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
  31 static bool
  32 midgard_is_live_in_instr(midgard_instruction *ins, int src)
  33 {
  34         if (ins->ssa_args.src0 == src)
  35                 return true;
  36
  37         if (ins->ssa_args.src1 == src)
  38                 return true;
  39
  40         return false;
  41 }
  42
  43 /* Determine if a variable is live in the successors of a block */
  44 static bool
  45 is_live_after_successors(compiler_context *ctx, midgard_block *bl, int src)
  46 {
  47         for (unsigned i = 0; i < bl->nr_successors; ++i) {
  48                 midgard_block *succ = bl->successors[i];
  49
  50                 /* If we already visited, the value we're seeking
  51                  * isn't down this path (or we would have short
  52                  * circuited */
  53
  54                 if (succ->visited) continue;
  55
  56                 /* Otherwise (it's visited *now*), check the block */
  57
  58                 succ->visited = true;
  59
  60                 /* Within this block, check if it's overwritten first */
  61                 bool block_done = false;
  62
  63                 mir_foreach_instr_in_block(succ, ins) {
  64                         if (midgard_is_live_in_instr(ins, src))
  65                                 return true;
  66
  67                         /* If written-before-use, we're gone */
  68
  69                         if (ins->ssa_args.dest == src && ins->mask == 0xF) {
  70                                 block_done = true;
  71                                 break;
  72                         }
  73                 }
  74
  75                 if (block_done)
  76                         continue;
  77
  78                 /* ...and also, check *its* successors */
  79                 if (is_live_after_successors(ctx, succ, src))
  80                         return true;
  81
  82         }
  83
  84         /* Welp. We're really not live. */
  85
  86         return false;
  87 }
  88
  89 bool
  90 mir_is_live_after(compiler_context *ctx, midgard_block *block, midgard_instruction *start, int src)
  91 {
  92         /* Check the rest of the block for liveness */
  93
  94         mir_foreach_instr_in_block_from(block, ins, mir_next_op(start)) {
  95                 if (midgard_is_live_in_instr(ins, src))
  96                         return true;
  97         }
  98
  99         /* Check the rest of the blocks for liveness recursively */
 100
 101         bool succ = is_live_after_successors(ctx, block, src);
 102
 103         mir_foreach_block(ctx, block) {
 104                 block->visited = false;
 105         }
 106
 107         return succ;
 108 }
 109
 110 /* Just a quick check -- is it written more than once? (I.e. are we definitely
 111  * not SSA?) */
 112
 113 bool
 114 mir_has_multiple_writes(compiler_context *ctx, int dest)
 115 {
 116         unsigned write_count = 0;
 117
 118         mir_foreach_instr_global(ctx, ins) {
 119                 if (ins->ssa_args.dest == dest)
 120                         write_count++;
 121         }
 122
 123         return write_count > 1;
 124 }