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      1 /*
      2  * Copyright  2010 Intel Corporation
      3  *
      4  * Permission is hereby granted, free of charge, to any person obtaining a
      5  * copy of this software and associated documentation files (the "Software"),
      6  * to deal in the Software without restriction, including without limitation
      7  * the rights to use, copy, modify, merge, publish, distribute, sublicense,
      8  * and/or sell copies of the Software, and to permit persons to whom the
      9  * Software is furnished to do so, subject to the following conditions:
     10  *
     11  * The above copyright notice and this permission notice (including the next
     12  * paragraph) shall be included in all copies or substantial portions of the
     13  * Software.
     14  *
     15  * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
     16  * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
     17  * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.  IN NO EVENT SHALL
     18  * THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
     19  * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
     20  * FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
     21  * DEALINGS IN THE SOFTWARE.
     22  */
     23 
     24 /**
     25  * \file opt_if_simplification.cpp
     26  *
     27  * Moves constant branches of if statements out to the surrounding
     28  * instruction stream, and inverts if conditionals to avoid empty
     29  * "then" blocks.
     30  */
     31 
     32 #include "ir.h"
     33 
     34 namespace {
     35 
     36 class ir_if_simplification_visitor : public ir_hierarchical_visitor {
     37 public:
     38    ir_if_simplification_visitor()
     39    {
     40       this->made_progress = false;
     41    }
     42 
     43    ir_visitor_status visit_leave(ir_if *);
     44    ir_visitor_status visit_enter(ir_assignment *);
     45 
     46    bool made_progress;
     47 };
     48 
     49 } /* unnamed namespace */
     50 
     51 /* We only care about the top level "if" instructions, so don't
     52  * descend into expressions.
     53  */
     54 ir_visitor_status
     55 ir_if_simplification_visitor::visit_enter(ir_assignment *ir)
     56 {
     57    (void) ir;
     58    return visit_continue_with_parent;
     59 }
     60 
     61 bool
     62 do_if_simplification(exec_list *instructions)
     63 {
     64    ir_if_simplification_visitor v;
     65 
     66    v.run(instructions);
     67    return v.made_progress;
     68 }
     69 
     70 
     71 ir_visitor_status
     72 ir_if_simplification_visitor::visit_leave(ir_if *ir)
     73 {
     74    /* If the if statement has nothing on either side, remove it. */
     75    if (ir->then_instructions.is_empty() &&
     76        ir->else_instructions.is_empty()) {
     77       ir->remove();
     78       this->made_progress = true;
     79       return visit_continue;
     80    }
     81 
     82    /* FINISHME: Ideally there would be a way to note that the condition results
     83     * FINISHME: in a constant before processing both of the other subtrees.
     84     * FINISHME: This can probably be done with some flags, but it would take
     85     * FINISHME: some work to get right.
     86     */
     87    ir_constant *condition_constant =
     88       ir->condition->constant_expression_value(ralloc_parent(ir));
     89    if (condition_constant) {
     90       /* Move the contents of the one branch of the conditional
     91        * that matters out.
     92        */
     93       if (condition_constant->value.b[0]) {
     94          ir->insert_before(&ir->then_instructions);
     95       } else {
     96          ir->insert_before(&ir->else_instructions);
     97       }
     98       ir->remove();
     99       this->made_progress = true;
    100       return visit_continue;
    101    }
    102 
    103    /* Turn:
    104     *
    105     *     if (cond) {
    106     *     } else {
    107     *         do_work();
    108     *     }
    109     *
    110     * into :
    111     *
    112     *     if (!cond)
    113     *         do_work();
    114     *
    115     * which avoids control flow for "else" (which is usually more
    116     * expensive than normal operations), and the "not" can usually be
    117     * folded into the generation of "cond" anyway.
    118     */
    119    if (ir->then_instructions.is_empty()) {
    120       ir->condition = new(ralloc_parent(ir->condition))
    121 	 ir_expression(ir_unop_logic_not, ir->condition);
    122       ir->else_instructions.move_nodes_to(&ir->then_instructions);
    123       this->made_progress = true;
    124    }
    125 
    126    return visit_continue;
    127 }
    128