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

[Jose Fonseca]

On 15/10/15 16:20, Roland Scheidegger wrote:
> 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).

Yep, that's my understanding of "unsafe fp math" too.

Jose