src/glsl/loop_unroll.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 "glsl_types.h"
  25 #include "loop_analysis.h"
  26 #include "ir_hierarchical_visitor.h"
  27
  28 #include "main/mtypes.h"
  29
  30 namespace {
  31
  32 class loop_unroll_visitor : public ir_hierarchical_visitor {
  33 public:
  34    loop_unroll_visitor(loop_state *state,
  35                        const struct gl_shader_compiler_options *options)
  36    {
  37       this->state = state;
  38       this->progress = false;
  39       this->options = options;
  40    }
  41
  42    virtual ir_visitor_status visit_leave(ir_loop *ir);
  43    void simple_unroll(ir_loop *ir, int iterations);
  44    void complex_unroll(ir_loop *ir, int iterations,
  45                        bool continue_from_then_branch);
  46    void splice_post_if_instructions(ir_if *ir_if, exec_list *splice_dest);
  47
  48    loop_state *state;
  49
  50    bool progress;
  51    const struct gl_shader_compiler_options *options;
  52 };
  53
  54 } /* anonymous namespace */
  55
  56 static bool
  57 is_break(ir_instruction *ir)
  58 {
  59    return ir != NULL && ir->ir_type == ir_type_loop_jump
  60                      && ((ir_loop_jump *) ir)->is_break();
  61 }
  62
  63 class loop_unroll_count : public ir_hierarchical_visitor {
  64 public:
  65    int nodes;
  66    bool unsupported_variable_indexing;
  67    bool array_indexed_by_induction_var_with_exact_iterations;
  68    /* If there are nested loops, the node count will be inaccurate. */
  69    bool nested_loop;
  70
  71    loop_unroll_count(exec_list *list, loop_variable_state *ls,
  72                      const struct gl_shader_compiler_options *options)
  73       : ls(ls), options(options)
  74    {
  75       nodes = 0;
  76       nested_loop = false;
  77       unsupported_variable_indexing = false;
  78       array_indexed_by_induction_var_with_exact_iterations = false;
  79
  80       run(list);
  81    }
  82
  83    virtual ir_visitor_status visit_enter(ir_assignment *)
  84    {
  85       nodes++;
  86       return visit_continue;
  87    }
  88
  89    virtual ir_visitor_status visit_enter(ir_expression *)
  90    {
  91       nodes++;
  92       return visit_continue;
  93    }
  94
  95    virtual ir_visitor_status visit_enter(ir_loop *)
  96    {
  97       nested_loop = true;
  98       return visit_continue;
  99    }
 100
 101    virtual ir_visitor_status visit_enter(ir_dereference_array *ir)
 102    {
 103       /* Check for arrays variably-indexed by a loop induction variable.
 104        * Unrolling the loop may convert that access into constant-indexing.
 105        *
 106        * Many drivers don't support particular kinds of variable indexing,
 107        * and have to resort to using lower_variable_index_to_cond_assign to
 108        * handle it.  This results in huge amounts of horrible code, so we'd
 109        * like to avoid that if possible.  Here, we just note that it will
 110        * happen.
 111        */
 112       if ((ir->array->type->is_array() || ir->array->type->is_matrix()) &&
 113           !ir->array_index->as_constant()) {
 114          ir_variable *array = ir->array->variable_referenced();
 115          loop_variable *lv = ls->get(ir->array_index->variable_referenced());
 116          if (array && lv && lv->is_induction_var()) {
 117             /* If an array is indexed by a loop induction variable, and the
 118              * array size is exactly the number of loop iterations, this is
 119              * probably a simple for-loop trying to access each element in
 120              * turn; the application may expect it to be unrolled.
 121              */
 122             if (int(array->type->length) == ls->limiting_terminator->iterations)
 123                array_indexed_by_induction_var_with_exact_iterations = true;
 124
 125             switch (array->data.mode) {
 126             case ir_var_auto:
 127             case ir_var_temporary:
 128             case ir_var_const_in:
 129             case ir_var_function_in:
 130             case ir_var_function_out:
 131             case ir_var_function_inout:
 132                if (options->EmitNoIndirectTemp)
 133                   unsupported_variable_indexing = true;
 134                break;
 135             case ir_var_uniform:
 136                if (options->EmitNoIndirectUniform)
 137                   unsupported_variable_indexing = true;
 138                break;
 139             case ir_var_shader_in:
 140                if (options->EmitNoIndirectInput)
 141                   unsupported_variable_indexing = true;
 142                break;
 143             case ir_var_shader_out:
 144                if (options->EmitNoIndirectOutput)
 145                   unsupported_variable_indexing = true;
 146                break;
 147             }
 148          }
 149       }
 150       return visit_continue;
 151    }
 152
 153 private:
 154    loop_variable_state *ls;
 155    const struct gl_shader_compiler_options *options;
 156 };
 157
 158
 159 /**
 160  * Unroll a loop which does not contain any jumps.  For example, if the input
 161  * is:
 162  *
 163  *     (loop (...) ...instrs...)
