glsl: Add a pass to propagate the "invariant" and "precise" qualifiers

Reviewed-by: Francisco Jerez <currojerez@riseup.net>

src/compiler/Makefile.sources

@@ -129,6 +129,7 @@ LIBGLSL_FILES = \
 	glsl/opt_tree_grafting.cpp \
 	glsl/opt_vectorize.cpp \
 	glsl/program.h \
+	glsl/propagate_invariance.cpp \
 	glsl/s_expression.cpp \
 	glsl/s_expression.h
 

src/compiler/glsl/glsl_parser_extras.cpp

@@ -1885,6 +1885,7 @@ do_common_optimization(exec_list *ir, bool linked,
       OPT(do_dead_functions, ir);
       OPT(do_structure_splitting, ir);
    }
+   propagate_invariance(ir);
    OPT(do_if_simplification, ir);
    OPT(opt_flatten_nested_if_blocks, ir);
    OPT(opt_conditional_discard, ir);

src/compiler/glsl/ir_optimization.h

@@ -139,6 +139,7 @@ bool lower_tess_level(gl_shader *shader);
 bool lower_vertex_id(gl_shader *shader);
 
 bool lower_subroutine(exec_list *instructions, struct _mesa_glsl_parse_state *state);
+void propagate_invariance(exec_list *instructions);
 
 ir_rvalue *
 compare_index_block(exec_list *instructions, ir_variable *index,

src/compiler/glsl/propagate_invariance.cpp

@@ -0,0 +1,125 @@
+/*
+ * Copyright © 2016 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.
+ */
+
+/**
+ * \file propagate_invariance.cpp
+ * Propagate the "invariant" and "precise" qualifiers to variables used to
+ * compute invariant or precise values.
+ *
+ * The GLSL spec (depending on what version you read) says, among the
+ * conditions for geting bit-for-bit the same values on an invariant output:
+ *
+ *    "All operations in the consuming expressions and any intermediate
+ *    expressions must be the same, with the same order of operands and same
+ *    associativity, to give the same order of evaluation."
+ *
+ * This effectively means that if a variable is used to compute an invariant
+ * value then that variable becomes invariant.  The same should apply to the
+ * "precise" qualifier.
+ */
+
+#include "ir.h"
+#include "ir_visitor.h"
+#include "ir_rvalue_visitor.h"
+#include "ir_optimization.h"
+#include "compiler/glsl_types.h"
+
+namespace {
+
+class ir_invariance_propagation_visitor : public ir_hierarchical_visitor {
+public:
+   ir_invariance_propagation_visitor()
+   {
+      this->progress = false;
+      this->dst_var = NULL;
+   }
+
+   virtual ~ir_invariance_propagation_visitor()
+   {
+      /* empty */
+   }
+
+   virtual ir_visitor_status visit_enter(ir_assignment *ir);
+   virtual ir_visitor_status visit_leave(ir_assignment *ir);
+   virtual ir_visitor_status visit(ir_dereference_variable *ir);
+
+   ir_variable *dst_var;
+   bool progress;
+};
+
+} /* unnamed namespace */
+
+ir_visitor_status
+ir_invariance_propagation_visitor::visit_enter(ir_assignment *ir)
+{
+   assert(this->dst_var == NULL);
+   ir_variable *var = ir->lhs->variable_referenced();
+   if (var->data.invariant || var->data.precise) {
+      this->dst_var = var;
+      return visit_continue;
+   } else {
+      return visit_continue_with_parent;
+   }
+}
+
+ir_visitor_status
+ir_invariance_propagation_visitor::visit_leave(ir_assignment *ir)
+{
+   this->dst_var = NULL;
+
+   return visit_continue;
+}
+
+ir_visitor_status
+ir_invariance_propagation_visitor::visit(ir_dereference_variable *ir)
+{
+   if (this->dst_var == NULL)
+      return visit_continue;
+
+   if (this->dst_var->data.invariant) {
+      if (!ir->var->data.invariant)
+         this->progress = true;
+
+      ir->var->data.invariant = true;
+   }
+
+   if (this->dst_var->data.precise) {
+      if (!ir->var->data.precise)
+         this->progress = true;
+
+      ir->var->data.precise = true;
+   }
+
+   return visit_continue;
+}
+
+void
+propagate_invariance(exec_list *instructions)
+{
+   ir_invariance_propagation_visitor visitor;
+
+   do {
+      visitor.progress = false;
+      visit_list_elements(&visitor, instructions);
+   } while (visitor.progress);
+}