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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 lower_if_to_cond_assign.cpp
   26  *
   27  * This flattens if-statements to conditional assignments if:
   28  *
   29  * - the GPU has limited or no flow control support
   30  *   (controlled by max_depth)
   31  *
   32  * - small conditional branches are more expensive than conditional assignments
   33  *   (controlled by min_branch_cost, that's the cost for a branch to be
   34  *    preserved)
   35  *
   36  * It can't handle other control flow being inside of its block, such
   37  * as calls or loops.  Hopefully loop unrolling and inlining will take
   38  * care of those.
   39  *
   40  * Drivers for GPUs with no control flow support should simply call
   41  *
   42  *    lower_if_to_cond_assign(instructions)
   43  *
   44  * to attempt to flatten all if-statements.
   45  *
   46  * Some GPUs (such as i965 prior to gen6) do support control flow, but have a
   47  * maximum nesting depth N.  Drivers for such hardware can call
   48  *
   49  *    lower_if_to_cond_assign(instructions, N)
   50  *
   51  * to attempt to flatten any if-statements appearing at depth > N.
   52  */
   53 
   54 #include "compiler/glsl_types.h"
   55 #include "ir.h"
   56 #include "util/set.h"
   57 #include "util/hash_table.h" /* Needed for the hashing functions */
   58 #include "main/macros.h" /* for MAX2 */
   59 
   60 namespace {
   61 
   62 class ir_if_to_cond_assign_visitor : public ir_hierarchical_visitor {
   63 public:
   64    ir_if_to_cond_assign_visitor(gl_shader_stage stage,
   65                                 unsigned max_depth,
   66                                 unsigned min_branch_cost)
   67    {
   68       this->progress = false;
   69       this->stage = stage;
   70       this->max_depth = max_depth;
   71       this->min_branch_cost = min_branch_cost;
   72       this->depth = 0;
   73 
   74       this->condition_variables = _mesa_pointer_set_create(NULL);
   75    }
   76 
   77    ~ir_if_to_cond_assign_visitor()
   78    {
   79       _mesa_set_destroy(this->condition_variables, NULL);
   80    }
   81 
   82    ir_visitor_status visit_enter(ir_if *);
   83    ir_visitor_status visit_leave(ir_if *);
   84 
   85    bool found_unsupported_op;
   86    bool found_expensive_op;
   87    bool found_dynamic_arrayref;
   88    bool is_then;
   89    bool progress;
   90    gl_shader_stage stage;
   91    unsigned then_cost;
   92    unsigned else_cost;
   93    unsigned min_branch_cost;
   94    unsigned max_depth;
   95    unsigned depth;
   96 
   97    struct set *condition_variables;
   98 };
   99 
  100 } /* anonymous namespace */
  101 
  102 bool
  103 lower_if_to_cond_assign(gl_shader_stage stage, exec_list *instructions,
  104                         unsigned max_depth, unsigned min_branch_cost)
  105 {
  106    if (max_depth == UINT_MAX)
  107       return false;
  108 
  109    ir_if_to_cond_assign_visitor v(stage, max_depth, min_branch_cost);
  110 
  111    visit_list_elements(&v, instructions);
  112 
  113    return v.progress;
  114 }
  115 
  116 static void
  117 check_ir_node(ir_instruction *ir, void *data)
  118 {
  119    ir_if_to_cond_assign_visitor *v = (ir_if_to_cond_assign_visitor *)data;
  120 
  121    switch (ir->ir_type) {
  122    case ir_type_call:
  123    case ir_type_discard:
  124    case ir_type_loop:
  125    case ir_type_loop_jump:
  126    case ir_type_return:
  127    case ir_type_emit_vertex:
  128    case ir_type_end_primitive:
  129    case ir_type_barrier:
  130       v->found_unsupported_op = true;
  131       break;
  132 
  133    case ir_type_dereference_variable: {
  134       ir_variable *var = ir->as_dereference_variable()->variable_referenced();
  135 
  136       /* Lowering branches with TCS output accesses breaks many piglit tests,
  137        * so don't touch them for now.
  138        */
  139       if (v->stage == MESA_SHADER_TESS_CTRL &&
  140           var->data.mode == ir_var_shader_out)
  141          v->found_unsupported_op = true;
  142       break;
  143    }
  144 
  145    /* SSBO, images, atomic counters are handled by ir_type_call */
  146    case ir_type_texture:
  147       v->found_expensive_op = true;
  148       break;
  149 
  150    case ir_type_dereference_array: {
  151       ir_dereference_array *deref = ir->as_dereference_array();
  152 
  153       if (deref->array_index->ir_type != ir_type_constant)
  154          v->found_dynamic_arrayref = true;
  155    } /* fall-through */
  156    case ir_type_expression:
  157    case ir_type_dereference_record:
  158       if (v->is_then)
  159          v->then_cost++;
  160       else
  161          v->else_cost++;
  162       break;
  163 
  164    default:
  165       break;
  166    }
  167 }
  168 
  169 static void
  170 move_block_to_cond_assign(void *mem_ctx,
  171                           ir_if *if_ir, ir_rvalue *cond_expr,
  172                           exec_list *instructions,
  173                           struct set *set)
  174 {
  175    foreach_in_list_safe(ir_instruction, ir, instructions) {
  176       if (ir->ir_type == ir_type_assignment) {
  177          ir_assignment *assign = (ir_assignment *)ir;
  178 
  179          if (_mesa_set_search(set, assign) == NULL) {
  180             _mesa_set_add(set, assign);
  181 
  182             /* If the LHS of the assignment is a condition variable that was
  183              * previously added, insert an additional assignment of false to
  184              * the variable.
