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

[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 */
> --
> 2.1.4
>