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