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

[Roland Scheidegger]

Am 15.10.2015 um 16:44 schrieb Marek Olšák:
> 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 */
>> --

Please add a comment why it's using t*a + (1-t)*b and not (a-b)*t + b.
Though it is yet another thing we should have some more control over in
tgsi. Because if you're willing to allow unsafe-fp-math, then you should
also be willing to accept the simpler formula (I'm quite sure
unsafe-fp-math would be allowed to turn one formula into the other).
But otherwise I guess this is ok - it is the formula specified by glsl
after all.

Roland