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

[Jason Ekstrand]

On Wed, Nov 29, 2017 at 6:57 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.
>
> On 16-bit scalar and vec3 16-bit the not paired component is read using
> only one byte_scattered_read message.
>

My gut tells me that, for vec3's, we'd be better off with a single untyped
read than one untyped read and one byte scattered read.  Also, are there
alignment issues with untyped surface reads/writes that might cause us
problems on vec3's?  I don't know what the alignment rules are for 16-bit
vec3's in Vulkan.


> v2: Removed use of stride = 2 on sources (Jason Ekstrand)
>     Rework optimization using unshuffle 16 reads (Chema Casanova)
> ---
>  src/intel/compiler/brw_fs_nir.cpp | 43 ++++++++++++++++++++++++++++++
> ---------
>  1 file changed, 33 insertions(+), 10 deletions(-)
>
> diff --git a/src/intel/compiler/brw_fs_nir.cpp
> b/src/intel/compiler/brw_fs_nir.cpp
> index fa7aa9c247..57e79853ef 100644
> --- a/src/intel/compiler/brw_fs_nir.cpp
> +++ b/src/intel/compiler/brw_fs_nir.cpp
> @@ -2354,16 +2354,39 @@ do_untyped_vector_read(const fs_builder &bld,
>           bld.ADD(read_offset, read_offset, brw_imm_ud(16));
>        }
>     } else 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,
> -                                                    1 /* dims */,
> -                                                    1,
> -                                                    16 /*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))
> );
> +      assert(dest.stride == 1);
> +
> +      int component_pairs = num_components / 2;
> +      /* Pairs of 16-bit components can be read with untyped read */
> +      if (component_pairs > 0) {
> +         fs_reg read_result = emit_untyped_read(bld, surf_index,
> +                                                offset_reg,
> +                                                1 /* dims */,
> +                                                component_pairs,
> +                                                BRW_PREDICATE_NONE);
> +         shuffle_32bit_load_result_to_16bit_data(bld,
> +               retype(dest, BRW_REGISTER_TYPE_HF),
> +               retype(read_result, BRW_REGISTER_TYPE_F),
>

I'd rather we use W and D rather than HF and F.  Rounding errors scare me.


> +               component_pairs * 2);
> +      }
> +      /* Last component of vec3 and scalar 16-bit read needs to be read
> +       * using one byte_scattered_read message
> +       */
> +      if (num_components % 2) {
> +         fs_reg read_offset = bld.vgrf(BRW_REGISTER_TYPE_UD);
> +         bld.ADD(read_offset,
> +                 offset_reg,
> +                 brw_imm_ud((num_components - 1) * type_sz(dest.type)));
> +         fs_reg read_result = emit_byte_scattered_read(bld, surf_index,
> +                                                       read_offset,
> +                                                       1 /* dims */,
> +                                                       1,
> +                                                       16 /* bit_size */,
> +
>  BRW_PREDICATE_NONE);
> +         read_result.type = dest.type;
> +         read_result.stride = 2;
> +
> +         bld.MOV(offset(dest, bld, num_components - 1), read_result);
>        }
>     } else {
>        unreachable("Unsupported type");
> --
> 2.14.3
>
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