src/panfrost/midgard/midgard_liveness.c

   1 /*
   2  * Copyright (C) 2018-2019 Alyssa Rosenzweig <alyssa@rosenzweig.io>
   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 FROM,
  20  * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
  21  * SOFTWARE.
  22  */
  23
  24 /* mir_is_live_after performs liveness analysis on the MIR, used primarily
  25  * as part of register allocation. TODO: Algorithmic improvements for
  26  * compiler performance (this is the worst algorithm possible -- see
  27  * backlog with Connor on IRC) */
  28
  29 #include "compiler.h"
  30 #include "util/u_memory.h"
  31
  32 /* Routines for liveness analysis */
  33
  34 static void
  35 liveness_gen(uint8_t *live, unsigned node, unsigned max, unsigned mask)
  36 {
  37         if (node >= max)
  38                 return;
  39
  40         live[node] |= mask;
  41 }
  42
  43 static void
  44 liveness_kill(uint8_t *live, unsigned node, unsigned max, unsigned mask)
  45 {
  46         if (node >= max)
  47                 return;
  48
  49         live[node] &= ~mask;
  50 }
  51
  52 /* Updates live_in for a single instruction */
  53
  54 void
  55 mir_liveness_ins_update(uint8_t *live, midgard_instruction *ins, unsigned max)
  56 {
  57         /* live_in[s] = GEN[s] + (live_out[s] - KILL[s]) */
  58
  59         liveness_kill(live, ins->dest, max, ins->mask);
  60
  61         mir_foreach_src(ins, src) {
  62                 unsigned node = ins->src[src];
  63                 unsigned mask = mir_mask_of_read_components(ins, node);
  64
  65                 liveness_gen(live, node, max, mask);
  66         }
  67 }
  68
  69 /* live_out[s] = sum { p in succ[s] } ( live_in[p] ) */
  70
  71 static void
  72 liveness_block_live_out(compiler_context *ctx, midgard_block *blk)
  73 {
  74         mir_foreach_successor(blk, succ) {
  75                 for (unsigned i = 0; i < ctx->temp_count; ++i)
  76                         blk->live_out[i] |= succ->live_in[i];
  77         }
  78 }
  79
  80 /* Liveness analysis is a backwards-may dataflow analysis pass. Within a block,
  81  * we compute live_out from live_in. The intrablock pass is linear-time. It
  82  * returns whether progress was made. */
  83
  84 static bool
  85 liveness_block_update(compiler_context *ctx, midgard_block *blk)
  86 {
  87         bool progress = false;
  88
  89         liveness_block_live_out(ctx, blk);
  90
  91         uint8_t *live = mem_dup(blk->live_out, ctx->temp_count);
  92
  93         mir_foreach_instr_in_block_rev(blk, ins)
  94                 mir_liveness_ins_update(live, ins, ctx->temp_count);
  95
  96         /* To figure out progress, diff live_in */
  97
  98         for (unsigned i = 0; (i < ctx->temp_count) && !progress; ++i)
  99                 progress |= (blk->live_in[i] != live[i]);
 100
 101         free(blk->live_in);
 102         blk->live_in = live;
 103
 104         return progress;
 105 }
 106
 107 /* Globally, liveness analysis uses a fixed-point algorithm based on a
 108  * worklist. We initialize a work list with the exit block. We iterate the work
 109  * list to compute live_in from live_out for each block on the work list,
 110  * adding the predecessors of the block to the work list if we made progress.
 111  */
 112
 113 void
 114 mir_compute_liveness(compiler_context *ctx)
 115 {
 116         /* List of midgard_block */
 117         struct set *work_list = _mesa_set_create(ctx,
 118                         _mesa_hash_pointer,
 119                         _mesa_key_pointer_equal);
 120
 121         /* Allocate */
 122
 123         mir_foreach_block(ctx, block) {
 124                 block->live_in = calloc(ctx->temp_count, 1);
 125                 block->live_out = calloc(ctx->temp_count, 1);
 126         }
 127
 128         /* Initialize the work list with the exit block */
 129         struct set_entry *cur;
 130
 131         midgard_block *exit = mir_exit_block(ctx);
 132         cur = _mesa_set_add(work_list, exit);
 133
 134         /* Iterate the work list */
 135
 136         do {
 137                 /* Pop off a block */
 138                 midgard_block *blk = (struct midgard_block *) cur->key;
 139                 _mesa_set_remove(work_list, cur);
 140
 141                 /* Update its liveness information */
 142                 bool progress = liveness_block_update(ctx, blk);
 143
 144                 /* If we made progress, we need to process the predecessors */
 145
 146                 if (progress || (blk == exit)) {
 147                         mir_foreach_predecessor(blk, pred)
 148                                 _mesa_set_add(work_list, pred);
 149                 }
 150         } while((cur = _mesa_set_next_entry(work_list, NULL)) != NULL);
 151 }
 152
 153 /* Determine if a variable is live in the successors of a block */
 154 static bool
 155 is_live_after_successors(compiler_context *ctx, midgard_block *bl, int src)
 156 {
 157         for (unsigned i = 0; i < bl->nr_successors; ++i) {
 158                 midgard_block *succ = bl->successors[i];
 159
 160                 /* If we already visited, the value we're seeking
 161                  * isn't down this path (or we would have short
 162                  * circuited */
 163
 164                 if (succ->visited) continue;
 165
 166                 /* Otherwise (it's visited *now*), check the block */
 167
 168                 succ->visited = true;
 169
 170                 /* Within this block, check if it's overwritten first */
 171                 unsigned overwritten_mask = 0;
 172
 173                 mir_foreach_instr_in_block(succ, ins) {
 174                         /* Did we read any components that we haven't overwritten yet? */
 175                         if (mir_mask_of_read_components(ins, src) & ~overwritten_mask)
 176                                 return true;
 177
 178                         /* If written-before-use, we're gone */
 179
 180                         if (ins->dest == src)
 181                                 overwritten_mask |= ins->mask;
 182                 }
 183
 184                 /* ...and also, check *its* successors */
 185                 if (is_live_after_successors(ctx, succ, src))
 186                         return true;
 187
 188         }
 189
 190         /* Welp. We're really not live. */
 191
 192         return false;
 193 }
 194
 195 bool
 196 mir_is_live_after(compiler_context *ctx, midgard_block *block, midgard_instruction *start, int src)
 197 {
 198         /* Check the rest of the block for liveness */
 199
 200         mir_foreach_instr_in_block_from(block, ins, mir_next_op(start)) {
 201                 if (mir_has_arg(ins, src))
 202                         return true;
 203         }
 204
 205         /* Check the rest of the blocks for liveness recursively */
 206
 207         bool succ = is_live_after_successors(ctx, block, src);
 208
 209         mir_foreach_block(ctx, block) {
 210                 block->visited = false;
 211         }
 212
 213         return succ;
 214 }