nir: Modernize the out-of-SSA pass

[mesa.git] / src / glsl / nir / nir_from_ssa.c

/*
 * Copyright © 2014 Intel Corporation
 *
 * 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:
 *    Jason Ekstrand (jason@jlekstrand.net)
 *
 */

#include "nir.h"
#include "nir_vla.h"

/*
 * This file implements an out-of-SSA pass as described in "Revisiting
 * Out-of-SSA Translation for Correctness, Code Quality, and Efficiency" by
 * Boissinot et. al.
 */

struct from_ssa_state {
   void *mem_ctx;
   void *dead_ctx;
   struct hash_table *merge_node_table;
   nir_instr *instr;
   nir_function_impl *impl;
};

/* Returns true if a dominates b */
static bool
ssa_def_dominates(nir_ssa_def *a, nir_ssa_def *b)
{
   if (a->live_index == 0) {
      /* SSA undefs always dominate */
      return true;
   } else if (b->live_index < a->live_index) {
      return false;
   } else if (a->parent_instr->block == b->parent_instr->block) {
      return a->live_index <= b->live_index;
   } else {
      return nir_block_dominates(a->parent_instr->block,
                                 b->parent_instr->block);
   }
}


/* The following data structure, which I have named merge_set is a way of
 * representing a set registers of non-interfering registers.  This is
 * based on the concept of a "dominence forest" presented in "Fast Copy
 * Coalescing and Live-Range Identification" by Budimlic et. al. but the
 * implementation concept is taken from  "Revisiting Out-of-SSA Translation
 * for Correctness, Code Quality, and Efficiency" by Boissinot et. al..
 *
 * Each SSA definition is associated with a merge_node and the association
 * is represented by a combination of a hash table and the "def" parameter
 * in the merge_node structure.  The merge_set stores a linked list of
 * merge_node's in dominence order of the ssa definitions.  (Since the
 * liveness analysis pass indexes the SSA values in dominence order for us,
 * this is an easy thing to keep up.)  It is assumed that no pair of the
 * nodes in a given set interfere.  Merging two sets or checking for
 * interference can be done in a single linear-time merge-sort walk of the
 * two lists of nodes.
 */
struct merge_set;

typedef struct {
   struct exec_node node;
   struct merge_set *set;
   nir_ssa_def *def;
} merge_node;

typedef struct merge_set {
   struct exec_list nodes;
   unsigned size;
   nir_register *reg;
} merge_set;

#if 0
static void
merge_set_dump(merge_set *set, FILE *fp)
{
   nir_ssa_def *dom[set->size];
   int dom_idx = -1;

   foreach_list_typed(merge_node, node, node, &set->nodes) {
      while (dom_idx >= 0 && !ssa_def_dominates(dom[dom_idx], node->def))
         dom_idx--;

      for (int i = 0; i <= dom_idx; i++)
         fprintf(fp, "  ");

      if (node->def->name)
         fprintf(fp, "ssa_%d /* %s */\n", node->def->index, node->def->name);
      else
         fprintf(fp, "ssa_%d\n", node->def->index);

      dom[++dom_idx] = node->def;
   }
}
#endif

static merge_node *
get_merge_node(nir_ssa_def *def, struct from_ssa_state *state)
{
   struct hash_entry *entry =
      _mesa_hash_table_search(state->merge_node_table, def);
   if (entry)
      return entry->data;

   merge_set *set = ralloc(state->dead_ctx, merge_set);
   exec_list_make_empty(&set->nodes);
   set->size = 1;
   set->reg = NULL;

   merge_node *node = ralloc(state->dead_ctx, merge_node);
   node->set = set;
   node->def = def;
   exec_list_push_head(&set->nodes, &node->node);

   _mesa_hash_table_insert(state->merge_node_table, def, node);

   return node;
}

static bool
merge_nodes_interfere(merge_node *a, merge_node *b)
{
   return nir_ssa_defs_interfere(a->def, b->def);
}

