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