[Mesa-dev] [PATCH v4 28/44] i965/fs: Use untyped_surface_read for 16-bit load_ssbo (v2)

[Jason Ekstrand]

On Tue, Dec 5, 2017 at 1:36 PM, Jose Maria Casanova Crespo <
jmcasanova at igalia.com> wrote:

> SSBO loads were using byte_scattered read messages as they allow
> reading 16-bit size components. byte_scattered messages can only
> operate one component at a time so we needed to emit as many messages
> as components.
>
> But for vec2 and vec4 of 16-bit, being multiple of 32-bit we can use the
> untyped_surface_read message to read pairs of 16-bit components using
> only one message. Once each pair is read it is unshuffled to return the
> proper 16-bit components. vec3 case is assimilated to vec4 but the 4th
> component is ignored.
>
> 16-bit scalars are read using one byte_scattered_read message.
>
> v2: Removed use of stride = 2 on sources (Jason Ekstrand)
>     Rework optimization using unshuffle 16 reads (Chema Casanova)
> v3: Use W and D types insead of HF and F in shuffle to avoid rounding
>     erros (Jason Ekstrand)
>     Use untyped_surface_read for 16-bit vec3. (Jason Ekstrand)
>
> CC: Jason Ekstrand <jason at jlekstrand.net>
> ---
>  src/intel/compiler/brw_fs_nir.cpp | 29 ++++++++++++++++++++++-------
>  1 file changed, 22 insertions(+), 7 deletions(-)
>
> diff --git a/src/intel/compiler/brw_fs_nir.cpp
> b/src/intel/compiler/brw_fs_nir.cpp
> index e11e75e6332..8deec082d59 100644
> --- a/src/intel/compiler/brw_fs_nir.cpp
> +++ b/src/intel/compiler/brw_fs_nir.cpp
> @@ -2303,16 +2303,31 @@ do_untyped_vector_read(const fs_builder &bld,
>                         unsigned num_components)
>  {
>     if (type_sz(dest.type) <= 2) {
> -      fs_reg read_offset = bld.vgrf(BRW_REGISTER_TYPE_UD);
> -      bld.MOV(read_offset, offset_reg);
> -      for (unsigned i = 0; i < num_components; i++) {
> -         fs_reg read_reg =
> -            emit_byte_scattered_read(bld, surf_index, read_offset,
> +      assert(dest.stride == 1);
> +
> +      if (num_components > 1) {
> +         /* Pairs of 16-bit components can be read with untyped read, for
> 16-bit
> +          * vec3 4th component is ignored.
> +          */
> +         fs_reg read_result =
> +            emit_untyped_read(bld, surf_index, offset_reg,
> +                              1 /* dims */, DIV_ROUND_UP(num_components,
> 2),
> +                              BRW_PREDICATE_NONE);
> +         shuffle_32bit_load_result_to_16bit_data(bld,
> +               retype(dest, BRW_REGISTER_TYPE_W),
> +               retype(read_result, BRW_REGISTER_TYPE_D),
> +               num_components);
> +      } else {
> +         assert(num_components == 1);
> +         /* scalar 16-bit are read using one byte_scattered_read message
> */
> +         fs_reg read_result =
> +            emit_byte_scattered_read(bld, surf_index, offset_reg,
>                                       1 /* dims */, 1,
>                                       type_sz(dest.type) * 8 /* bit_size
> */,
>                                       BRW_PREDICATE_NONE);
> -         bld.MOV(offset(dest, bld, i), subscript(read_reg, dest.type, 0));
> -         bld.ADD(read_offset, read_offset, brw_imm_ud(type_sz(dest.type))
> );
> +         read_result.type = dest.type;
> +         read_result.stride = 2;
> +         bld.MOV(dest, read_result);
>

If read_reg has a 32-bit type, you could use subscript here.  Meh.

Reviewed-by: Jason Ekstrand <jason at jlekstrand.net>


>        }
>     } else if (type_sz(dest.type) == 4) {
>        fs_reg read_result = emit_untyped_read(bld, surf_index, offset_reg,
> --
> 2.11.0
>
>
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