src/compiler/nir/nir_opt_remove_phis.c

   1 /*
   2  * Copyright © 2015 Connor Abbott
   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  *    Connor Abbott (cwabbott0@gmail.com)
  25  *
  26  */
  27
  28 #include "nir.h"
  29
  30 static nir_alu_instr *
  31 get_parent_mov(nir_ssa_def *ssa)
  32 {
  33    if (ssa->parent_instr->type != nir_instr_type_alu)
  34       return NULL;
  35
  36    nir_alu_instr *alu = nir_instr_as_alu(ssa->parent_instr);
  37    return (alu->op == nir_op_imov || alu->op == nir_op_fmov) ? alu : NULL;
  38 }
  39
  40 static bool
  41 matching_mov(nir_alu_instr *mov1, nir_ssa_def *ssa)
  42 {
  43    if (!mov1)
  44       return false;
  45
  46    nir_alu_instr *mov2 = get_parent_mov(ssa);
  47    return mov2 && nir_alu_srcs_equal(mov1, mov2, 0, 0);
  48 }
  49
  50 /*
  51  * This is a pass for removing phi nodes that look like:
  52  * a = phi(b, b, b, ...)
  53  *
  54  * Note that we can't ignore undef sources here, or else we may create a
  55  * situation where the definition of b isn't dominated by its uses. We're
  56  * allowed to do this since the definition of b must dominate all of the
  57  * phi node's predecessors, which means it must dominate the phi node as well
  58  * as all of the phi node's uses. In essence, the phi node acts as a copy
  59  * instruction. b can't be another phi node in the same block, since the only
  60  * time when phi nodes can source other phi nodes defined in the same block is
  61  * at the loop header, and in that case one of the sources of the phi has to
  62  * be from before the loop and that source can't be b.
  63  */
  64
  65 static bool
  66 remove_phis_block(nir_block *block)
  67 {
  68    bool progress = false;
  69
  70    nir_foreach_instr_safe(instr, block) {
  71       if (instr->type != nir_instr_type_phi)
  72          break;
  73
  74       nir_phi_instr *phi = nir_instr_as_phi(instr);
  75
  76       nir_ssa_def *def = NULL;
  77       nir_alu_instr *mov = NULL;
  78       bool srcs_same = true;
  79
  80       nir_foreach_phi_src(src, phi) {
  81          assert(src->src.is_ssa);
  82
  83          /* For phi nodes at the beginning of loops, we may encounter some
  84           * sources from backedges that point back to the destination of the
  85           * same phi, i.e. something like:
  86           *
  87           * a = phi(a, b, ...)
  88           *
  89           * We can safely ignore these sources, since if all of the normal
  90           * sources point to the same definition, then that definition must
  91           * still dominate the phi node, and the phi will still always take
  92           * the value of that definition.
  93           */
  94          if (src->src.ssa == &phi->dest.ssa)
  95             continue;
  96
  97          if (def == NULL) {
  98             def  = src->src.ssa;
  99             mov = get_parent_mov(def);
 100          } else {
 101             if (src->src.ssa != def && !matching_mov(mov, src->src.ssa)) {
 102                srcs_same = false;
 103                break;
 104             }
 105          }
 106       }
 107
 108       if (!srcs_same)
 109          continue;
 110
 111       /* We must have found at least one definition, since there must be at
 112        * least one forward edge.
 113        */
 114       assert(def != NULL);
 115
 116       assert(phi->dest.is_ssa);
 117       nir_ssa_def_rewrite_uses(&phi->dest.ssa, nir_src_for_ssa(def));
 118       nir_instr_remove(instr);
 119
 120       progress = true;
 121    }
 122
 123    return progress;
 124 }
 125
 126 static bool
 127 remove_phis_impl(nir_function_impl *impl)
 128 {
 129    bool progress = false;
 130
 131    nir_foreach_block(block, impl) {
 132       progress |= remove_phis_block(block);
 133    }
 134
 135    if (progress) {
 136       nir_metadata_preserve(impl, nir_metadata_block_index |
 137                                   nir_metadata_dominance);
 138    }
 139
 140    return progress;
 141 }
 142
 143 bool
 144 nir_opt_remove_phis(nir_shader *shader)
 145 {
 146    bool progress = false;
 147
 148    nir_foreach_function(function, shader)
 149       if (function->impl)
 150          progress = remove_phis_impl(function->impl) || progress;
 151
 152    return progress;
 153 }
 154