glsl: Add a lowering pass to remove reads of shader output variables.

This is similar to Gallium's existing glsl_to_tgsi::remove_output_read
lowering pass, but done entirely inside the GLSL compiler.

Signed-off-by: Vincent Lejeune <vljn@ovi.com>
Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Signed-off-by: Dave Airlie <airlied@redhat.com>

src/glsl/Makefile.sources

@@ -60,6 +60,7 @@ LIBGLSL_CXX_SOURCES := \
 	lower_vec_index_to_cond_assign.cpp \
 	lower_vec_index_to_swizzle.cpp \
 	lower_vector.cpp \
+	lower_output_reads.cpp \
 	opt_algebraic.cpp \
 	opt_constant_folding.cpp \
 	opt_constant_propagation.cpp \

src/glsl/ir_optimization.h

@@ -72,6 +72,7 @@ bool lower_variable_index_to_cond_assign(exec_list *instructions,
     bool lower_input, bool lower_output, bool lower_temp, bool lower_uniform);
 bool lower_quadop_vector(exec_list *instructions, bool dont_lower_swz);
 bool lower_clip_distance(exec_list *instructions);
+void lower_output_reads(exec_list *instructions);
 bool optimize_redundant_jumps(exec_list *instructions);
 
 ir_rvalue *

src/glsl/lower_output_reads.cpp

@@ -0,0 +1,141 @@
+/*
+ * Copyright © 2012 Vincent Lejeune
+ * Copyright © 2012 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 "ir.h"
+#include "program/hash_table.h"
+
+/**
+ * \file lower_output_reads.cpp
+ *
+ * In GLSL, shader output variables (such as varyings) can be both read and
+ * written.  However, on some hardware, reading an output register causes
+ * trouble.
+ *
+ * This pass creates temporary shadow copies of every (used) shader output,
+ * and replaces all accesses to use those instead.  It also adds code to the
+ * main() function to copy the final values to the actual shader outputs.
+ */
+
+class output_read_remover : public ir_hierarchical_visitor {
+protected:
+   /**
+    * A hash table mapping from the original ir_variable shader outputs
+    * (ir_var_out mode) to the new temporaries to be used instead.
+    */
+   hash_table *replacements;
+
+   void *mem_ctx;
+public:
+   output_read_remover();
+   ~output_read_remover();
+   virtual ir_visitor_status visit(class ir_dereference_variable *);
+   virtual ir_visitor_status visit_leave(class ir_return *);
+   virtual ir_visitor_status visit_leave(class ir_function_signature *);
+};
+
+output_read_remover::output_read_remover()
+{
+   mem_ctx = ralloc_context(NULL);
+   replacements =
+      hash_table_ctor(0, hash_table_pointer_hash, hash_table_pointer_compare);
+}
+
+output_read_remover::~output_read_remover()
+{
+   hash_table_dtor(replacements);
+   ralloc_free(mem_ctx);
+}
+
+ir_visitor_status
+output_read_remover::visit(ir_dereference_variable *ir)
+{
+   if (ir->var->mode != ir_var_out)
+      return visit_continue;
+
+   ir_variable *temp = (ir_variable *) hash_table_find(replacements, ir->var);
+
+   /* If we don't have an existing temporary, create one. */
+   if (temp == NULL) {
+      void *var_ctx = ralloc_parent(ir->var);
+      temp = new(var_ctx) ir_variable(ir->var->type, ir->var->name,
+                                      ir_var_temporary);
+      hash_table_insert(replacements, temp, ir->var);
+   }
+
+   /* Update the dereference to use the temporary */
+   ir->var = temp;
+
+   return visit_continue;
+}
+
+/**
+ * Create an assignment to copy a temporary value back to the actual output.
+ */
+static ir_assignment *
+copy(void *ctx, ir_variable *output, ir_variable *temp)
+{
+   ir_dereference_variable *lhs = new(ctx) ir_dereference_variable(output);
+   ir_dereference_variable *rhs = new(ctx) ir_dereference_variable(temp);
+   return new(ctx) ir_assignment(lhs, rhs);
+}
+
+/** Insert a copy-back assignment before a "return" statement */
+static void
+emit_return_copy(const void *key, void *data, void *closure)
+{
+   ir_return *ir = (ir_return *) closure;
+   ir->insert_before(copy(ir, (ir_variable *) key, (ir_variable *) data));
+}
+
+/** Insert a copy-back assignment at the end of the main() function */
+static void
+emit_main_copy(const void *key, void *data, void *closure)
+{
+   ir_function_signature *sig = (ir_function_signature *) closure;
+   sig->body.push_tail(copy(sig, (ir_variable *) key, (ir_variable *) data));
+}
+
+ir_visitor_status
+output_read_remover::visit_leave(ir_return *ir)
+{
+   hash_table_call_foreach(replacements, emit_return_copy, ir);
+   return visit_continue;
+}
+
+ir_visitor_status
+output_read_remover::visit_leave(ir_function_signature *sig)
+{
+   if (strcmp(sig->function_name(), "main") != 0)
+      return visit_continue;
+
+   hash_table_call_foreach(replacements, emit_main_copy, sig);
+   return visit_continue;
+}
+
+void
+lower_output_reads(exec_list *instructions)
+{
+   output_read_remover v;
+   visit_list_elements(&v, instructions);
+}