nir: Allow outputs reads and add the relevant intrinsics.

Normally, we rely on nir_lower_outputs_to_temporaries to create shadow
variables for outputs, buffering the results and writing them all out
at the end of the program.  However, this is infeasible for tessellation
control shader outputs.

Tessellation control shaders can generate multiple output vertices, and
write per-vertex outputs.  These are arrays indexed by the vertex
number; each thread only writes one element, but can read any other
element - including those being concurrently written by other threads.
The barrier() intrinsic synchronizes between threads.

Even if we tried to shadow every output element (which is of dubious
value), we'd have to read updated values in at barrier() time, which
means we need to allow output reads.

Most stages should continue using nir_lower_outputs_to_temporaries(),
but in theory drivers could choose not to if they really wanted.

v2: Rebase to accomodate Jason's review feedback.

Signed-off-by: Kenneth Graunke <[email protected]>
Reviewed-by: Jason Ekstrand <[email protected]>

4 files changed, 21 insertions, 8 deletions

diff --git a/src/glsl/nir/nir_intrinsics.h b/src/glsl/nir/nir_intrinsics.h
index 26ac7ce9cd7..b8d7d6c68cb 100644
--- a/src/glsl/nir/nir_intrinsics.h
+++ b/src/glsl/nir/nir_intrinsics.h
@@ -255,6 +255,8 @@ LOAD(ubo, 1, 1, NIR_INTRINSIC_CAN_ELIMINATE | NIR_INTRINSIC_CAN_REORDER)
 LOAD(input, 0, 1, NIR_INTRINSIC_CAN_ELIMINATE | NIR_INTRINSIC_CAN_REORDER)
 LOAD(per_vertex_input, 1, 1, NIR_INTRINSIC_CAN_ELIMINATE | NIR_INTRINSIC_CAN_REORDER)
 LOAD(ssbo, 1, 1, NIR_INTRINSIC_CAN_ELIMINATE)
+LOAD(output, 0, 1, NIR_INTRINSIC_CAN_ELIMINATE)
+LOAD(per_vertex_output, 1, 1, NIR_INTRINSIC_CAN_ELIMINATE)
 
 /*
  * Stores work the same way as loads, except now the first register input is
diff --git a/src/glsl/nir/nir_lower_io.c b/src/glsl/nir/nir_lower_io.c
index b7b599da6d4..8a4177fb9f0 100644
--- a/src/glsl/nir/nir_lower_io.c
+++ b/src/glsl/nir/nir_lower_io.c
@@ -161,6 +161,15 @@ load_op(struct lower_io_state *state,
                              nir_intrinsic_load_input;
       }
       break;
+   case nir_var_shader_out:
+      if (per_vertex) {
+         op = has_indirect ? nir_intrinsic_load_per_vertex_output_indirect :
+                             nir_intrinsic_load_per_vertex_output;
+      } else {
+         op = has_indirect ? nir_intrinsic_load_output_indirect :
+                             nir_intrinsic_load_output;
+      }
+      break;
    case nir_var_uniform:
       op = has_indirect ? nir_intrinsic_load_uniform_indirect :
                           nir_intrinsic_load_uniform;
@@ -191,13 +200,16 @@ nir_lower_io_block(nir_block *block, void *void_state)
       if (state->mode != -1 && state->mode != mode)
          continue;
 
+      if (mode != nir_var_shader_in &&
+          mode != nir_var_shader_out &&
+          mode != nir_var_uniform)
+         continue;
+
       switch (intrin->intrinsic) {
       case nir_intrinsic_load_var: {
-         if (mode != nir_var_shader_in && mode != nir_var_uniform)
-            continue;
-
          bool per_vertex =
-            is_per_vertex_input(state, intrin->variables[0]->var);
+            is_per_vertex_input(state, intrin->variables[0]->var) ||
+            is_per_vertex_output(state, intrin->variables[0]->var);
 
          nir_ssa_def *indirect;
          nir_ssa_def *vertex_index;
@@ -241,8 +253,7 @@ nir_lower_io_block(nir_block *block, void *void_state)
       }
 
       case nir_intrinsic_store_var: {
-         if (intrin->variables[0]->var->data.mode != nir_var_shader_out)
-            continue;
+         assert(mode == nir_var_shader_out);
 
          nir_ssa_def *indirect;
          nir_ssa_def *vertex_index;
diff --git a/src/glsl/nir/nir_print.c b/src/glsl/nir/nir_print.c
index 23fcafeb7e8..f7f5fdf3181 100644
--- a/src/glsl/nir/nir_print.c
+++ b/src/glsl/nir/nir_print.c
@@ -448,6 +448,8 @@ print_intrinsic_instr(nir_intrinsic_instr *instr, print_state *state)
    case nir_intrinsic_load_per_vertex_input_indirect:
       var_list = &state->shader->inputs;
       break;
+   case nir_intrinsic_load_output:
+   case nir_intrinsic_load_output_indirect:
    case nir_intrinsic_store_output:
    case nir_intrinsic_store_output_indirect:
    case nir_intrinsic_store_per_vertex_output:
diff --git a/src/glsl/nir/nir_validate.c b/src/glsl/nir/nir_validate.c
index 51c2529dc38..ed374b921fa 100644
--- a/src/glsl/nir/nir_validate.c
+++ b/src/glsl/nir/nir_validate.c
@@ -405,7 +405,6 @@ validate_intrinsic_instr(nir_intrinsic_instr *instr, validate_state *state)
              (instr->variables[0]->var->data.mode == nir_var_uniform &&
               glsl_get_base_type(type) == GLSL_TYPE_SUBROUTINE));
       assert(instr->num_components == glsl_get_vector_elements(type));
-      assert(instr->variables[0]->var->data.mode != nir_var_shader_out);
       break;
    }
    case nir_intrinsic_store_var: {
@@ -426,7 +425,6 @@ validate_intrinsic_instr(nir_intrinsic_instr *instr, validate_state *state)
       assert(instr->variables[0]->var->data.mode != nir_var_shader_in &&
              instr->variables[0]->var->data.mode != nir_var_uniform &&
              instr->variables[0]->var->data.mode != nir_var_shader_storage);
-      assert(instr->variables[1]->var->data.mode != nir_var_shader_out);
       break;
    default:
       break;