src/glsl/nir/nir_opt_dead_cf.c

   1 /*
   2  * Copyright © 2014 Connor Abbott
   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 DEALINGS
  21  * IN THE SOFTWARE.
  22  *
  23  * Authors:
  24  *    Connor Abbott (cwabbott0@gmail.com)
  25  *
  26  */
  27
  28 #include "nir.h"
  29 #include "nir_control_flow.h"
  30
  31 /*
  32  * This file implements an optimization that deletes statically unreachable
  33  * code. In NIR, one way this can happen if if an if statement has a constant
  34  * condition:
  35  *
  36  * if (true) {
  37  *    ...
  38  * }
  39  *
  40  * We delete the if statement and paste the contents of the always-executed
  41  * branch into the surrounding control flow, possibly removing more code if
  42  * the branch had a jump at the end.
  43  *
  44  * The other way is that control flow can end in a jump so that code after it
  45  * never gets executed. In particular, this can happen after optimizing
  46  * something like:
  47  *
  48  * if (true) {
  49  *    ...
  50  *    break;
  51  * }
  52  * ...
  53  *
  54  * We also consider the case where both branches of an if end in a jump, e.g.:
  55  *
  56  * if (...) {
  57  *    break;
  58  * } else {
  59  *    continue;
  60  * }
  61  * ...
  62  *
  63  */
  64
  65 static void
  66 remove_after_cf_node(nir_cf_node *node)
  67 {
  68    nir_cf_node *end = node;
  69    while (!nir_cf_node_is_last(end))
  70       end = nir_cf_node_next(end);
  71
  72    nir_cf_list list;
  73    nir_cf_extract(&list, nir_after_cf_node(node), nir_after_cf_node(end));
  74    nir_cf_delete(&list);
  75 }
  76
  77 static void
  78 opt_constant_if(nir_if *if_stmt, bool condition)
  79 {
  80    void *mem_ctx = ralloc_parent(if_stmt);
  81
  82    /* First, we need to remove any phi nodes after the if by rewriting uses to
  83     * point to the correct source.
  84     */
  85    nir_block *after = nir_cf_node_as_block(nir_cf_node_next(&if_stmt->cf_node));
  86    nir_block *last_block =
  87       nir_cf_node_as_block(condition ? nir_if_last_then_node(if_stmt)
  88                                      : nir_if_last_else_node(if_stmt));
  89
  90    nir_foreach_instr_safe(after, instr) {
  91       if (instr->type != nir_instr_type_phi)
  92          break;
  93
  94       nir_phi_instr *phi = nir_instr_as_phi(instr);
  95       nir_ssa_def *def = NULL;
  96       nir_foreach_phi_src(phi, phi_src) {
  97          if (phi_src->pred != last_block)
  98             continue;
  99
 100          assert(phi_src->src.is_ssa);
 101          def = phi_src->src.ssa;
 102       }
 103
 104       assert(def);
 105       assert(phi->dest.is_ssa);
 106       nir_ssa_def_rewrite_uses(&phi->dest.ssa, nir_src_for_ssa(def), mem_ctx);
 107       nir_instr_remove(instr);
 108    }
 109
 110    /* The control flow list we're about to paste in may include a jump at the
 111     * end, and in that case we have to delete the rest of the control flow
 112     * list after the if since it's unreachable and the validator will balk if
 113     * we don't.