/* Merges b into a */
static merge_set *
merge_merge_sets(merge_set *a, merge_set *b)
{
   struct exec_node *an = exec_list_get_head(&a->nodes);
   struct exec_node *bn = exec_list_get_head(&b->nodes);
   while (!exec_node_is_tail_sentinel(bn)) {
      merge_node *a_node = exec_node_data(merge_node, an, node);
      merge_node *b_node = exec_node_data(merge_node, bn, node);

      if (exec_node_is_tail_sentinel(an) ||
          a_node->def->live_index > b_node->def->live_index) {
         struct exec_node *next = bn->next;
         exec_node_remove(bn);
         exec_node_insert_node_before(an, bn);
         exec_node_data(merge_node, bn, node)->set = a;
         bn = next;
      } else {
         an = an->next;
      }
   }

   a->size += b->size;
   b->size = 0;

   return a;
}

/* Checks for any interference between two merge sets
 *
 * This is an implementation of Algorithm 2 in "Revisiting Out-of-SSA
 * Translation for Correctness, Code Quality, and Efficiency" by
 * Boissinot et. al.
 */
static bool
merge_sets_interfere(merge_set *a, merge_set *b)
{
   NIR_VLA(merge_node *, dom, a->size + b->size);
   int dom_idx = -1;

   struct exec_node *an = exec_list_get_head(&a->nodes);
   struct exec_node *bn = exec_list_get_head(&b->nodes);
   while (!exec_node_is_tail_sentinel(an) ||
          !exec_node_is_tail_sentinel(bn)) {

      merge_node *current;
      if (exec_node_is_tail_sentinel(an)) {
         current = exec_node_data(merge_node, bn, node);
         bn = bn->next;
      } else if (exec_node_is_tail_sentinel(bn)) {
         current = exec_node_data(merge_node, an, node);
         an = an->next;
      } else {
         merge_node *a_node = exec_node_data(merge_node, an, node);
         merge_node *b_node = exec_node_data(merge_node, bn, node);

         if (a_node->def->live_index <= b_node->def->live_index) {
            current = a_node;
            an = an->next;
         } else {
            current = b_node;
            bn = bn->next;
         }
      }

      while (dom_idx >= 0 &&
             !ssa_def_dominates(dom[dom_idx]->def, current->def))
         dom_idx--;

      if (dom_idx >= 0 && merge_nodes_interfere(current, dom[dom_idx]))
         return true;

      dom[++dom_idx] = current;
   }

   return false;
}

static bool
add_parallel_copy_to_end_of_block(nir_block *block, void *void_state)
{
   struct from_ssa_state *state = void_state;

   bool need_end_copy = false;
   if (block->successors[0]) {
      nir_instr *instr = nir_block_first_instr(block->successors[0]);
      if (instr && instr->type == nir_instr_type_phi)
         need_end_copy = true;
   }

   if (block->successors[1]) {
      nir_instr *instr = nir_block_first_instr(block->successors[1]);
      if (instr && instr->type == nir_instr_type_phi)
         need_end_copy = true;
   }

   if (need_end_copy) {
      /* If one of our successors has at least one phi node, we need to
       * create a parallel copy at the end of the block but before the jump
       * (if there is one).
       */
      nir_parallel_copy_instr *pcopy =
         nir_parallel_copy_instr_create(state->dead_ctx);

      nir_instr *last_instr = nir_block_last_instr(block);
      if (last_instr && last_instr->type == nir_instr_type_jump) {
         nir_instr_insert_before(last_instr, &pcopy->instr);
      } else {
         nir_instr_insert_after_block(block, &pcopy->instr);
      }
   }

   return true;
}

static nir_parallel_copy_instr *
get_parallel_copy_at_end_of_block(nir_block *block)
{
   nir_instr *last_instr = nir_block_last_instr(block);
   if (last_instr == NULL)
      return NULL;