  185              */
  186             const bool assign_to_cv =
  187                _mesa_set_search(
  188                   set, assign->lhs->variable_referenced()) != NULL;
  189 
  190             if (!assign->condition) {
  191                if (assign_to_cv) {
  192                   assign->rhs =
  193                      new(mem_ctx) ir_expression(ir_binop_logic_and,
  194                                                 glsl_type::bool_type,
  195                                                 cond_expr->clone(mem_ctx, NULL),
  196                                                 assign->rhs);
  197                } else {
  198                   assign->condition = cond_expr->clone(mem_ctx, NULL);
  199                }
  200             } else {
  201                assign->condition =
  202                   new(mem_ctx) ir_expression(ir_binop_logic_and,
  203                                              glsl_type::bool_type,
  204                                              cond_expr->clone(mem_ctx, NULL),
  205                                              assign->condition);
  206             }
  207          }
  208       }
  209 
  210       /* Now, move from the if block to the block surrounding it. */
  211       ir->remove();
  212       if_ir->insert_before(ir);
  213    }
  214 }
  215 
  216 ir_visitor_status
  217 ir_if_to_cond_assign_visitor::visit_enter(ir_if *)
  218 {
  219    this->depth++;
  220 
  221    return visit_continue;
  222 }
  223 
  224 ir_visitor_status
  225 ir_if_to_cond_assign_visitor::visit_leave(ir_if *ir)
  226 {
  227    bool must_lower = this->depth-- > this->max_depth;
  228 
  229    /* Only flatten when beyond the GPU's maximum supported nesting depth. */
  230    if (!must_lower && this->min_branch_cost == 0)
  231       return visit_continue;
  232 
  233    this->found_unsupported_op = false;
  234    this->found_expensive_op = false;
  235    this->found_dynamic_arrayref = false;
  236    this->then_cost = 0;
  237    this->else_cost = 0;
  238 
  239    ir_assignment *assign;
  240 
  241    /* Check that both blocks don't contain anything we can't support. */
  242    this->is_then = true;
  243    foreach_in_list(ir_instruction, then_ir, &ir->then_instructions) {
  244       visit_tree(then_ir, check_ir_node, this);
  245    }
  246 
  247    this->is_then = false;
  248    foreach_in_list(ir_instruction, else_ir, &ir->else_instructions) {
  249       visit_tree(else_ir, check_ir_node, this);
  250    }
  251 
  252    if (this->found_unsupported_op)
  253       return visit_continue; /* can't handle inner unsupported opcodes */
  254 
  255    /* Skip if the branch cost is high enough or if there's an expensive op.
  256     *
  257     * Also skip if non-constant array indices were encountered, since those
  258     * can be out-of-bounds for a not-taken branch, and so generating an
  259     * assignment would be incorrect. In the case of must_lower, it's up to the
  260     * backend to deal with any potential fall-out (perhaps by translating the
  261     * assignments to hardware-predicated moves).
  262     */
  263    if (!must_lower &&
  264        (this->found_expensive_op ||
  265         this->found_dynamic_arrayref ||
  266         MAX2(this->then_cost, this->else_cost) >= this->min_branch_cost))
  267       return visit_continue;
  268 
  269    void *mem_ctx = ralloc_parent(ir);
  270 
  271    /* Store the condition to a variable.  Move all of the instructions from
  272     * the then-clause of the if-statement.  Use the condition variable as a
  273     * condition for all assignments.
  274     */
  275    ir_variable *const then_var =
  276       new(mem_ctx) ir_variable(glsl_type::bool_type,
  277                                "if_to_cond_assign_then",
  278                                ir_var_temporary);
  279    ir->insert_before(then_var);
  280 
  281    ir_dereference_variable *then_cond =
  282       new(mem_ctx) ir_dereference_variable(then_var);
  283 
  284    assign = new(mem_ctx) ir_assignment(then_cond, ir->condition);
  285    ir->insert_before(assign);
  286 
  287    move_block_to_cond_assign(mem_ctx, ir, then_cond,
  288                              &ir->then_instructions,
  289                              this->condition_variables);
  290 
  291    /* Add the new condition variable to the hash table.  This allows us to
  292     * find this variable when lowering other (enclosing) if-statements.
  293     */
  294    _mesa_set_add(this->condition_variables, then_var);
  295 
  296    /* If there are instructions in the else-clause, store the inverse of the
  297     * condition to a variable.  Move all of the instructions from the
  298     * else-clause if the if-statement.  Use the (inverse) condition variable
  299     * as a condition for all assignments.
  300     */
  301    if (!ir->else_instructions.is_empty()) {
  302       ir_variable *const else_var =
  303          new(mem_ctx) ir_variable(glsl_type::bool_type,
  304                                   "if_to_cond_assign_else",
  305                                   ir_var_temporary);
  306       ir->insert_before(else_var);
  307 
  308       ir_dereference_variable *else_cond =
  309          new(mem_ctx) ir_dereference_variable(else_var);
  310 
  311       ir_rvalue *inverse =
  312          new(mem_ctx) ir_expression(ir_unop_logic_not,
  313                                     then_cond->clone(mem_ctx, NULL));
  314 
  315       assign = new(mem_ctx) ir_assignment(else_cond, inverse);
  316       ir->insert_before(assign);
  317 
  318       move_block_to_cond_assign(mem_ctx, ir, else_cond,
  319                                 &ir->else_instructions,
  320                                 this->condition_variables);
  321 
  322       /* Add the new condition variable to the hash table.  This allows us to
  323        * find this variable when lowering other (enclosing) if-statements.
  324        */
  325       _mesa_set_add(this->condition_variables, else_var);
  326    }
  327 
  328    ir->remove();
  329 
  330    this->progress = true;
  331 
  332    return visit_continue;
  333 }