[Mesa-dev] [PATCH 02/26] nir: Use writemasked store_vars in glsl_to_nir.

[Jason Ekstrand]

Yeah, I like this.

On Wed, Dec 2, 2015 at 4:15 PM, Kenneth Graunke <kenneth at whitecape.org> wrote:
> Instead of performing the read-modify-write cycle in glsl->nir, we can
> simply emit a partial writemask.  For locals, nir_lower_vars_to_ssa will
> do the equivalent read-modify-write cycle for us, so we continue to get
> the same SSA values we had before.
>
> Because glsl_to_nir calls nir_lower_outputs_to_temporaries, all outputs
> are shadowed with temporary values, and written out as whole vectors at
> the end of the shader.  So, most consumers will still not see partial
> writemasks.
>
> However, nir_lower_outputs_to_temporaries bails for tessellation control
> shader outputs.  So those remain actual variables, and stores to those
> variables now get a writemask.  nir_lower_io passes that through.  This
> means that TCS outputs should actually work now.
>
> This is a functional change for tessellation control shaders.
>
> Signed-off-by: Kenneth Graunke <kenneth at whitecape.org>
> ---
>  src/glsl/nir/glsl_to_nir.cpp | 39 +++++++++------------------------------
>  1 file changed, 9 insertions(+), 30 deletions(-)
>
> diff --git a/src/glsl/nir/glsl_to_nir.cpp b/src/glsl/nir/glsl_to_nir.cpp
> index 5c84b0d..0d33eaa 100644
> --- a/src/glsl/nir/glsl_to_nir.cpp
> +++ b/src/glsl/nir/glsl_to_nir.cpp
> @@ -1044,44 +1044,23 @@ nir_visitor::visit(ir_assignment *ir)
>     nir_ssa_def *src = evaluate_rvalue(ir->rhs);
>
>     if (ir->write_mask != (1 << num_components) - 1 && ir->write_mask != 0) {
> -      /*
> -       * We have no good way to update only part of a variable, so just load
> -       * the LHS and do a vec operation to combine the old with the new, and
> -       * then store it
> -       * back into the LHS. Copy propagation should get rid of the mess.
> +      /* GLSL IR will give us the input to the write-masked assignment in a
> +       * single packed vector.  So, for example, if the writemask is xzw, then
> +       * we have to swizzle x -> x, y -> z, and z -> w and get the y component
> +       * from the load.
>         */
> -
> -      nir_intrinsic_instr *load =
> -         nir_intrinsic_instr_create(this->shader, nir_intrinsic_load_var);
> -      load->num_components = ir->lhs->type->vector_elements;
> -      nir_ssa_dest_init(&load->instr, &load->dest, num_components, NULL);
> -      load->variables[0] = lhs_deref;
> -      ralloc_steal(load, load->variables[0]);
> -      nir_builder_instr_insert(&b, &load->instr);
> -
> -      nir_ssa_def *srcs[4];
> -
> +      unsigned swiz[4];
>        unsigned component = 0;
> -      for (unsigned i = 0; i < ir->lhs->type->vector_elements; i++) {
> -         if (ir->write_mask & (1 << i)) {
> -            /* GLSL IR will give us the input to the write-masked assignment
> -             * in a single packed vector.  So, for example, if the
> -             * writemask is xzw, then we have to swizzle x -> x, y -> z,
> -             * and z -> w and get the y component from the load.
> -             */
> -            srcs[i] = nir_channel(&b, src, component++);
> -         } else {
> -            srcs[i] = nir_channel(&b, &load->dest.ssa, i);
> -         }
> +      for (unsigned i = 0; i < 4; i++) {
> +         swiz[i] = ir->write_mask & (1 << i) ? component++ : 0;
>        }
> -
> -      src = nir_vec(&b, srcs, ir->lhs->type->vector_elements);
> +      src = nir_swizzle(&b, src, swiz, num_components, !supports_ints);
>     }
>
>     nir_intrinsic_instr *store =
>        nir_intrinsic_instr_create(this->shader, nir_intrinsic_store_var);
>     store->num_components = ir->lhs->type->vector_elements;
> -   store->const_index[0] = (1 << store->num_components) - 1;
> +   store->const_index[0] = ir->write_mask;
>     nir_deref *store_deref = nir_copy_deref(store, &lhs_deref->deref);
>     store->variables[0] = nir_deref_as_var(store_deref);
>     store->src[0] = nir_src_for_ssa(src);
> --
> 2.6.2
>
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