   /* The last instruction may be a jump in which case the parallel copy is
    * right before it.
    */
   if (last_instr->type == nir_instr_type_jump)
      last_instr = nir_instr_prev(last_instr);

   if (last_instr && last_instr->type == nir_instr_type_parallel_copy)
      return nir_instr_as_parallel_copy(last_instr);
   else
      return NULL;
}

/** Isolate phi nodes with parallel copies
 *
 * In order to solve the dependency problems with the sources and
 * destinations of phi nodes, we first isolate them by adding parallel
 * copies to the beginnings and ends of basic blocks.  For every block with
 * phi nodes, we add a parallel copy immediately following the last phi
 * node that copies the destinations of all of the phi nodes to new SSA
 * values.  We also add a parallel copy to the end of every block that has
 * a successor with phi nodes that, for each phi node in each successor,
 * copies the corresponding sorce of the phi node and adjust the phi to
 * used the destination of the parallel copy.
 *
 * In SSA form, each value has exactly one definition.  What this does is
 * ensure that each value used in a phi also has exactly one use.  The
 * destinations of phis are only used by the parallel copy immediately
 * following the phi nodes and.  Thanks to the parallel copy at the end of
 * the predecessor block, the sources of phi nodes are are the only use of
 * that value.  This allows us to immediately assign all the sources and
 * destinations of any given phi node to the same register without worrying
 * about interference at all.  We do coalescing to get rid of the parallel
 * copies where possible.
 *
 * Before this pass can be run, we have to iterate over the blocks with
 * add_parallel_copy_to_end_of_block to ensure that the parallel copies at
 * the ends of blocks exist.  We can create the ones at the beginnings as
 * we go, but the ones at the ends of blocks need to be created ahead of
 * time because of potential back-edges in the CFG.
 */
static bool
isolate_phi_nodes_block(nir_block *block, void *void_state)
{
   struct from_ssa_state *state = void_state;

   nir_instr *last_phi_instr = NULL;
   nir_foreach_instr(block, instr) {
      /* Phi nodes only ever come at the start of a block */
      if (instr->type != nir_instr_type_phi)
         break;

      last_phi_instr = instr;
   }

   /* If we don't have any phi's, then there's nothing for us to do. */
   if (last_phi_instr == NULL)
      return true;

   /* If we have phi nodes, we need to create a parallel copy at the
    * start of this block but after the phi nodes.
    */
   nir_parallel_copy_instr *block_pcopy =
      nir_parallel_copy_instr_create(state->dead_ctx);
   nir_instr_insert_after(last_phi_instr, &block_pcopy->instr);

   nir_foreach_instr(block, instr) {
      /* Phi nodes only ever come at the start of a block */
      if (instr->type != nir_instr_type_phi)
         break;

      nir_phi_instr *phi = nir_instr_as_phi(instr);
      assert(phi->dest.is_ssa);
      nir_foreach_phi_src(phi, src) {
         nir_parallel_copy_instr *pcopy =
            get_parallel_copy_at_end_of_block(src->pred);
         assert(pcopy);

         nir_parallel_copy_entry *entry = rzalloc(state->dead_ctx,
                                                  nir_parallel_copy_entry);
         nir_ssa_dest_init(&pcopy->instr, &entry->dest,
                           phi->dest.ssa.num_components, src->src.ssa->name);
         exec_list_push_tail(&pcopy->entries, &entry->node);

         assert(src->src.is_ssa);
         nir_instr_rewrite_src(&pcopy->instr, &entry->src, src->src);

         nir_instr_rewrite_src(&phi->instr, &src->src,
                               nir_src_for_ssa(&entry->dest.ssa));
      }

      nir_parallel_copy_entry *entry = rzalloc(state->dead_ctx,
                                               nir_parallel_copy_entry);
      nir_ssa_dest_init(&block_pcopy->instr, &entry->dest,
                        phi->dest.ssa.num_components, phi->dest.ssa.name);
      exec_list_push_tail(&block_pcopy->entries, &entry->node);

