src/compiler/nir/nir_liveness.c

   1 /*
   2  * Copyright © 2014 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 DEALINGS
  21  * IN THE SOFTWARE.
  22  *
  23  * Authors:
  24  *    Jason Ekstrand (jason@jlekstrand.net)
  25  */
  26
  27 #include "nir.h"
  28 #include "nir_worklist.h"
  29 #include "nir_vla.h"
  30
  31 /*
  32  * Basic liveness analysis.  This works only in SSA form.
  33  *
  34  * This liveness pass treats phi nodes as being melded to the space between
  35  * blocks so that the destinations of a phi are in the livein of the block
  36  * in which it resides and the sources are in the liveout of the
  37  * corresponding block.  By formulating the liveness information in this
  38  * way, we ensure that the definition of any variable dominates its entire
  39  * live range.  This is true because the only way that the definition of an
  40  * SSA value may not dominate a use is if the use is in a phi node and the
  41  * uses in phi no are in the live-out of the corresponding predecessor
  42  * block but not in the live-in of the block containing the phi node.
  43  */
  44
  45 struct live_ssa_defs_state {
  46    unsigned num_ssa_defs;
  47    unsigned bitset_words;
  48
  49    nir_block_worklist worklist;
  50 };
  51
  52 static bool
  53 index_ssa_def(nir_ssa_def *def, void *void_state)
  54 {
  55    struct live_ssa_defs_state *state = void_state;
  56
  57    if (def->parent_instr->type == nir_instr_type_ssa_undef)
  58       def->live_index = 0;
  59    else
  60       def->live_index = state->num_ssa_defs++;
  61
  62    return true;
  63 }
  64
  65 static bool
  66 index_ssa_definitions_block(nir_block *block, void *state)
  67 {
  68    nir_foreach_instr(block, instr)
  69       nir_foreach_ssa_def(instr, index_ssa_def, state);
  70
  71    return true;
  72 }
  73
  74 /* Initialize the liveness data to zero and add the given block to the
  75  * worklist.
  76  */
  77 static bool
  78 init_liveness_block(nir_block *block, void *void_state)
  79 {
  80    struct live_ssa_defs_state *state = void_state;
  81
  82    block->live_in = reralloc(block, block->live_in, BITSET_WORD,
  83                              state->bitset_words);
  84    memset(block->live_in, 0, state->bitset_words * sizeof(BITSET_WORD));
  85
  86    block->live_out = reralloc(block, block->live_out, BITSET_WORD,
  87                               state->bitset_words);
  88    memset(block->live_out, 0, state->bitset_words * sizeof(BITSET_WORD));
  89
  90    nir_block_worklist_push_head(&state->worklist, block);
  91
  92    return true;
  93 }
  94
  95 static bool
  96 set_src_live(nir_src *src, void *void_live)
  97 {
  98    BITSET_WORD *live = void_live;
  99
 100    if (!src->is_ssa)
 101       return true;
 102
 103    if (src->ssa->live_index == 0)
 104       return true;   /* undefined variables are never live */
 105
 106    BITSET_SET(live, src->ssa->live_index);
 107
 108    return true;
 109 }
 110
 111 static bool
 112 set_ssa_def_dead(nir_ssa_def *def, void *void_live)
 113 {
 114    BITSET_WORD *live = void_live;
 115
 116    BITSET_CLEAR(live, def->live_index);
 117
 118    return true;
 119 }
 120
 121 /** Propagates the live in of succ across the edge to the live out of pred
 122  *
 123  * Phi nodes exist "between" blocks and all the phi nodes at the start of a
 124  * block act "in parallel".  When we propagate from the live_in of one
 125  * block to the live out of the other, we have to kill any writes from phis
 126  * and make live any sources.
 127  *
 128  * Returns true if updating live out of pred added anything
 129  */
 130 static bool
 131 propagate_across_edge(nir_block *pred, nir_block *succ,
 132                       struct live_ssa_defs_state *state)
 133 {
 134    NIR_VLA(BITSET_WORD, live, state->bitset_words);
 135    memcpy(live, succ->live_in, state->bitset_words * sizeof *live);
 136
 137    nir_foreach_instr(succ, instr) {
 138       if (instr->type != nir_instr_type_phi)
 139          break;
 140       nir_phi_instr *phi = nir_instr_as_phi(instr);
 141
 142       assert(phi->dest.is_ssa);
 143       set_ssa_def_dead(&phi->dest.ssa, live);
 144    }
 145
 146    nir_foreach_instr(succ, instr) {
 147       if (instr->type != nir_instr_type_phi)
 148          break;
 149       nir_phi_instr *phi = nir_instr_as_phi(instr);
 150
 151       nir_foreach_phi_src(phi, src) {
 152          if (src->pred == pred) {
 153             set_src_live(&src->src, live);
 154             break;
 155          }
 156       }
 157    }
 158
 159    BITSET_WORD progress = 0;
 160    for (unsigned i = 0; i < state->bitset_words; ++i) {
 161       progress |= live[i] & ~pred->live_out[i];
 162       pred->live_out[i] |= live[i];
 163    }
 164    return progress != 0;
 165 }
 166
 167 void
 168 nir_live_ssa_defs_impl(nir_function_impl *impl)
 169 {
 170    struct live_ssa_defs_state state;
 171
 172    /* We start at 1 because we reserve the index value of 0 for ssa_undef
 173     * instructions.  Those are never live, so their liveness information
 174     * can be compacted into a single bit.