 164  *
 165  * And the iteration count is 3, the output will be:
 166  *
 167  *     ...instrs... ...instrs... ...instrs...
 168  */
 169 void
 170 loop_unroll_visitor::simple_unroll(ir_loop *ir, int iterations)
 171 {
 172    void *const mem_ctx = ralloc_parent(ir);
 173
 174    for (int i = 0; i < iterations; i++) {
 175       exec_list copy_list;
 176
 177       copy_list.make_empty();
 178       clone_ir_list(mem_ctx, &copy_list, &ir->body_instructions);
 179
 180       ir->insert_before(&copy_list);
 181    }
 182
 183    /* The loop has been replaced by the unrolled copies.  Remove the original
 184     * loop from the IR sequence.
 185     */
 186    ir->remove();
 187
 188    this->progress = true;
 189 }
 190
 191
 192 /**
 193  * Unroll a loop whose last statement is an ir_if.  If \c
 194  * continue_from_then_branch is true, the loop is repeated only when the
 195  * "then" branch of the if is taken; otherwise it is repeated only when the
 196  * "else" branch of the if is taken.
 197  *
 198  * For example, if the input is:
 199  *
 200  *     (loop (...)
 201  *      ...body...
 202  *      (if (cond)
 203  *          (...then_instrs...)
 204  *        (...else_instrs...)))
 205  *
 206  * And the iteration count is 3, and \c continue_from_then_branch is true,
 207  * then the output will be:
 208  *
 209  *     ...body...
 210  *     (if (cond)
 211  *         (...then_instrs...
 212  *          ...body...
 213  *          (if (cond)
 214  *              (...then_instrs...
 215  *               ...body...
 216  *               (if (cond)
 217  *                   (...then_instrs...)
 218  *                 (...else_instrs...)))
 219  *            (...else_instrs...)))
 220  *       (...else_instrs))
 221  */
 222 void
 223 loop_unroll_visitor::complex_unroll(ir_loop *ir, int iterations,
 224                                     bool continue_from_then_branch)
 225 {
 226    void *const mem_ctx = ralloc_parent(ir);
 227    ir_instruction *ir_to_replace = ir;
 228
 229    for (int i = 0; i < iterations; i++) {
 230       exec_list copy_list;
 231
 232       copy_list.make_empty();
 233       clone_ir_list(mem_ctx, &copy_list, &ir->body_instructions);
 234
 235       ir_if *ir_if = ((ir_instruction *) copy_list.get_tail())->as_if();
 236       assert(ir_if != NULL);
 237
 238       ir_to_replace->insert_before(&copy_list);
 239       ir_to_replace->remove();
 240
 241       /* placeholder that will be removed in the next iteration */
 242       ir_to_replace =
 243          new(mem_ctx) ir_loop_jump(ir_loop_jump::jump_continue);
 244
 245       exec_list *const list = (continue_from_then_branch)
 246          ? &ir_if->then_instructions : &ir_if->else_instructions;
 247
 248       list->push_tail(ir_to_replace);
 249    }
 250
 251    ir_to_replace->remove();
 252
 253    this->progress = true;
 254 }
 255
 256
 257 /**
 258  * Move all of the instructions which follow \c ir_if to the end of
 259  * \c splice_dest.
 260  *
 261  * For example, in the code snippet:
 262  *
 263  *     (if (cond)
 264  *         (...then_instructions...
 265  *          break)
 266  *       (...else_instructions...))
 267  *     ...post_if_instructions...
 268  *
 269  * If \c ir_if points to the "if" instruction, and \c splice_dest points to
 270  * (...else_instructions...), the code snippet is transformed into:
 271  *
 272  *     (if (cond)
 273  *         (...then_instructions...
 274  *          break)
 275  *       (...else_instructions...
 276  *        ...post_if_instructions...))
 277  */
 278 void
 279 loop_unroll_visitor::splice_post_if_instructions(ir_if *ir_if,
 280                                                  exec_list *splice_dest)
 281 {
 282    while (!ir_if->get_next()->is_tail_sentinel()) {
 283       ir_instruction *move_ir = (ir_instruction *) ir_if->get_next();
 284
 285       move_ir->remove();
 286       splice_dest->push_tail(move_ir);
 287    }
 288 }
 289
 290
 291 ir_visitor_status
 292 loop_unroll_visitor::visit_leave(ir_loop *ir)
 293 {
 294    loop_variable_state *const ls = this->state->get(ir);
 295    int iterations;
 296
 297    /* If we've entered a loop that hasn't been analyzed, something really,
 298     * really bad has happened.