      nir_ssa_def_rewrite_uses(&phi->dest.ssa,
                               nir_src_for_ssa(&entry->dest.ssa),
                               state->mem_ctx);

      nir_instr_rewrite_src(&block_pcopy->instr, &entry->src,
                            nir_src_for_ssa(&phi->dest.ssa));
   }

   return true;
}

static bool
coalesce_phi_nodes_block(nir_block *block, void *void_state)
{
   struct from_ssa_state *state = void_state;

   nir_foreach_instr(block, instr) {
      /* Phi nodes only ever come at the start of a block */
      if (instr->type != nir_instr_type_phi)
         break;

      nir_phi_instr *phi = nir_instr_as_phi(instr);

      assert(phi->dest.is_ssa);
      merge_node *dest_node = get_merge_node(&phi->dest.ssa, state);

      nir_foreach_phi_src(phi, src) {
         assert(src->src.is_ssa);
         merge_node *src_node = get_merge_node(src->src.ssa, state);
         if (src_node->set != dest_node->set)
            merge_merge_sets(dest_node->set, src_node->set);
      }
   }

   return true;
}

static void
agressive_coalesce_parallel_copy(nir_parallel_copy_instr *pcopy,
                                 struct from_ssa_state *state)
{
   nir_foreach_parallel_copy_entry(pcopy, entry) {
      if (!entry->src.is_ssa)
         continue;

      /* Since load_const instructions are SSA only, we can't replace their
       * destinations with registers and, therefore, can't coalesce them.
       */
      if (entry->src.ssa->parent_instr->type == nir_instr_type_load_const)
         continue;

      /* Don't try and coalesce these */
      if (entry->dest.ssa.num_components != entry->src.ssa->num_components)
         continue;

      merge_node *src_node = get_merge_node(entry->src.ssa, state);
      merge_node *dest_node = get_merge_node(&entry->dest.ssa, state);

      if (src_node->set == dest_node->set)
         continue;

      if (!merge_sets_interfere(src_node->set, dest_node->set))
         merge_merge_sets(src_node->set, dest_node->set);
   }
}

static bool
agressive_coalesce_block(nir_block *block, void *void_state)
{
   struct from_ssa_state *state = void_state;

   nir_parallel_copy_instr *start_pcopy = NULL;
   nir_foreach_instr(block, instr) {
      /* Phi nodes only ever come at the start of a block */
      if (instr->type != nir_instr_type_phi) {
         if (instr->type != nir_instr_type_parallel_copy)
            break; /* The parallel copy must be right after the phis */

         start_pcopy = nir_instr_as_parallel_copy(instr);

         agressive_coalesce_parallel_copy(start_pcopy, state);

         break;
      }
   }

   nir_parallel_copy_instr *end_pcopy =
      get_parallel_copy_at_end_of_block(block);

   if (end_pcopy && end_pcopy != start_pcopy)
      agressive_coalesce_parallel_copy(end_pcopy, state);

   return true;
}

static bool
rewrite_ssa_def(nir_ssa_def *def, void *void_state)
{
   struct from_ssa_state *state = void_state;
   nir_register *reg;

   struct hash_entry *entry =
      _mesa_hash_table_search(state->merge_node_table, def);
   if (entry) {
      /* In this case, we're part of a phi web.  Use the web's register. */
      merge_node *node = (merge_node *)entry->data;

      /* If it doesn't have a register yet, create one.  Note that all of
       * the things in the merge set should be the same so it doesn't
       * matter which node's definition we use.
       */
      if (node->set->reg == NULL) {
         node->set->reg = nir_local_reg_create(state->impl);
         node->set->reg->name = def->name;
         node->set->reg->num_components = def->num_components;
         node->set->reg->num_array_elems = 0;
      }

      reg = node->set->reg;
   } else {
      /* We leave load_const SSA values alone.  They act as immediates to
       * the backend.  If it got coalesced into a phi, that's ok.
       */
      if (def->parent_instr->type == nir_instr_type_load_const)
         return true;

      reg = nir_local_reg_create(state->impl);
      reg->name = def->name;
      reg->num_components = def->num_components;
      reg->num_array_elems = 0;

