src/compiler/glsl/opt_constant_folding.cpp

   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_constant_folding.cpp
  26  * Replace constant-valued expressions with references to constant values.
  27  */
  28
  29 #include "ir.h"
  30 #include "ir_visitor.h"
  31 #include "ir_rvalue_visitor.h"
  32 #include "ir_optimization.h"
  33 #include "compiler/glsl_types.h"
  34
  35 namespace {
  36
  37 /**
  38  * Visitor class for replacing expressions with ir_constant values.
  39  */
  40
  41 class ir_constant_folding_visitor : public ir_rvalue_visitor {
  42 public:
  43    ir_constant_folding_visitor()
  44    {
  45       this->progress = false;
  46    }
  47
  48    virtual ~ir_constant_folding_visitor()
  49    {
  50       /* empty */
  51    }
  52
  53    virtual ir_visitor_status visit_enter(ir_discard *ir);
  54    virtual ir_visitor_status visit_enter(ir_assignment *ir);
  55    virtual ir_visitor_status visit_enter(ir_call *ir);
  56
  57    virtual void handle_rvalue(ir_rvalue **rvalue);
  58
  59    bool progress;
  60 };
  61
  62 } /* unnamed namespace */
  63
  64 bool
  65 ir_constant_fold(ir_rvalue **rvalue)
  66 {
  67    if (*rvalue == NULL || (*rvalue)->ir_type == ir_type_constant)
  68       return false;
  69
  70    /* Note that we do rvalue visitoring on leaving.  So if an
  71     * expression has a non-constant operand, no need to go looking
  72     * down it to find if it's constant.  This cuts the time of this
  73     * pass down drastically.
  74     */
  75    ir_expression *expr = (*rvalue)->as_expression();
  76    if (expr) {
  77       for (unsigned int i = 0; i < expr->get_num_operands(); i++) {
  78          if (!expr->operands[i]->as_constant())
  79             return false;
  80       }
  81    }
  82
  83    /* Ditto for swizzles. */
  84    ir_swizzle *swiz = (*rvalue)->as_swizzle();
  85    if (swiz && !swiz->val->as_constant())
  86       return false;
  87
  88    /* Ditto for array dereferences */
  89    ir_dereference_array *array_ref = (*rvalue)->as_dereference_array();
  90    if (array_ref && (!array_ref->array->as_constant() ||
  91                      !array_ref->array_index->as_constant()))
  92       return false;
  93
  94    /* No constant folding can be performed on variable dereferences.  We need
  95     * to explicitly avoid them, as calling constant_expression_value() on a
  96     * variable dereference will return a clone of var->constant_value.  This
  97     * would make us propagate the value into the tree, which isn't our job.
  98     */
  99    ir_dereference_variable *var_ref = (*rvalue)->as_dereference_variable();
 100    if (var_ref)
 101       return false;
 102
 103    ir_constant *constant = (*rvalue)->constant_expression_value();
 104    if (constant) {
 105       *rvalue = constant;
 106       return true;
 107    }
 108    return false;
 109 }
 110
 111 void
 112 ir_constant_folding_visitor::handle_rvalue(ir_rvalue **rvalue)
 113 {
 114    if (ir_constant_fold(rvalue))
 115       this->progress = true;
 116 }
 117
 118 ir_visitor_status
 119 ir_constant_folding_visitor::visit_enter(ir_discard *ir)
 120 {
 121    if (ir->condition) {
 122       ir->condition->accept(this);
 123       handle_rvalue(&ir->condition);
 124
 125       ir_constant *const_val = ir->condition->as_constant();
 126       /* If the condition is constant, either remove the condition or
 127        * remove the never-executed assignment.
 128        */
 129       if (const_val) {
 130          if (const_val->value.b[0])
 131             ir->condition = NULL;
 132          else
 133             ir->remove();
 134          this->progress = true;
 135       }
 136    }
 137
 138    return visit_continue_with_parent;
 139 }
 140
 141 ir_visitor_status
 142 ir_constant_folding_visitor::visit_enter(ir_assignment *ir)
 143 {
 144    ir->rhs->accept(this);
 145    handle_rvalue(&ir->rhs);
 146
 147    if (ir->condition) {
 148       ir->condition->accept(this);
 149       handle_rvalue(&ir->condition);
 150
 151       ir_constant *const_val = ir->condition->as_constant();
 152       /* If the condition is constant, either remove the condition or
 153        * remove the never-executed assignment.
 154        */
 155       if (const_val) {
 156          if (const_val->value.b[0])
 157             ir->condition = NULL;
 158          else
 159             ir->remove();
 160          this->progress = true;
 161       }
 162    }
 163
 164    /* Don't descend into the LHS because we want it to stay as a
 165     * variable dereference.  FINISHME: We probably should to get array
 166     * indices though.
 167     */
 168    return visit_continue_with_parent;
 169 }
 170
 171 ir_visitor_status
 172 ir_constant_folding_visitor::visit_enter(ir_call *ir)
 173 {
 174    /* Attempt to constant fold parameters */
 175    foreach_two_lists(formal_node, &ir->callee->parameters,
 176                      actual_node, &ir->actual_parameters) {
 177       ir_rvalue *param_rval = (ir_rvalue *) actual_node;
 178       ir_variable *sig_param = (ir_variable *) formal_node;
 179
 180       if (sig_param->data.mode == ir_var_function_in
 181           || sig_param->data.mode == ir_var_const_in) {
 182          ir_rvalue *new_param = param_rval;
 183
 184          handle_rvalue(&new_param);
 185          if (new_param != param_rval) {
 186             param_rval->replace_with(new_param);
 187          }
 188       }
 189    }
 190
 191    /* Next, see if the call can be replaced with an assignment of a constant */
 192    ir_constant *const_val = ir->constant_expression_value();
 193
 194    if (const_val != NULL) {
 195       ir_assignment *assignment =
 196          new(ralloc_parent(ir)) ir_assignment(ir->return_deref, const_val);
 197       ir->replace_with(assignment);
 198    }
 199
 200    return visit_continue_with_parent;
 201 }
 202
 203 bool
 204 do_constant_folding(exec_list *instructions)
 205 {
 206    ir_constant_folding_visitor constant_folding;
 207
 208    visit_list_elements(&constant_folding, instructions);
 209
 210    return constant_folding.progress;
 211 }