ir3_merge_regs.c

/*
 * Copyright (C) 2021 Valve 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.
 */

#include "ir3_compiler.h"
#include "ir3_ra.h"
#include "ralloc.h"

/* This pass "merges" compatible phi-web SSA values. First, we insert a bunch
 * of parallelcopy's to trivially turn the program into CSSA form. Then we
 * try to "merge" SSA def's into "merge sets" which could be allocated to a
 * single register in order to eliminate copies. First we merge phi nodes,
 * which should always succeed because of the parallelcopy's we inserted, and
 * then we try to coalesce the copies we introduced.
 *
 * The merged registers are used for three purposes:
 *
 * 1. We always use the same pvtmem slot for spilling all SSA defs in each
 * merge set. This prevents us from having to insert memory-to-memory copies
 * in the spiller and makes sure we don't insert unecessary copies.
 * 2. When two values are live at the same time, part of the same merge
 * set, and they overlap each other in the merge set, they always occupy
 * overlapping physical registers in RA. This reduces register pressure and
 * copies in several important scenarios:
 *	- When sources of a collect are used later by something else, we don't
 *	have to introduce copies.
 *	- We can handle sequences of extracts that "explode" a vector into its
 *	components without any additional copying.
 * 3. We use the merge sets for affinities in register allocation: That is, we
 * try to allocate all the definitions in the same merge set to the
 * same/compatible registers. This helps us e.g. allocate sources of a collect
 * to contiguous registers without too much special code in RA.
 *
 * In a "normal" register allocator, or when spilling, we'd just merge
 * registers in the same merge set to the same register, but with SSA-based
 * register allocation we may have to split the live interval.
 *
 * The implementation is based on "Revisiting Out-of-SSA Translation for
 * Correctness, CodeQuality, and Efficiency," and is broadly similar to the
 * implementation in nir_from_ssa, with the twist that we also try to coalesce
 * META_SPLIT and META_COLLECT. This makes this pass more complicated but
 * prevents us from needing to handle these specially in RA and the spiller,
 * which are already complicated enough. This also forces us to implement that
 * value-comparison optimization they explain, as without it we wouldn't be
 * able to coalesce META_SPLIT even in the simplest of cases.
 */

/* In order to dynamically reconstruct the dominance forest, we need the
 * instructions ordered by a preorder traversal of the dominance tree:
 */

static unsigned
index_instrs(struct ir3_block *block, unsigned index)
{
   foreach_instr (instr, &block->instr_list)
      instr->ip = index++;

   for (unsigned i = 0; i < block->dom_children_count; i++)
      index = index_instrs(block->dom_children[i], index);

   return index;
}

/* Definitions within a merge set are ordered by instr->ip as set above: */

static bool
def_after(struct ir3_register *a, struct ir3_register *b)
{
   return a->instr->ip > b->instr->ip;
}

static bool
def_dominates(struct ir3_register *a, struct ir3_register *b)
{
   if (def_after(a, b)) {
      return false;
   } else if (a->instr->block == b->instr->block) {
      return def_after(b, a);
   } else {
      return ir3_block_dominates(a->instr->block, b->instr->block);
   }
}

/* This represents a region inside a register. The offset is relative to the
 * start of the register, and offset + size <= size(reg).
 */
struct def_value {
   struct ir3_register *reg;
   unsigned offset, size;
};

/* Chase any copies to get the source of a region inside a register. This is
 * Value(a) in the paper.
 */
static struct def_value
chase_copies(struct def_value value)
{
   while (true) {
      struct ir3_instruction *instr = value.reg->instr;
      if (instr->opc == OPC_META_SPLIT) {
         value.offset += instr->split.off * reg_elem_size(value.reg);
         value.reg = instr->srcs[0]->def;
      } else if (instr->opc == OPC_META_COLLECT) {
         if (value.offset % reg_elem_size(value.reg) != 0 ||
             value.size > reg_elem_size(value.reg) ||
             value.offset + value.size > reg_size(value.reg))
            break;
         struct ir3_register *src =
            instr->srcs[value.offset / reg_elem_size(value.reg)];
         if (!src->def)
            break;
         value.offset = 0;
         value.reg = src->def;
      } else {
         /* TODO: parallelcopy */
         break;
      }
   }

   return value;
}

/* This represents an entry in the merge set, and consists of a register +
 * offset from the merge set base.
 */
struct merge_def {
   struct ir3_register *reg;
   unsigned offset;
};

static bool
can_skip_interference(const struct merge_def *a, const struct merge_def *b)
{
   unsigned a_start = a->offset;
   unsigned b_start = b->offset;
   unsigned a_end = a_start + reg_size(a->reg);
   unsigned b_end = b_start + reg_size(b->reg);