      /* This register comes from an SSA definition that is defined and not
       * part of a phi-web.  Therefore, we know it has a single unique
       * definition that dominates all of its uses; we can copy the
       * parent_instr from the SSA def safely.
       */
      if (def->parent_instr->type != nir_instr_type_ssa_undef)
         reg->parent_instr = def->parent_instr;
   }

   nir_ssa_def_rewrite_uses(def, nir_src_for_reg(reg), state->mem_ctx);
   assert(def->uses->entries == 0 && def->if_uses->entries == 0);

   if (def->parent_instr->type == nir_instr_type_ssa_undef)
      return true;

   assert(def->parent_instr->type != nir_instr_type_load_const);

   /* At this point we know a priori that this SSA def is part of a
    * nir_dest.  We can use exec_node_data to get the dest pointer.
    */
   nir_dest *dest = exec_node_data(nir_dest, def, ssa);

   _mesa_set_destroy(dest->ssa.uses, NULL);
   _mesa_set_destroy(dest->ssa.if_uses, NULL);

   *dest = nir_dest_for_reg(reg);

   _mesa_set_add(reg->defs, state->instr);

   return true;
}

/* Resolves ssa definitions to registers.  While we're at it, we also
 * remove phi nodes and ssa_undef instructions
 */
static bool
resolve_registers_block(nir_block *block, void *void_state)
{
   struct from_ssa_state *state = void_state;

   nir_foreach_instr_safe(block, instr) {
      state->instr = instr;
      nir_foreach_ssa_def(instr, rewrite_ssa_def, state);

      if (instr->type == nir_instr_type_ssa_undef ||
          instr->type == nir_instr_type_phi) {
         nir_instr_remove(instr);
         ralloc_steal(state->dead_ctx, instr);
      }
   }
   state->instr = NULL;

   return true;
}

static void
emit_copy(nir_parallel_copy_instr *pcopy, nir_src src, nir_src dest_src,
          void *mem_ctx)
{
   assert(!dest_src.is_ssa &&
          dest_src.reg.indirect == NULL &&
          dest_src.reg.base_offset == 0);

   if (src.is_ssa)
      assert(src.ssa->num_components >= dest_src.reg.reg->num_components);
   else
      assert(src.reg.reg->num_components >= dest_src.reg.reg->num_components);

   nir_alu_instr *mov = nir_alu_instr_create(mem_ctx, nir_op_imov);
   nir_src_copy(&mov->src[0].src, &src, mem_ctx);
   mov->dest.dest = nir_dest_for_reg(dest_src.reg.reg);
   mov->dest.write_mask = (1 << dest_src.reg.reg->num_components) - 1;

   nir_instr_insert_before(&pcopy->instr, &mov->instr);
}

/* Resolves a single parallel copy operation into a sequence of mov's
 *
 * This is based on Algorithm 1 from "Revisiting Out-of-SSA Translation for
 * Correctness, Code Quality, and Efficiency" by Boissinot et. al..
 * However, I never got the algorithm to work as written, so this version
 * is slightly modified.
 *
 * The algorithm works by playing this little shell game with the values.
 * We start by recording where every source value is and which source value
 * each destination value should receive.  We then grab any copy whose
 * destination is "empty", i.e. not used as a source, and do the following:
 *  - Find where its source value currently lives
 *  - Emit the move instruction
 *  - Set the location of the source value to the destination
 *  - Mark the location containing the source value
 *  - Mark the destination as no longer needing to be copied
 *
 * When we run out of "empty" destinations, we have a cycle and so we
 * create a temporary register, copy to that register, and mark the value
 * we copied as living in that temporary.  Now, the cycle is broken, so we
 * can continue with the above steps.
 */
static void
resolve_parallel_copy(nir_parallel_copy_instr *pcopy,
                      struct from_ssa_state *state)
{
   unsigned num_copies = 0;
   nir_foreach_parallel_copy_entry(pcopy, entry) {
      /* Sources may be SSA */
      if (!entry->src.is_ssa && entry->src.reg.reg == entry->dest.reg.reg)
         continue;