 299     */
 300    if (ls == NULL) {
 301       assert(ls != NULL);
 302       return visit_continue;
 303    }
 304
 305    /* Don't try to unroll loops where the number of iterations is not known
 306     * at compile-time.
 307     */
 308    if (ls->limiting_terminator == NULL)
 309       return visit_continue;
 310
 311    iterations = ls->limiting_terminator->iterations;
 312
 313    const int max_iterations = options->MaxUnrollIterations;
 314
 315    /* Don't try to unroll loops that have zillions of iterations either.
 316     */
 317    if (iterations > max_iterations)
 318       return visit_continue;
 319
 320    /* Don't try to unroll nested loops and loops with a huge body.
 321     */
 322    loop_unroll_count count(&ir->body_instructions, ls, options);
 323
 324    bool loop_too_large =
 325       count.nested_loop || count.nodes * iterations > max_iterations * 5;
 326
 327    if (loop_too_large && !count.unsupported_variable_indexing &&
 328        !count.array_indexed_by_induction_var_with_exact_iterations)
 329       return visit_continue;
 330
 331    /* Note: the limiting terminator contributes 1 to ls->num_loop_jumps.
 332     * We'll be removing the limiting terminator before we unroll.
 333     */
 334    assert(ls->num_loop_jumps > 0);
 335    unsigned predicted_num_loop_jumps = ls->num_loop_jumps - 1;
 336
 337    if (predicted_num_loop_jumps > 1)
 338       return visit_continue;
 339
 340    if (predicted_num_loop_jumps == 0) {
 341       ls->limiting_terminator->ir->remove();
 342       simple_unroll(ir, iterations);
 343       return visit_continue;
 344    }
 345
 346    ir_instruction *last_ir = (ir_instruction *) ir->body_instructions.get_tail();
 347    assert(last_ir != NULL);
 348
 349    if (is_break(last_ir)) {
 350       /* If the only loop-jump is a break at the end of the loop, the loop
 351        * will execute exactly once.  Remove the break and use the simple
 352        * unroller with an iteration count of 1.
 353        */
 354       last_ir->remove();
 355
 356       ls->limiting_terminator->ir->remove();
 357       simple_unroll(ir, 1);
 358       return visit_continue;
 359    }
 360
 361    /* recognize loops in the form produced by ir_lower_jumps */
 362    foreach_in_list(ir_instruction, cur_ir, &ir->body_instructions) {
 363       /* Skip the limiting terminator, since it will go away when we
 364        * unroll.
 365        */
 366       if (cur_ir == ls->limiting_terminator->ir)
 367          continue;
 368
 369       ir_if *ir_if = cur_ir->as_if();
 370       if (ir_if != NULL) {
 371          /* Determine which if-statement branch, if any, ends with a
 372           * break.  The branch that did *not* have the break will get a
 373           * temporary continue inserted in each iteration of the loop
 374           * unroll.
 375           *
 376           * Note that since ls->num_loop_jumps is <= 1, it is impossible
 377           * for both branches to end with a break.
 378           */
 379          ir_instruction *ir_if_last =
 380             (ir_instruction *) ir_if->then_instructions.get_tail();
 381
 382          if (is_break(ir_if_last)) {
 383             ls->limiting_terminator->ir->remove();
 384             splice_post_if_instructions(ir_if, &ir_if->else_instructions);
 385             ir_if_last->remove();
 386             complex_unroll(ir, iterations, false);
 387             return visit_continue;
 388          } else {
 389             ir_if_last =
 390                (ir_instruction *) ir_if->else_instructions.get_tail();
 391
 392             if (is_break(ir_if_last)) {
 393                ls->limiting_terminator->ir->remove();
 394                splice_post_if_instructions(ir_if, &ir_if->then_instructions);
 395                ir_if_last->remove();
 396                complex_unroll(ir, iterations, true);
 397                return visit_continue;
 398             }
 399          }
 400       }
 401    }
 402
 403    /* Did not find the break statement.  It must be in a complex if-nesting,
 404     * so don't try to unroll.
 405     */
 406    return visit_continue;
 407 }
 408
 409
 410 bool
 411 unroll_loops(exec_list *instructions, loop_state *ls,
 412              const struct gl_shader_compiler_options *options)
 413 {
 414    loop_unroll_visitor v(ls, options);
 415
 416    v.run(instructions);
 417
 418    return v.progress;
 419 }