nir: add some helpers for doing linking

The initial helpers add support for removing unused varyings between
stages.

V2:
- Moved the io mask helper function into this file rather than
  nir.h so it's not used elsewhere considering it doesn't handle
  all corner cases.
- Use bitmask rather than hash table to handle tcs outputs (Ken)

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>

src/compiler/Makefile.sources

@@ -202,6 +202,7 @@ NIR_FILES = \
 	nir/nir_instr_set.h \
 	nir/nir_intrinsics.c \
 	nir/nir_intrinsics.h \
+	nir/nir_linking_helpers.c \
 	nir/nir_liveness.c \
 	nir/nir_loop_analyze.c \
 	nir/nir_loop_analyze.h \

src/compiler/nir/nir.h

@@ -2417,6 +2417,9 @@ void nir_shader_gather_info(nir_shader *shader, nir_function_impl *entrypoint);
 void nir_assign_var_locations(struct exec_list *var_list, unsigned *size,
                               int (*type_size)(const struct glsl_type *));
 
+/* Some helpers to do very simple linking */
+bool nir_remove_unused_varyings(nir_shader *producer, nir_shader *consumer);
+
 typedef enum {
    /* If set, this forces all non-flat fragment shader inputs to be
     * interpolated as if with the "sample" qualifier.  This requires

src/compiler/nir/nir_linking_helpers.c

@@ -0,0 +1,147 @@
+/*
+ * Copyright © 2015 Intel Corporation
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a
+ * copy of this software and associated documentation files (the "Software"),
+ * to deal in the Software without restriction, including without limitation
+ * the rights to use, copy, modify, merge, publish, distribute, sublicense,
+ * and/or sell copies of the Software, and to permit persons to whom the
+ * Software is furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice (including the next
+ * paragraph) shall be included in all copies or substantial portions of the
+ * Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.  IN NO EVENT SHALL
+ * THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
+ * FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS
+ * IN THE SOFTWARE.
+ */
+
+#include "nir.h"
+#include "util/set.h"
+#include "util/hash_table.h"
+
+/* This file contains various little helpers for doing simple linking in
+ * NIR.  Eventually, we'll probably want a full-blown varying packing
+ * implementation in here.  Right now, it just deletes unused things.
+ */
+
+/**
+ * Returns the bits in the inputs_read, outputs_written, or
+ * system_values_read bitfield corresponding to this variable.
+ */
+static uint64_t
+get_variable_io_mask(nir_variable *var, gl_shader_stage stage)
+{
+   /* TODO: add support for tess patches */
+   if (var->data.patch || var->data.location < 0)
+      return 0;
+
+   assert(var->data.mode == nir_var_shader_in ||
+          var->data.mode == nir_var_shader_out ||
+          var->data.mode == nir_var_system_value);
+   assert(var->data.location >= 0);
+
+   const struct glsl_type *type = var->type;
+   if (nir_is_per_vertex_io(var, stage)) {
+      assert(glsl_type_is_array(type));
+      type = glsl_get_array_element(type);
+   }
+
+   unsigned slots = glsl_count_attribute_slots(type, false);
+   return ((1ull << slots) - 1) << var->data.location;
+}
+
+static uint64_t
+tcs_output_read_bitmask(nir_shader *shader)
+{
+   uint64_t read = 0;
+
+   nir_foreach_function(function, shader) {
+      if (function->impl) {
+         nir_foreach_block(block, function->impl) {
+            nir_foreach_instr(instr, block) {
+               if (instr->type != nir_instr_type_intrinsic)
+                  continue;
+
+               nir_intrinsic_instr *intrin_instr =
+                  nir_instr_as_intrinsic(instr);
+               if (intrin_instr->intrinsic == nir_intrinsic_load_var &&
+                   intrin_instr->variables[0]->var->data.mode ==
+                   nir_var_shader_out) {
+                  read |= get_variable_io_mask(intrin_instr->variables[0]->var,
+                                               shader->stage);
+               }
+            }
+         }
+      }
+   }
+
+   return read;
+}
+
+static bool
+remove_unused_io_vars(nir_shader *shader, struct exec_list *var_list,
+                      uint64_t used_by_other_stage)
+{
+   bool progress = false;
+
+   nir_foreach_variable_safe(var, var_list) {
+      /* TODO: add patch support */
+      if (var->data.patch)
+         continue;
+
+      if (var->data.location < VARYING_SLOT_VAR0 && var->data.location >= 0)
+         continue;
+
+      if (var->data.always_active_io)
+         continue;
+
+      if (!(used_by_other_stage & get_variable_io_mask(var, shader->stage))) {
+         /* This one is invalid, make it a global variable instead */
+         var->data.location = 0;
+         var->data.mode = nir_var_global;
+
+         exec_node_remove(&var->node);
+         exec_list_push_tail(&shader->globals, &var->node);
+
+         progress = true;
+      }
+   }
+
+   return progress;
+}
+
+bool
+nir_remove_unused_varyings(nir_shader *producer, nir_shader *consumer)
+{
+   assert(producer->stage != MESA_SHADER_FRAGMENT);
+   assert(consumer->stage != MESA_SHADER_VERTEX);
+
+   uint64_t read = 0, written = 0;
+
+   nir_foreach_variable(var, &producer->outputs)
+      written |= get_variable_io_mask(var, producer->stage);
+
+   nir_foreach_variable(var, &consumer->inputs)
+      read |= get_variable_io_mask(var, consumer->stage);
+
+   /* Each TCS invocation can read data written by other TCS invocations,
+    * so even if the outputs are not used by the TES we must also make
+    * sure they are not read by the TCS before demoting them to globals.
+    */
+   if (producer->stage == MESA_SHADER_TESS_CTRL)
+      read |= tcs_output_read_bitmask(producer);
+
+   bool progress = false;
+   progress = remove_unused_io_vars(producer, &producer->outputs, read);
+
+   progress =
+      remove_unused_io_vars(consumer, &consumer->inputs, written) || progress;
+
+   return progress;
+}