      num_copies++;
   }

   if (num_copies == 0) {
      /* Hooray, we don't need any copies! */
      nir_instr_remove(&pcopy->instr);
      return;
   }

   /* The register/source corresponding to the given index */
   NIR_VLA_ZERO(nir_src, values, num_copies * 2);

   /* The current location of a given piece of data.  We will use -1 for "null" */
   NIR_VLA_FILL(int, loc, num_copies * 2, -1);

   /* The piece of data that the given piece of data is to be copied from.  We will use -1 for "null" */
   NIR_VLA_FILL(int, pred, num_copies * 2, -1);

   /* The destinations we have yet to properly fill */
   NIR_VLA(int, to_do, num_copies * 2);
   int to_do_idx = -1;

   /* Now we set everything up:
    *  - All values get assigned a temporary index
    *  - Current locations are set from sources
    *  - Predicessors are recorded from sources and destinations
    */
   int num_vals = 0;
   nir_foreach_parallel_copy_entry(pcopy, entry) {
      /* Sources may be SSA */
      if (!entry->src.is_ssa && entry->src.reg.reg == entry->dest.reg.reg)
         continue;

      int src_idx = -1;
      for (int i = 0; i < num_vals; ++i) {
         if (nir_srcs_equal(values[i], entry->src))
            src_idx = i;
      }
      if (src_idx < 0) {
         src_idx = num_vals++;
         values[src_idx] = entry->src;
      }

      nir_src dest_src = nir_src_for_reg(entry->dest.reg.reg);

      int dest_idx = -1;
      for (int i = 0; i < num_vals; ++i) {
         if (nir_srcs_equal(values[i], dest_src)) {
            /* Each destination of a parallel copy instruction should be
             * unique.  A destination may get used as a source, so we still
             * have to walk the list.  However, the predecessor should not,
             * at this point, be set yet, so we should have -1 here.
             */
            assert(pred[i] == -1);
            dest_idx = i;
         }
      }
      if (dest_idx < 0) {
         dest_idx = num_vals++;
         values[dest_idx] = dest_src;
      }

      loc[src_idx] = src_idx;
      pred[dest_idx] = src_idx;

      to_do[++to_do_idx] = dest_idx;
   }

   /* Currently empty destinations we can go ahead and fill */
   NIR_VLA(int, ready, num_copies * 2);
   int ready_idx = -1;

   /* Mark the ones that are ready for copying.  We know an index is a
    * destination if it has a predecessor and it's ready for copying if
    * it's not marked as containing data.
    */
   for (int i = 0; i < num_vals; i++) {
      if (pred[i] != -1 && loc[i] == -1)
         ready[++ready_idx] = i;
   }

   while (to_do_idx >= 0) {
      while (ready_idx >= 0) {
         int b = ready[ready_idx--];
         int a = pred[b];
         emit_copy(pcopy, values[loc[a]], values[b], state->mem_ctx);

         /* If any other copies want a they can find it at b */
         loc[a] = b;

         /* b has been filled, mark it as not needing to be copied */
         pred[b] = -1;

         /* If a needs to be filled, it's ready for copying now */
         if (pred[a] != -1)
            ready[++ready_idx] = a;
      }
      int b = to_do[to_do_idx--];
      if (pred[b] == -1)
         continue;

