src/glsl/loop_controls.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 #include <limits.h>
  25 #include "main/compiler.h"
  26 #include "glsl_types.h"
  27 #include "loop_analysis.h"
  28 #include "ir_hierarchical_visitor.h"
  29
  30 /**
  31  * Find an initializer of a variable outside a loop
  32  *
  33  * Works backwards from the loop to find the pre-loop value of the variable.
  34  * This is used, for example, to find the initial value of loop induction
  35  * variables.
  36  *
  37  * \param loop  Loop where \c var is an induction variable
  38  * \param var   Variable whose initializer is to be found
  39  *
  40  * \return
  41  * The \c ir_rvalue assigned to the variable outside the loop.  May return
  42  * \c NULL if no initializer can be found.
  43  */
  44 ir_rvalue *
  45 find_initial_value(ir_loop *loop, ir_variable *var)
  46 {
  47    for (exec_node *node = loop->prev;
  48         !node->is_head_sentinel();
  49         node = node->prev) {
  50       ir_instruction *ir = (ir_instruction *) node;
  51
  52       switch (ir->ir_type) {
  53       case ir_type_call:
  54       case ir_type_loop:
  55       case ir_type_loop_jump:
  56       case ir_type_return:
  57       case ir_type_if:
  58          return NULL;
  59
  60       case ir_type_function:
  61       case ir_type_function_signature:
  62          assert(!"Should not get here.");
  63          return NULL;
  64
  65       case ir_type_assignment: {
  66          ir_assignment *assign = ir->as_assignment();
  67          ir_variable *assignee = assign->lhs->whole_variable_referenced();
  68
  69          if (assignee == var)
  70             return (assign->condition != NULL) ? NULL : assign->rhs;
  71
  72          break;
  73       }
  74
  75       default:
  76          break;
  77       }
  78    }
  79
  80    return NULL;
  81 }
  82
  83
  84 int
  85 calculate_iterations(ir_rvalue *from, ir_rvalue *to, ir_rvalue *increment,
  86                      enum ir_expression_operation op)
  87 {
  88    if (from == NULL || to == NULL || increment == NULL)
  89       return -1;
  90
  91    void *mem_ctx = ralloc_context(NULL);
  92
  93    ir_expression *const sub =
  94       new(mem_ctx) ir_expression(ir_binop_sub, from->type, to, from);
  95
  96    ir_expression *const div =
  97       new(mem_ctx) ir_expression(ir_binop_div, sub->type, sub, increment);
  98
  99    ir_constant *iter = div->constant_expression_value();
 100
 101    if (iter == NULL)
 102       return -1;
 103
 104    if (!iter->type->is_integer()) {
 105       ir_rvalue *cast =
 106          new(mem_ctx) ir_expression(ir_unop_f2i, glsl_type::int_type, iter,
 107                                     NULL);
 108
 109       iter = cast->constant_expression_value();
 110    }
 111
 112    int iter_value = iter->get_int_component(0);
 113
 114    /* Make sure that the calculated number of iterations satisfies the exit
 115     * condition.  This is needed to catch off-by-one errors and some types of
 116     * ill-formed loops.  For example, we need to detect that the following
 117     * loop does not have a maximum iteration count.
 118     *
 119     *    for (float x = 0.0; x != 0.9; x += 0.2)
 120     *        ;
 121     */
 122    const int bias[] = { -1, 0, 1 };
 123    bool valid_loop = false;
 124
 125    for (unsigned i = 0; i < Elements(bias); i++) {
 126       iter = (increment->type->is_integer())
 127          ? new(mem_ctx) ir_constant(iter_value + bias[i])
 128          : new(mem_ctx) ir_constant(float(iter_value + bias[i]));
 129
 130       ir_expression *const mul =
 131          new(mem_ctx) ir_expression(ir_binop_mul, increment->type, iter,
 132                                     increment);
 133
 134       ir_expression *const add =
 135          new(mem_ctx) ir_expression(ir_binop_add, mul->type, mul, from);
 136
 137       ir_expression *const cmp =
 138          new(mem_ctx) ir_expression(op, glsl_type::bool_type, add, to);
 139
 140       ir_constant *const cmp_result = cmp->constant_expression_value();
 141
 142       assert(cmp_result != NULL);
 143       if (cmp_result->get_bool_component(0)) {
 144          iter_value += bias[i];
 145          valid_loop = true;
 146          break;
 147       }
 148    }
 149
 150    ralloc_free(mem_ctx);
 151    return (valid_loop) ? iter_value : -1;
 152 }
 153
 154
 155 class loop_control_visitor : public ir_hierarchical_visitor {
 156 public:
 157    loop_control_visitor(loop_state *state)
 158    {
 159       this->state = state;
 160       this->progress = false;
 161    }
 162
 163    virtual ir_visitor_status visit_leave(ir_loop *ir);
 164
 165    loop_state *state;
 166
 167    bool progress;
 168 };
 169
 170
 171 ir_visitor_status
 172 loop_control_visitor::visit_leave(ir_loop *ir)
 173 {
 174    loop_variable_state *const ls = this->state->get(ir);
 175
 176    /* If we've entered a loop that hasn't been analyzed, something really,
 177     * really bad has happened.
