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 #include "nir_builder.h"
  30
  31 static nir_alu_instr *
  32 get_parent_mov(nir_ssa_def *ssa)
  33 {
  34    if (ssa->parent_instr->type != nir_instr_type_alu)
  35       return NULL;
  36
  37    nir_alu_instr *alu = nir_instr_as_alu(ssa->parent_instr);
  38    return (alu->op == nir_op_imov || alu->op == nir_op_fmov) ? alu : NULL;
  39 }
  40
  41 static bool
  42 matching_mov(nir_alu_instr *mov1, nir_ssa_def *ssa)
  43 {
  44    if (!mov1)
  45       return false;
  46
  47    nir_alu_instr *mov2 = get_parent_mov(ssa);
  48    return mov2 && nir_alu_srcs_equal(mov1, mov2, 0, 0);
  49 }
  50
  51 /*
  52  * This is a pass for removing phi nodes that look like:
  53  * a = phi(b, b, b, ...)
  54  *
  55  * Note that we can't ignore undef sources here, or else we may create a
  56  * situation where the definition of b isn't dominated by its uses. We're
  57  * allowed to do this since the definition of b must dominate all of the
  58  * phi node's predecessors, which means it must dominate the phi node as well
  59  * as all of the phi node's uses. In essence, the phi node acts as a copy
  60  * instruction. b can't be another phi node in the same block, since the only
  61  * time when phi nodes can source other phi nodes defined in the same block is
  62  * at the loop header, and in that case one of the sources of the phi has to
  63  * be from before the loop and that source can't be b.
  64  */
  65
  66 static bool
  67 remove_phis_block(nir_block *block, nir_builder *b)
  68 {
  69    bool progress = false;
  70
  71    nir_foreach_instr_safe(instr, block) {
  72       if (instr->type != nir_instr_type_phi)
  73          break;
  74
  75       nir_phi_instr *phi = nir_instr_as_phi(instr);
  76
  77       nir_ssa_def *def = NULL;
  78       nir_alu_instr *mov = NULL;
  79       bool srcs_same = true;
  80
  81       nir_foreach_phi_src(src, phi) {
  82          assert(src->src.is_ssa);
  83
  84          /* For phi nodes at the beginning of loops, we may encounter some
  85           * sources from backedges that point back to the destination of the
  86           * same phi, i.e. something like:
  87           *
  88           * a = phi(a, b, ...)
  89           *
  90           * We can safely ignore these sources, since if all of the normal
  91           * sources point to the same definition, then that definition must
  92           * still dominate the phi node, and the phi will still always take
  93           * the value of that definition.
  94           */
  95          if (src->src.ssa == &phi->dest.ssa)
  96             continue;
  97
  98          if (def == NULL) {
  99             def  = src->src.ssa;
 100             mov = get_parent_mov(def);
 101          } else {
 102             if (src->src.ssa != def && !matching_mov(mov, src->src.ssa)) {
 103                srcs_same = false;
 104                break;
 105             }
 106          }
 107       }
 108
 109       if (!srcs_same)
 110          continue;
 111
 112       /* We must have found at least one definition, since there must be at
 113        * least one forward edge.
 114        */
 115       assert(def != NULL);
 116
 117       if (mov) {
 118          /* If the sources were all movs from the same source with the same
 119           * swizzle, then we can't just pick a random move because it may not
 120           * dominate the phi node. Instead, we need to emit our own move after
 121           * the phi which uses the shared source, and rewrite uses of the phi
 122           * to use the move instead. This is ok, because while the movs may
 123           * not all dominate the phi node, their shared source does.
 124           */
 125
 126          b->cursor = nir_after_phis(block);
 127          def = mov->op == nir_op_imov ?
 128             nir_imov_alu(b, mov->src[0], def->num_components) :
 129             nir_fmov_alu(b, mov->src[0], def->num_components);
 130       }
 131
 132       assert(phi->dest.is_ssa);
 133       nir_ssa_def_rewrite_uses(&phi->dest.ssa, nir_src_for_ssa(def));
 134       nir_instr_remove(instr);
 135
 136       progress = true;
 137    }
 138
 139    return progress;
 140 }
 141
 142 static bool
 143 nir_opt_remove_phis_impl(nir_function_impl *impl)
 144 {
 145    bool progress = false;
 146    nir_builder bld;
 147    nir_builder_init(&bld, impl);
 148
 149    nir_foreach_block(block, impl) {
 150       progress |= remove_phis_block(block, &bld);
 151    }
 152
 153    if (progress) {
 154       nir_metadata_preserve(impl, nir_metadata_block_index |
 155                                   nir_metadata_dominance);
 156    }
 157
 158    return progress;
 159 }
 160
 161 bool
 162 nir_opt_remove_phis(nir_shader *shader)
 163 {
 164    bool progress = false;
 165
 166    nir_foreach_function(function, shader)
 167       if (function->impl)
 168          progress = nir_opt_remove_phis_impl(function->impl) || progress;
 169
 170    return progress;
 171 }
 172