      /* If we got here, then we don't have any more trivial copies that we
       * can do.  We have to break a cycle, so we create a new temporary
       * register for that purpose.  Normally, if going out of SSA after
       * register allocation, you would want to avoid creating temporary
       * registers.  However, we are going out of SSA before register
       * allocation, so we would rather not create extra register
       * dependencies for the backend to deal with.  If it wants, the
       * backend can coalesce the (possibly multiple) temporaries.
       */
      assert(num_vals < num_copies * 2);
      nir_register *reg = nir_local_reg_create(state->impl);
      reg->name = "copy_temp";
      reg->num_array_elems = 0;
      if (values[b].is_ssa)
         reg->num_components = values[b].ssa->num_components;
      else
         reg->num_components = values[b].reg.reg->num_components;
      values[num_vals].is_ssa = false;
      values[num_vals].reg.reg = reg;

      emit_copy(pcopy, values[b], values[num_vals], state->mem_ctx);
      loc[b] = num_vals;
      ready[++ready_idx] = b;
      num_vals++;
   }

   nir_instr_remove(&pcopy->instr);
}

/* Resolves the parallel copies in a block.  Each block can have at most
 * two:  One at the beginning, right after all the phi noces, and one at
 * the end (or right before the final jump if it exists).
 */
static bool
resolve_parallel_copies_block(nir_block *block, void *void_state)
{
   struct from_ssa_state *state = void_state;

   /* At this point, we have removed all of the phi nodes.  If a parallel
    * copy existed right after the phi nodes in this block, it is now the
    * first instruction.
    */
   nir_instr *first_instr = nir_block_first_instr(block);
   if (first_instr == NULL)
      return true; /* Empty, nothing to do. */

   if (first_instr->type == nir_instr_type_parallel_copy) {
      nir_parallel_copy_instr *pcopy = nir_instr_as_parallel_copy(first_instr);

      resolve_parallel_copy(pcopy, state);
   }

   /* It's possible that the above code already cleaned up the end parallel
    * copy.  However, doing so removed it form the instructions list so we
    * won't find it here.  Therefore, it's safe to go ahead and just look
    * for one and clean it up if it exists.
    */
   nir_parallel_copy_instr *end_pcopy =
      get_parallel_copy_at_end_of_block(block);
   if (end_pcopy)
      resolve_parallel_copy(end_pcopy, state);

   return true;
}

static void
nir_convert_from_ssa_impl(nir_function_impl *impl)
{
   struct from_ssa_state state;

   state.mem_ctx = ralloc_parent(impl);
   state.dead_ctx = ralloc_context(NULL);
   state.impl = impl;
   state.merge_node_table = _mesa_hash_table_create(NULL, _mesa_hash_pointer,
                                                    _mesa_key_pointer_equal);

   nir_foreach_block(impl, add_parallel_copy_to_end_of_block, &state);
   nir_foreach_block(impl, isolate_phi_nodes_block, &state);

   /* Mark metadata as dirty before we ask for liveness analysis */
   nir_metadata_preserve(impl, nir_metadata_block_index |
                               nir_metadata_dominance);

   nir_metadata_require(impl, nir_metadata_live_variables |
                              nir_metadata_dominance);

   nir_foreach_block(impl, coalesce_phi_nodes_block, &state);
   nir_foreach_block(impl, agressive_coalesce_block, &state);

   nir_foreach_block(impl, resolve_registers_block, &state);

   nir_foreach_block(impl, resolve_parallel_copies_block, &state);

   nir_metadata_preserve(impl, nir_metadata_block_index |
                               nir_metadata_dominance);

   /* Clean up dead instructions and the hash tables */
   _mesa_hash_table_destroy(state.merge_node_table, NULL);
   ralloc_free(state.dead_ctx);
}

void
nir_convert_from_ssa(nir_shader *shader)
{
   nir_foreach_overload(shader, overload) {
      if (overload->impl)
         nir_convert_from_ssa_impl(overload->impl);
   }
}