 178     */
 179    if (ls == NULL) {
 180       assert(ls != NULL);
 181       return visit_continue;
 182    }
 183
 184    /* Search the loop terminating conditions for one of the form 'i < c' where
 185     * i is a loop induction variable, c is a constant, and < is any relative
 186     * operator.
 187     */
 188    int max_iterations = ls->max_iterations;
 189
 190    if(ir->from && ir->to && ir->increment)
 191       max_iterations = calculate_iterations(ir->from, ir->to, ir->increment, (ir_expression_operation)ir->cmp);
 192
 193    if(max_iterations < 0)
 194       max_iterations = INT_MAX;
 195
 196    foreach_list(node, &ls->terminators) {
 197       loop_terminator *t = (loop_terminator *) node;
 198       ir_if *if_stmt = t->ir;
 199
 200       /* If-statements can be either 'if (expr)' or 'if (deref)'.  We only care
 201        * about the former here.
 202        */
 203       ir_expression *cond = if_stmt->condition->as_expression();
 204       if (cond == NULL)
 205          continue;
 206
 207       switch (cond->operation) {
 208       case ir_binop_less:
 209       case ir_binop_greater:
 210       case ir_binop_lequal:
 211       case ir_binop_gequal: {
 212          /* The expressions that we care about will either be of the form
 213           * 'counter < limit' or 'limit < counter'.  Figure out which is
 214           * which.
 215           */
 216          ir_rvalue *counter = cond->operands[0]->as_dereference_variable();
 217          ir_constant *limit = cond->operands[1]->as_constant();
 218          enum ir_expression_operation cmp = cond->operation;
 219
 220          if (limit == NULL) {
 221             counter = cond->operands[1]->as_dereference_variable();
 222             limit = cond->operands[0]->as_constant();
 223
 224             switch (cmp) {
 225             case ir_binop_less:    cmp = ir_binop_gequal;  break;
 226             case ir_binop_greater: cmp = ir_binop_lequal;  break;
 227             case ir_binop_lequal:  cmp = ir_binop_greater; break;
 228             case ir_binop_gequal:  cmp = ir_binop_less;    break;
 229             default: assert(!"Should not get here.");
 230             }
 231          }
 232
 233          if ((counter == NULL) || (limit == NULL))
 234             break;
 235
 236          ir_variable *var = counter->variable_referenced();
 237
 238          ir_rvalue *init = find_initial_value(ir, var);
 239
 240          foreach_list(iv_node, &ls->induction_variables) {
 241             loop_variable *lv = (loop_variable *) iv_node;
 242
 243             if (lv->var == var) {
 244                const int iterations = calculate_iterations(init, limit,
 245                                                            lv->increment,
 246                                                            cmp);
 247                if (iterations >= 0) {
 248                   /* If the new iteration count is lower than the previously
 249                    * believed iteration count, update the loop control values.
 250                    */
 251                   if (iterations < max_iterations) {
 252                      ir->from = init->clone(ir, NULL);
 253                      ir->to = limit->clone(ir, NULL);
 254                      ir->increment = lv->increment->clone(ir, NULL);
 255                      ir->counter = lv->var;
 256                      ir->cmp = cmp;
 257
 258                      max_iterations = iterations;
 259                   }
 260
 261                   /* Remove the conditional break statement.  The loop
 262                    * controls are now set such that the exit condition will be
 263                    * satisfied.
 264                    */
 265                   if_stmt->remove();
 266
 267                   assert(ls->num_loop_jumps > 0);
 268                   ls->num_loop_jumps--;
 269
 270                   this->progress = true;
 271                }
 272
 273                break;
 274             }
 275          }
 276          break;
 277       }
 278
 279       default:
 280          break;
 281       }
 282    }
 283
 284    /* If we have proven the one of the loop exit conditions is satisifed before
 285     * running the loop once, remove the loop.
 286     */
 287    if (max_iterations == 0)
 288       ir->remove();
 289    else
 290       ls->max_iterations = max_iterations;
 291
 292    return visit_continue;
 293 }
 294
 295
 296 bool
 297 set_loop_controls(exec_list *instructions, loop_state *ls)
 298 {
 299    loop_control_visitor v(ls);
 300
 301    v.run(instructions);
 302
 303    return v.progress;
 304 }