intel/fs: Add support for CS to group invocations in quads

When using quads, instead of mapping the elements to the next 4 local
invocation indices, we map the two next in the "current" row and two
next in the "next row".  A side effect is that a thread will execute
the indices in a different order.

We now perform the lowering of both local invocation ID and index
together -- and don't rely anymore on lowering done by
nir_lower_system_values.  That is convenient when doing the math for
quads, because we need X and Y to get the right invocation index.

When the pass progresses, fold the constants and clean up to reduce
the noise from the indexing math.

This implements the derivative_group_quadsNV semantics from
NV_compute_shader_derivatives.

v2: Take subgroup_id into account, otherwise only values in the first
    subgroup would be used. (Jason)

v3: Calculate invocation index and ID together, to avoid duplicating
    some math in the quads case when both index and ID are used. (Jason)

v4: Don't call cleanup passes as part of the lowering, let that to the
    call site. (Jason)
    Change calculation to use less instructions. (Jason)

Reviewed-by: Ian Romanick <[email protected]> (v3)
Reviewed-by: Jason Ekstrand <[email protected]>

1 files changed, 5 insertions, 0 deletions

diff --git a/src/intel/compiler/brw_fs.cpp b/src/intel/compiler/brw_fs.cpp
index 0c2439d9daf..a637ee3422f 100644
--- a/src/intel/compiler/brw_fs.cpp
+++ b/src/intel/compiler/brw_fs.cpp
@@ -8017,6 +8017,11 @@ compile_cs_to_nir(const struct brw_compiler *compiler,
    nir_shader *shader = nir_shader_clone(mem_ctx, src_shader);
    shader = brw_nir_apply_sampler_key(shader, compiler, &key->tex, true);
    brw_nir_lower_cs_intrinsics(shader, dispatch_width);
+
+   /* Clean up after the local index and ID calculations. */
+   nir_opt_constant_folding(shader);
+   nir_opt_dce(shader);
+
    return brw_postprocess_nir(shader, compiler, true);
 }