src/compiler/nir/nir_opt_remove_phis.c

/*
 * Copyright © 2015 Connor Abbott
 *
 * Permission is hereby granted, free of charge, to any person obtaining a
 * copy of this software and associated documentation files (the "Software"),
 * to deal in the Software without restriction, including without limitation
 * the rights to use, copy, modify, merge, publish, distribute, sublicense,
 * and/or sell copies of the Software, and to permit persons to whom the
 * Software is furnished to do so, subject to the following conditions:
 *
 * The above copyright notice and this permission notice (including the next
 * paragraph) shall be included in all copies or substantial portions of the
 * Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.  IN NO EVENT SHALL
 * THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
 * FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS
 * IN THE SOFTWARE.
 *
 * Authors:
 *    Connor Abbott (cwabbott0@gmail.com)
 *
 */

#include "nir.h"
#include "nir_builder.h"

static bool
phi_srcs_equal(nir_def *a, nir_def *b)
{
   if (a == b)
      return true;

   if (a->parent_instr->type != b->parent_instr->type)
      return false;

   if (a->parent_instr->type != nir_instr_type_alu &&
       a->parent_instr->type != nir_instr_type_load_const)
      return false;

   if (!nir_instrs_equal(a->parent_instr, b->parent_instr))
      return false;

   /* nir_instrs_equal ignores exact/fast_math */
   if (a->parent_instr->type == nir_instr_type_alu) {
      nir_alu_instr *a_alu = nir_instr_as_alu(a->parent_instr);
      nir_alu_instr *b_alu = nir_instr_as_alu(b->parent_instr);
      if (a_alu->exact != b_alu->exact || a_alu->fp_fast_math != b_alu->fp_fast_math)
         return false;
   }

   return true;
}

static bool
src_dominates_block(nir_src *src, void *state)
{
   nir_block *block = state;
   return nir_block_dominates(src->ssa->parent_instr->block, block);
}

static bool
can_rematerialize_phi_src(nir_block *imm_dom, nir_def *def)
{
   if (def->parent_instr->type == nir_instr_type_alu) {
      return nir_foreach_src(def->parent_instr, src_dominates_block, imm_dom);
   } else if (def->parent_instr->type == nir_instr_type_load_const) {
      return true;
   }
   return false;
}

/*
 * This is a pass for removing phi nodes that look like:
 * a = phi(b, b, b, ...)
 *
 * Note that we can't always ignore undef sources here, or else we may create a
 * situation where the definition of b isn't dominated by its uses. We're
 * allowed to do this since the definition of b must dominate all of the
 * phi node's predecessors, which means it must dominate the phi node as well
 * as all of the phi node's uses. In essence, the phi node acts as a copy
 * instruction. b can't be another phi node in the same block, since the only
 * time when phi nodes can source other phi nodes defined in the same block is
 * at the loop header, and in that case one of the sources of the phi has to
 * be from before the loop and that source can't be b.
 */

static bool
remove_phis_instr(nir_builder *b, nir_phi_instr *phi, void *unused)
{
   nir_block *block = phi->instr.block;
   nir_def *def = NULL;
   bool needs_remat = false;

   /* Skip unreachable phis, they should be removed by nir_opt_dead_cf. */
   if (nir_block_is_unreachable(block))
      return false;

   nir_foreach_phi_src(src, phi) {
      /* For phi nodes at the beginning of loops, we may encounter some
       * sources from backedges that point back to the destination of the
       * same phi, i.e. something like:
       *
       * a = phi(a, b, ...)
       *
       * We can safely ignore these sources, since if all of the normal
       * sources point to the same definition, then that definition must
       * still dominate the phi node, and the phi will still always take
       * the value of that definition.
       */
      if (src->src.ssa == &phi->def)
         continue;

      /* Ignore undef sources. */
      if (nir_src_is_undef(src->src))
         continue;

      if (def == NULL) {
         def = src->src.ssa;
         if (!nir_block_dominates(def->parent_instr->block, block->imm_dom)) {
            if (!can_rematerialize_phi_src(block->imm_dom, def))
               return false;
            needs_remat = true;
         }
      } else if (!phi_srcs_equal(src->src.ssa, def)) {
         return false;
      }
   }

   if (!def) {
      /* In this case, the phi had no non undef sources. So turn it into an undef. */
      b->cursor = nir_after_phis(block);
      def = nir_undef(b, phi->def.num_components, phi->def.bit_size);
   } else if (needs_remat) {
      b->cursor = nir_after_block_before_jump(block->imm_dom);
      nir_instr *remat = nir_instr_clone(b->shader, def->parent_instr);
      nir_builder_instr_insert(b, remat);
      def = nir_instr_def(remat);
   }

   nir_def_replace(&phi->def, def);
   return true;
}

bool
nir_opt_remove_phis(nir_shader *shader)
{
   nir_foreach_function_impl(impl, shader)
      nir_metadata_require(impl, nir_metadata_dominance);

   return nir_shader_phi_pass(shader, remove_phis_instr,
                              nir_metadata_control_flow, NULL);
}

bool
nir_remove_single_src_phis_block(nir_block *block)
{
   assert(block->predecessors->entries <= 1);
   bool progress = false;
   nir_foreach_phi_safe(phi, block) {
      nir_def *def = NULL;
      nir_foreach_phi_src(src, phi) {
         def = src->src.ssa;
         break;
      }

      if (!def) {
         nir_builder b = nir_builder_create(nir_cf_node_get_function(&block->cf_node));
         b.cursor = nir_after_phis(block);
         def = nir_undef(&b, phi->def.num_components, phi->def.bit_size);
      }

      nir_def_replace(&phi->def, def);
      progress = true;
   }
   return progress;
}