nir: Add a pass to lower outputs to temporary variables

This pass can be used as a helper for NIR producers so they don't have to
worry about creating the temporaries themselves.

src/glsl/Makefile.sources

@@ -40,6 +40,7 @@ NIR_FILES = \
 	nir/nir_lower_locals_to_regs.c \
 	nir/nir_lower_idiv.c \
 	nir/nir_lower_io.c \
+	nir/nir_lower_outputs_to_temporaries.c \
 	nir/nir_lower_phis_to_scalar.c \
 	nir/nir_lower_samplers.cpp \
 	nir/nir_lower_system_values.c \

src/glsl/nir/nir.h

@@ -1652,6 +1652,8 @@ void nir_lower_global_vars_to_local(nir_shader *shader);
 
 void nir_lower_locals_to_regs(nir_shader *shader);
 
+void nir_lower_outputs_to_temporaries(nir_shader *shader);
+
 void nir_assign_var_locations(struct exec_list *var_list,
                               unsigned *size,
                               int (*type_size)(const struct glsl_type *));

src/glsl/nir/nir_lower_outputs_to_temporaries.c

@@ -0,0 +1,97 @@
+/*
+ * 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.
+ *
+ * Authors:
+ *    Jason Ekstrand (jason@jlekstrand.net)
+ *
+ */
+
+/*
+ * Implements a pass that lowers output variables to a temporary plus an
+ * output variable with a single copy at each exit point of the shader.
+ * This way the output variable is only ever written.
+ *
+ * Because valid NIR requires that output variables are never read, this
+ * pass is more of a helper for NIR producers and must be run before the
+ * shader is ever validated.
+ */
+
+#include "nir.h"
+
+static void
+emit_output_copies(nir_shader *shader, nir_variable *temp, nir_variable *output)
+{
+   nir_foreach_overload(shader, overload) {
+      if (!overload->impl || strcmp(overload->function->name, "main"))
+         continue;
+
+      struct set_entry *block_entry;
+      set_foreach(overload->impl->end_block->predecessors, block_entry) {
+         struct nir_block *block = (void *)block_entry->key;
+
+         nir_intrinsic_instr *copy =
+            nir_intrinsic_instr_create(shader, nir_intrinsic_copy_var);
+         copy->variables[0] = nir_deref_var_create(copy, output);
+         copy->variables[1] = nir_deref_var_create(copy, temp);
+
+         nir_instr *last_instr = nir_block_last_instr(block);
+         if (last_instr && last_instr->type == nir_instr_type_jump)
+            nir_instr_insert_before(last_instr, &copy->instr);
+         else
+            nir_instr_insert_after_block(block, &copy->instr);
+      }
+   }
+}
+
+void
+nir_lower_outputs_to_temporaries(nir_shader *shader)
+{
+   struct exec_list old_outputs;
+
+   exec_list_move_nodes_to(&shader->outputs, &old_outputs);
+
+   /* Walk over all of the outputs turn each output into a temporary and
+    * make a new variable for the actual output.
+    */
+   foreach_list_typed(nir_variable, var, node, &old_outputs) {
+      nir_variable *output = ralloc(shader, nir_variable);
+      memcpy(output, var, sizeof *output);
+
+      /* The orignal is now the temporary */
+      nir_variable *temp = var;
+
+      /* Move the original name over to the new output */
+      if (output->name)
+         ralloc_steal(output, output->name);
+
+      /* Give the temporary a new name with @out-temp appended */
+      temp->name = ralloc_asprintf(temp, "%s@out-temp", output->name);
+      temp->data.mode = nir_var_global;
+      temp->constant_initializer = NULL;
+
+      exec_list_push_tail(&shader->outputs, &output->node);
+
+      emit_output_copies(shader, temp, output);
+   }
+
+   exec_list_append(&shader->globals, &old_outputs); 
+}