[Mesa-dev] [PATCH] ac/nir: mark some texture intrinsics as convergent

[Bas Nieuwenhuizen]

On Thu, May 30, 2019 at 6:50 PM Rhys Perry <pendingchaos02 at gmail.com> wrote:
>
> Otherwise LLVM can sink them and their texture coordinate calculations
> into divergent branches.
>
> Cc: <mesa-stable at lists.freedesktop.org>
> Signed-off-by: Rhys Perry <pendingchaos02 at gmail.com>
> ---
>  src/amd/common/ac_nir_to_llvm.c | 29 +++++++++++++++++++++++++++++
>  1 file changed, 29 insertions(+)
>
> diff --git a/src/amd/common/ac_nir_to_llvm.c b/src/amd/common/ac_nir_to_llvm.c
> index 265e3b636c4..d2dc617de36 100644
> --- a/src/amd/common/ac_nir_to_llvm.c
> +++ b/src/amd/common/ac_nir_to_llvm.c
> @@ -1316,6 +1316,30 @@ static nir_deref_instr *get_tex_texture_deref(const nir_tex_instr *instr)
>         return texture_deref_instr;
>  }
>
> +static bool has_implicit_derivatives(const nir_tex_instr *instr)
> +{
> +       switch (instr->op) {
> +       case nir_texop_txs:
> +       case nir_texop_query_levels:
> +       case nir_texop_texture_samples:
> +       case nir_texop_samples_identical:
> +               return false;
> +       default:
> +               break;
> +       }
> +       for (unsigned i = 0; i < instr->num_srcs; i++) {
> +               switch (instr->src[i].src_type) {
> +               case nir_tex_src_lod:
> +               case nir_tex_src_ddx:
> +               case nir_tex_src_ddy:
> +                       return false;
> +               default:
> +                       break;
> +               }
> +       }
> +       return true;
> +}

txf, tg4 and friends do not provide any of lod/ddx/ddy do they?

> +
>  static LLVMValueRef build_tex_intrinsic(struct ac_nir_context *ctx,
>                                         const nir_tex_instr *instr,
>                                         struct ac_image_args *args)
> @@ -1394,6 +1418,11 @@ static LLVMValueRef build_tex_intrinsic(struct ac_nir_context *ctx,
>         }
>
>         args->attributes = AC_FUNC_ATTR_READNONE;
> +       /* Prevent texture instructions with implicit derivatives from being
> +        * sinked into branches. */
> +       if (has_implicit_derivatives(instr))
> +               args->attributes |= AC_FUNC_ATTR_CONVERGENT;
> +
>         return ac_build_image_opcode(&ctx->ac, args);
>  }
>
> --
> 2.21.0
>