 114     */
 115
 116    if (!exec_list_is_empty(&last_block->instr_list)) {
 117       nir_instr *last_instr = nir_block_last_instr(last_block);
 118       if (last_instr->type == nir_instr_type_jump)
 119          remove_after_cf_node(&if_stmt->cf_node);
 120    }
 121
 122    /* Finally, actually paste in the then or else branch and delete the if. */
 123    struct exec_list *cf_list = condition ? &if_stmt->then_list
 124                                          : &if_stmt->else_list;
 125
 126    nir_cf_list list;
 127    nir_cf_extract(&list, nir_before_cf_list(cf_list),
 128                   nir_after_cf_list(cf_list));
 129    nir_cf_reinsert(&list, nir_after_cf_node(&if_stmt->cf_node));
 130    nir_cf_node_remove(&if_stmt->cf_node);
 131 }
 132
 133 static bool
 134 dead_cf_block(nir_block *block)
 135 {
 136    nir_if *following_if = nir_block_get_following_if(block);
 137    if (!following_if)
 138       return false;
 139
 140   nir_const_value *const_value =
 141      nir_src_as_const_value(following_if->condition);
 142
 143   if (!const_value)
 144      return false;
 145
 146    opt_constant_if(following_if, const_value->u[0] != 0);
 147    return true;
 148 }
 149
 150 static bool
 151 ends_in_jump(nir_block *block)
 152 {
 153    if (exec_list_is_empty(&block->instr_list))
 154       return false;
 155
 156    nir_instr *instr = nir_block_last_instr(block);
 157    return instr->type == nir_instr_type_jump;
 158 }
 159
 160 static bool
 161 dead_cf_list(struct exec_list *list, bool *list_ends_in_jump)
 162 {
 163    bool progress = false;
 164    *list_ends_in_jump = false;
 165
 166    nir_cf_node *prev = NULL;
 167
 168    foreach_list_typed(nir_cf_node, cur, node, list) {
 169       switch (cur->type) {
 170       case nir_cf_node_block: {
 171          nir_block *block = nir_cf_node_as_block(cur);
 172          if (dead_cf_block(block)) {
 173             /* We just deleted the if after this block, so we may have
 174              * deleted the block before or after it -- which one is an
 175              * implementation detail. Therefore, to recover the place we were
 176              * at, we have to use the previous cf_node.
 177              */
 178
 179             if (prev) {
 180                cur = nir_cf_node_next(prev);
 181             } else {
 182                cur = exec_node_data(nir_cf_node, exec_list_get_head(list),
 183                                     node);
 184             }
 185
 186             block = nir_cf_node_as_block(cur);
 187
 188             progress = true;
 189          }
 190
 191          if (ends_in_jump(block)) {
 192             *list_ends_in_jump = true;
 193
 194             if (!exec_node_is_tail_sentinel(cur->node.next)) {
 195                remove_after_cf_node(cur);
 196                return true;
 197             }
 198          }
 199
 200          break;
 201       }
 202
 203       case nir_cf_node_if: {
 204          nir_if *if_stmt = nir_cf_node_as_if(cur);
 205          bool then_ends_in_jump, else_ends_in_jump;
 206          progress |= dead_cf_list(&if_stmt->then_list, &then_ends_in_jump);
 207          progress |= dead_cf_list(&if_stmt->else_list, &else_ends_in_jump);
 208
 209          if (then_ends_in_jump && else_ends_in_jump) {
 210             *list_ends_in_jump = true;
 211             nir_block *next = nir_cf_node_as_block(nir_cf_node_next(cur));
 212             if (!exec_list_is_empty(&next->instr_list) ||
 213                 !exec_node_is_tail_sentinel(next->cf_node.node.next)) {
 214                remove_after_cf_node(cur);
 215                return true;
 216             }
 217          }
 218
 219          break;
 220       }
 221
 222       case nir_cf_node_loop: {
 223          nir_loop *loop = nir_cf_node_as_loop(cur);
 224          bool dummy;
 225          progress |= dead_cf_list(&loop->body, &dummy);
 226
 227          break;
 228       }
 229
 230       default:
 231          unreachable("unknown cf node type");
 232       }
 233
 234       prev = cur;
 235    }
 236
 237    return progress;
 238 }
 239
 240 static bool
 241 opt_dead_cf_impl(nir_function_impl *impl)
 242 {
 243    bool dummy;
 244    bool progress = dead_cf_list(&impl->body, &dummy);
 245
 246    if (progress)
 247       nir_metadata_preserve(impl, nir_metadata_none);
 248
 249    return progress;
 250 }
 251
 252 bool
 253 nir_opt_dead_cf(nir_shader *shader)
 254 {
 255    bool progress = false;
 256
 257    nir_foreach_overload(shader, overload)
 258       if (overload->impl)
 259          progress |= opt_dead_cf_impl(overload->impl);
 260
 261    return progress;
 262 }