   /* Registers that don't overlap never interfere */
   if (a_end <= b_start || b_end <= a_start)
      return true;

   /* Disallow skipping interference unless one definition contains the
    * other. This restriction is important for register allocation, because
    * it means that at any given point in the program, the live values in a
    * given merge set will form a tree. If they didn't, then one live value
    * would partially overlap another, and they would have overlapping live
    * ranges because they're live at the same point. This simplifies register
    * allocation and spilling.
    */
   if (!((a_start <= b_start && a_end >= b_end) ||
         (b_start <= a_start && b_end >= a_end)))
      return false;

   /* For each register, chase the intersection of a and b to find the
    * ultimate source.
    */
   unsigned start = MAX2(a_start, b_start);
   unsigned end = MIN2(a_end, b_end);
   struct def_value a_value = chase_copies((struct def_value){
      .reg = a->reg,
      .offset = start - a_start,
      .size = end - start,
   });
   struct def_value b_value = chase_copies((struct def_value){
      .reg = b->reg,
      .offset = start - b_start,
      .size = end - start,
   });
   return a_value.reg == b_value.reg && a_value.offset == b_value.offset;
}

static struct ir3_merge_set *
get_merge_set(struct ir3_register *def)
{
   if (def->merge_set)
      return def->merge_set;

   struct ir3_merge_set *set = ralloc(def, struct ir3_merge_set);
   set->preferred_reg = ~0;
   set->interval_start = ~0;
   set->size = reg_size(def);
   set->alignment = (def->flags & IR3_REG_HALF) ? 1 : 2;
   set->regs_count = 1;
   set->regs = ralloc(set, struct ir3_register *);
   set->regs[0] = def;

   return set;
}

/* Merges b into a */
static struct ir3_merge_set *
merge_merge_sets(struct ir3_merge_set *a, struct ir3_merge_set *b, int b_offset)
{
   if (b_offset < 0)
      return merge_merge_sets(b, a, -b_offset);

   struct ir3_register **new_regs =
      rzalloc_array(a, struct ir3_register *, a->regs_count + b->regs_count);

   unsigned a_index = 0, b_index = 0, new_index = 0;
   for (; a_index < a->regs_count || b_index < b->regs_count; new_index++) {
      if (b_index < b->regs_count &&
          (a_index == a->regs_count ||
           def_after(a->regs[a_index], b->regs[b_index]))) {
         new_regs[new_index] = b->regs[b_index++];
         new_regs[new_index]->merge_set_offset += b_offset;
      } else {
         new_regs[new_index] = a->regs[a_index++];
      }
      new_regs[new_index]->merge_set = a;
   }

   assert(new_index == a->regs_count + b->regs_count);

   /* Technically this should be the lcm, but because alignment is only 1 or
    * 2 so far this should be ok.
    */
   a->alignment = MAX2(a->alignment, b->alignment);
   a->regs_count += b->regs_count;
   ralloc_free(a->regs);
   a->regs = new_regs;
   a->size = MAX2(a->size, b->size + b_offset);

   return a;
}

static bool
merge_sets_interfere(struct ir3_liveness *live, struct ir3_merge_set *a,
                     struct ir3_merge_set *b, int b_offset)
{
   if (b_offset < 0)
      return merge_sets_interfere(live, b, a, -b_offset);

   struct merge_def dom[a->regs_count + b->regs_count];
   unsigned a_index = 0, b_index = 0;
   int dom_index = -1;

   /* Reject trying to merge the sets if the alignment doesn't work out */
   if (b_offset % a->alignment != 0)
      return true;

   while (a_index < a->regs_count || b_index < b->regs_count) {
      struct merge_def current;
      if (a_index == a->regs_count) {
         current.reg = b->regs[b_index];
         current.offset = current.reg->merge_set_offset + b_offset;
         b_index++;
      } else if (b_index == b->regs_count) {
         current.reg = a->regs[a_index];
         current.offset = current.reg->merge_set_offset;
         a_index++;
      } else {
         if (def_after(b->regs[b_index], a->regs[a_index])) {
            current.reg = a->regs[a_index];
            current.offset = current.reg->merge_set_offset;
            a_index++;
         } else {
            current.reg = b->regs[b_index];
            current.offset = current.reg->merge_set_offset + b_offset;
            b_index++;
         }
      }

      while (dom_index >= 0 &&
             !def_dominates(dom[dom_index].reg, current.reg)) {
         dom_index--;
      }

