * emit_data)
>>   {
>> -   LLVMValueRef tmp;
>> -   tmp = lp_build_emit_llvm_binary(bld_base, TGSI_OPCODE_SUB,
>> -                                   emit_data->args[1],
>> -                                   emit_data->args[2]);
>> -   emit_data->output[emit_data->chan] = lp_build_emit_llvm_ternary(bld_base,
>> -                    TGSI_OPCODE_MAD, emit_data->args[0], tmp, emit_data->args[2]);
>> +   struct lp_build_context *bld = &bld_base->base;
>> +   LLVMValueRef inv, a, b;
>> +
>> +   inv = lp_build_sub(bld, bld_base->base.one, emit_data->args[0]);
>> +   a = lp_build_mul(bld, emit_data->args[1], emit_data->args[0]);
>> +   b = lp_build_mul(bld, emit_data->args[2], inv);
>> +   emit_data->output[emit_data->chan] = lp_build_add(bld, a, b);
>>   }
>>
>>   /* TGSI_OPCODE_MAD */
>> --
>> 2.1.4
>>
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