src/glsl/opt_if_simplification.cpp

/*
 * Copyright © 2010 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 opt_if_simplification.cpp
 *
 * Moves constant branches of if statements out to the surrounding
 * instruction stream, and inverts if conditionals to avoid empty
 * "then" blocks.
 */

#include "ir.h"

namespace {

class ir_if_simplification_visitor : public ir_hierarchical_visitor {
public:
   ir_if_simplification_visitor()
   {
      this->made_progress = false;
   }

   ir_visitor_status visit_leave(ir_if *);
   ir_visitor_status visit_enter(ir_assignment *);

   bool made_progress;
};

} /* unnamed namespace */

/* We only care about the top level "if" instructions, so don't
 * descend into expressions.
 */
ir_visitor_status
ir_if_simplification_visitor::visit_enter(ir_assignment *ir)
{
   (void) ir;
   return visit_continue_with_parent;
}

bool
do_if_simplification(exec_list *instructions)
{
   ir_if_simplification_visitor v;

   v.run(instructions);
   return v.made_progress;
}


ir_visitor_status
ir_if_simplification_visitor::visit_leave(ir_if *ir)
{
   /* If the if statement has nothing on either side, remove it. */
   if (ir->then_instructions.is_empty() &&
       ir->else_instructions.is_empty()) {
      ir->remove();
      this->made_progress = true;
      return visit_continue;
   }

   /* FINISHME: Ideally there would be a way to note that the condition results
    * FINISHME: in a constant before processing both of the other subtrees.
    * FINISHME: This can probably be done with some flags, but it would take
    * FINISHME: some work to get right.
    */
   ir_constant *condition_constant = ir->condition->constant_expression_value();
   if (condition_constant) {
      /* Move the contents of the one branch of the conditional
       * that matters out.
       */
      if (condition_constant->value.b[0]) {
         ir->insert_before(&ir->then_instructions);
      } else {
         ir->insert_before(&ir->else_instructions);
      }
      ir->remove();
      this->made_progress = true;
      return visit_continue;
   }

   /* Turn:
    *
    *     if (cond) {
    *     } else {
    *         do_work();
    *     }
    *
    * into :
    *
    *     if (!cond)
    *         do_work();
    *
    * which avoids control flow for "else" (which is usually more
    * expensive than normal operations), and the "not" can usually be
    * folded into the generation of "cond" anyway.
    */
   if (ir->then_instructions.is_empty()) {
      ir->condition = new(ralloc_parent(ir->condition))
	 ir_expression(ir_unop_logic_not, ir->condition);
      ir->else_instructions.move_nodes_to(&ir->then_instructions);
      this->made_progress = true;
   }

   return visit_continue;
}