 175     */
 176    state.num_ssa_defs = 1;
 177    nir_foreach_block(impl, index_ssa_definitions_block, &state);
 178
 179    nir_block_worklist_init(&state.worklist, impl->num_blocks, NULL);
 180
 181    /* We now know how many unique ssa definitions we have and we can go
 182     * ahead and allocate live_in and live_out sets and add all of the
 183     * blocks to the worklist.
 184     */
 185    state.bitset_words = BITSET_WORDS(state.num_ssa_defs);
 186    nir_foreach_block(impl, init_liveness_block, &state);
 187
 188    /* We're now ready to work through the worklist and update the liveness
 189     * sets of each of the blocks.  By the time we get to this point, every
 190     * block in the function implementation has been pushed onto the
 191     * worklist in reverse order.  As long as we keep the worklist
 192     * up-to-date as we go, everything will get covered.
 193     */
 194    while (!nir_block_worklist_is_empty(&state.worklist)) {
 195       /* We pop them off in the reverse order we pushed them on.  This way
 196        * the first walk of the instructions is backwards so we only walk
 197        * once in the case of no control flow.
 198        */
 199       nir_block *block = nir_block_worklist_pop_head(&state.worklist);
 200
 201       memcpy(block->live_in, block->live_out,
 202              state.bitset_words * sizeof(BITSET_WORD));
 203
 204       nir_if *following_if = nir_block_get_following_if(block);
 205       if (following_if)
 206          set_src_live(&following_if->condition, block->live_in);
 207
 208       nir_foreach_instr_reverse(block, instr) {
 209          /* Phi nodes are handled seperately so we want to skip them.  Since
 210           * we are going backwards and they are at the beginning, we can just
 211           * break as soon as we see one.
 212           */
 213          if (instr->type == nir_instr_type_phi)
 214             break;
 215
 216          nir_foreach_ssa_def(instr, set_ssa_def_dead, block->live_in);
 217          nir_foreach_src(instr, set_src_live, block->live_in);
 218       }
 219
 220       /* Walk over all of the predecessors of the current block updating
 221        * their live in with the live out of this one.  If anything has
 222        * changed, add the predecessor to the work list so that we ensure
 223        * that the new information is used.
 224        */
 225       struct set_entry *entry;
 226       set_foreach(block->predecessors, entry) {
 227          nir_block *pred = (nir_block *)entry->key;
 228          if (propagate_across_edge(pred, block, &state))
 229             nir_block_worklist_push_tail(&state.worklist, pred);
 230       }
 231    }
 232
 233    nir_block_worklist_fini(&state.worklist);
 234 }
 235
 236 static bool
 237 src_does_not_use_def(nir_src *src, void *def)
 238 {
 239    return !src->is_ssa || src->ssa != (nir_ssa_def *)def;
 240 }
 241
 242 static bool
 243 search_for_use_after_instr(nir_instr *start, nir_ssa_def *def)
 244 {
 245    /* Only look for a use strictly after the given instruction */
 246    struct exec_node *node = start->node.next;
 247    while (!exec_node_is_tail_sentinel(node)) {
 248       nir_instr *instr = exec_node_data(nir_instr, node, node);
 249       if (!nir_foreach_src(instr, src_does_not_use_def, def))
 250          return true;
 251       node = node->next;
 252    }
 253    return false;
 254 }
 255
 256 /* Returns true if def is live at instr assuming that def comes before
 257  * instr in a pre DFS search of the dominance tree.
 258  */
 259 static bool
 260 nir_ssa_def_is_live_at(nir_ssa_def *def, nir_instr *instr)
 261 {
 262    if (BITSET_TEST(instr->block->live_out, def->live_index)) {
 263       /* Since def dominates instr, if def is in the liveout of the block,
 264        * it's live at instr
 265        */
 266       return true;
 267    } else {
 268       if (BITSET_TEST(instr->block->live_in, def->live_index) ||
 269           def->parent_instr->block == instr->block) {
 270          /* In this case it is either live coming into instr's block or it
 271           * is defined in the same block.  In this case, we simply need to
 272           * see if it is used after instr.
 273           */
 274          return search_for_use_after_instr(instr, def);
 275       } else {
 276          return false;
 277       }
 278    }
 279 }
 280
 281 bool
 282 nir_ssa_defs_interfere(nir_ssa_def *a, nir_ssa_def *b)
 283 {
 284    if (a->parent_instr == b->parent_instr) {
 285       /* Two variables defined at the same time interfere assuming at
 286        * least one isn't dead.
 287        */
 288       return true;
 289    } else if (a->live_index == 0 || b->live_index == 0) {
 290       /* If either variable is an ssa_undef, then there's no interference */
 291       return false;
 292    } else if (a->live_index < b->live_index) {
 293       return nir_ssa_def_is_live_at(a, b->parent_instr);
 294    } else {
 295       return nir_ssa_def_is_live_at(b, a->parent_instr);
 296    }
 297 }