      /* TODO: in the original paper, just dom[dom_index] needs to be
       * checked for interference. We implement the value-chasing extension
       * as well as support for sub-registers, which complicates this
       * significantly because it's no longer the case that if a dominates b
       * dominates c and a and b don't interfere then we only need to check
       * interference between b and c to be sure a and c don't interfere --
       * this means we may have to check for interference against values
       * higher in the stack then dom[dom_index]. In the paper there's a
       * description of a way to do less interference tests with the
       * value-chasing extension, but we'd have to come up with something
       * ourselves for handling the similar problems that come up with
       * allowing values to contain subregisters. For now we just test
       * everything in the stack.
       */
      for (int i = 0; i <= dom_index; i++) {
         if (can_skip_interference(&current, &dom[i]))
            continue;

         /* Ok, now we actually have to check interference. Since we know
          * that dom[i] dominates current, this boils down to checking
          * whether dom[i] is live after current.
          */
         if (ir3_def_live_after(live, dom[i].reg, current.reg->instr))
            return true;
      }

      dom[++dom_index] = current;
   }

   return false;
}

static void
try_merge_defs(struct ir3_liveness *live, struct ir3_register *a,
               struct ir3_register *b, unsigned b_offset)
{
   struct ir3_merge_set *a_set = get_merge_set(a);
   struct ir3_merge_set *b_set = get_merge_set(b);

   if (a_set == b_set) {
      /* Note: Even in this case we may not always successfully be able to
       * coalesce this copy, if the offsets don't line up. But in any
       * case, we can't do anything.
       */
      return;
   }

   int b_set_offset = a->merge_set_offset + b_offset - b->merge_set_offset;

   if (!merge_sets_interfere(live, a_set, b_set, b_set_offset))
      merge_merge_sets(a_set, b_set, b_set_offset);
}

static void
coalesce_phi(struct ir3_liveness *live, struct ir3_instruction *phi)
{
   for (unsigned i = 0; i < phi->srcs_count; i++) {
      if (phi->srcs[i]->def)
         try_merge_defs(live, phi->dsts[0], phi->srcs[i]->def, 0);
   }
}

static void
aggressive_coalesce_parallel_copy(struct ir3_liveness *live,
                                  struct ir3_instruction *pcopy)
{
   for (unsigned i = 0; i < pcopy->dsts_count; i++) {
      if (!(pcopy->srcs[i]->flags & IR3_REG_SSA))
         continue;
      try_merge_defs(live, pcopy->dsts[i], pcopy->srcs[i]->def, 0);
   }
}

static void
aggressive_coalesce_split(struct ir3_liveness *live,
                          struct ir3_instruction *split)
{
   try_merge_defs(live, split->srcs[0]->def, split->dsts[0],
                  split->split.off * reg_elem_size(split->dsts[0]));
}

static void
aggressive_coalesce_collect(struct ir3_liveness *live,
                            struct ir3_instruction *collect)
{
   for (unsigned i = 0, offset = 0; i < collect->srcs_count;
        offset += reg_elem_size(collect->srcs[i]), i++) {
      if (!(collect->srcs[i]->flags & IR3_REG_SSA))
         continue;
      try_merge_defs(live, collect->dsts[0], collect->srcs[i]->def, offset);
   }
}

static void
create_parallel_copy(struct ir3_block *block)
{
   for (unsigned i = 0; i < 2; i++) {
      if (!block->successors[i])
         continue;

      struct ir3_block *succ = block->successors[i];

      unsigned pred_idx = ir3_block_get_pred_index(succ, block);

      unsigned phi_count = 0;
      foreach_instr (phi, &succ->instr_list) {
         if (phi->opc != OPC_META_PHI)
            break;

         /* Avoid undef */
         if ((phi->srcs[pred_idx]->flags & IR3_REG_SSA) &&
             !phi->srcs[pred_idx]->def)
            continue;

