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