         /* We don't support critical edges. If we were to support them,
          * we'd need to insert parallel copies after the phi node to solve
          * the lost-copy problem.
          */
         assert(i == 0 && !block->successors[1]);
         phi_count++;
      }

      if (phi_count == 0)
         continue;

      struct ir3_register *src[phi_count];
      unsigned j = 0;
      foreach_instr (phi, &succ->instr_list) {
         if (phi->opc != OPC_META_PHI)
            break;
         if ((phi->srcs[pred_idx]->flags & IR3_REG_SSA) &&
             !phi->srcs[pred_idx]->def)
            continue;
         src[j++] = phi->srcs[pred_idx];
      }
      assert(j == phi_count);

      struct ir3_instruction *pcopy =
         ir3_instr_create(block, OPC_META_PARALLEL_COPY, phi_count, phi_count);

      for (j = 0; j < phi_count; j++) {
         struct ir3_register *reg = __ssa_dst(pcopy);
         reg->flags |= src[j]->flags & (IR3_REG_HALF | IR3_REG_ARRAY);
         reg->size = reg_elems(src[j]);
      }

      for (j = 0; j < phi_count; j++) {
         pcopy->srcs[pcopy->srcs_count++] =
            ir3_reg_clone(block->shader, src[j]);
      }

      j = 0;
      foreach_instr (phi, &succ->instr_list) {
         if (phi->opc != OPC_META_PHI)
            break;
         if ((phi->srcs[pred_idx]->flags & IR3_REG_SSA) &&
             !phi->srcs[pred_idx]->def)
            continue;
         phi->srcs[pred_idx]->def = pcopy->dsts[j];
         phi->srcs[pred_idx]->flags = pcopy->dsts[j]->flags;
         j++;
      }
      assert(j == phi_count);
   }
}

void
ir3_create_parallel_copies(struct ir3 *ir)
{
   foreach_block (block, &ir->block_list) {
      create_parallel_copy(block);
   }
}

static void
index_merge_sets(struct ir3 *ir)
{
   unsigned offset = 0;
   foreach_block (block, &ir->block_list) {
      foreach_instr (instr, &block->instr_list) {
         for (unsigned i = 0; i < instr->dsts_count; i++) {
            struct ir3_register *dst = instr->dsts[i];

            unsigned dst_offset;
            struct ir3_merge_set *merge_set = dst->merge_set;
            unsigned size = reg_size(dst);
            if (merge_set) {
               if (merge_set->interval_start == ~0) {
                  merge_set->interval_start = offset;
                  offset += merge_set->size;
               }
               dst_offset = merge_set->interval_start + dst->merge_set_offset;
            } else {
               dst_offset = offset;
               offset += size;
            }

            dst->interval_start = dst_offset;
            dst->interval_end = dst_offset + size;
         }
      }
   }
}

#define RESET      "\x1b[0m"
#define BLUE       "\x1b[0;34m"
#define SYN_SSA(x) BLUE x RESET

static void
dump_merge_sets(struct ir3 *ir)
{
   printf("merge sets:\n");
   struct set *merge_sets = _mesa_pointer_set_create(NULL);
   foreach_block (block, &ir->block_list) {
      foreach_instr (instr, &block->instr_list) {
         for (unsigned i = 0; i < instr->dsts_count; i++) {
            struct ir3_register *dst = instr->dsts[i];

            struct ir3_merge_set *merge_set = dst->merge_set;
            if (!merge_set || _mesa_set_search(merge_sets, merge_set))
               continue;

            printf("merge set, size %u, align %u:\n", merge_set->size,
                   merge_set->alignment);
            for (unsigned j = 0; j < merge_set->regs_count; j++) {
               struct ir3_register *reg = merge_set->regs[j];
               printf("\t" SYN_SSA("ssa_%u") ":%u, offset %u\n",
                      reg->instr->serialno, reg->name, reg->merge_set_offset);
            }

            _mesa_set_add(merge_sets, merge_set);
         }
      }
   }

   ralloc_free(merge_sets);
}

void
ir3_merge_regs(struct ir3_liveness *live, struct ir3 *ir)
{
   index_instrs(ir3_start_block(ir), 0);

   /* First pass: coalesce phis, which must be together. */
   foreach_block (block, &ir->block_list) {
      foreach_instr (instr, &block->instr_list) {
         if (instr->opc != OPC_META_PHI)
            break;

         coalesce_phi(live, instr);
      }
   }

   /* Second pass: aggressively coalesce parallelcopy, split, collect */
   foreach_block (block, &ir->block_list) {
      foreach_instr (instr, &block->instr_list) {
         switch (instr->opc) {
         case OPC_META_SPLIT:
            aggressive_coalesce_split(live, instr);
            break;
         case OPC_META_COLLECT:
            aggressive_coalesce_collect(live, instr);
            break;
         case OPC_META_PARALLEL_COPY:
            aggressive_coalesce_parallel_copy(live, instr);
            break;
         default:
            break;
         }
      }
   }

   index_merge_sets(ir);

   if (ir3_shader_debug & IR3_DBG_RAMSGS)
      dump_merge_sets(ir);
}