third_party_mesa3d/src/asahi/compiler/agx_register_allocate.c

/*
 * Copyright (C) 2021 Alyssa Rosenzweig <alyssa@rosenzweig.io>
 *
 * 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 "agx_compiler.h"
#include "agx_builder.h"

/* SSA-based register allocator */

struct ra_ctx {
   agx_context *shader;
   agx_block *block;
   uint8_t *ssa_to_reg;
   uint8_t *ncomps;
   BITSET_WORD *visited;
   BITSET_WORD *used_regs;

   /* For affinities */
   agx_instr **src_to_collect;

   /* Maximum number of registers that RA is allowed to use */
   unsigned bound;
};

/** Returns number of registers written by an instruction */
unsigned
agx_write_registers(agx_instr *I, unsigned d)
{
   unsigned size = agx_size_align_16(I->dest[d].size);

   switch (I->op) {
   case AGX_OPCODE_ITER:
      assert(1 <= I->channels && I->channels <= 4);
      return I->channels * size;

   case AGX_OPCODE_DEVICE_LOAD:
   case AGX_OPCODE_TEXTURE_LOAD:
   case AGX_OPCODE_TEXTURE_SAMPLE:
   case AGX_OPCODE_LD_TILE:
      return util_bitcount(I->mask) * size;

   case AGX_OPCODE_LDCF:
      return 6;
   case AGX_OPCODE_COLLECT:
      return I->nr_srcs * agx_size_align_16(I->src[0].size);
   default:
      return size;
   }
}

static inline enum agx_size
agx_split_width(const agx_instr *I)
{
   enum agx_size width = ~0;

   agx_foreach_dest(I, d) {
      if (I->dest[d].type == AGX_INDEX_NULL)
         continue;
      else if (width != ~0)
         assert(width == I->dest[d].size);
      else
         width = I->dest[d].size;
   }

   assert(width != ~0 && "should have been DCE'd");
   return width;
}

static unsigned
find_regs(BITSET_WORD *used_regs, unsigned count, unsigned align, unsigned max)
{
   assert(count >= 1);

   for (unsigned reg = 0; reg < max; reg += align) {
      if (!BITSET_TEST_RANGE(used_regs, reg, reg + count - 1))
         return reg;
   }

   /* Couldn't find a free register, dump the state of the register file */
   fprintf(stderr, "Failed to find register of size %u aligned %u max %u.\n",
           count, align, max);

   fprintf(stderr, "Register file:\n");
   for (unsigned i = 0; i < BITSET_WORDS(max); ++i)
      fprintf(stderr, "    %08X\n", used_regs[i]);

   unreachable("Could not find a free register");
}

/*
 * Loop over live-in values at the start of the block and mark their registers
 * as in-use. We process blocks in dominance order, so this handles everything
 * but loop headers.
 *
 * For loop headers, this handles the forward edges but not the back edge.
 * However, that's okay: we don't want to reserve the registers that are
 * defined within the loop, because then we'd get a contradiction. Instead we
 * leave them available and then they become fixed points of a sort.
 */
static void
reserve_live_in(struct ra_ctx *rctx)
{
      int i;
      BITSET_FOREACH_SET(i, rctx->block->live_in, rctx->shader->alloc) {
         /* Skip values defined in loops when processing the loop header */
         if (!BITSET_TEST(rctx->visited, i))
            continue;

         for (unsigned j = 0; j < rctx->ncomps[i]; ++j)
            BITSET_SET(rctx->used_regs, rctx->ssa_to_reg[i] + j);
      }
}

static void
assign_regs(struct ra_ctx *rctx, agx_index v, unsigned reg)
{
   assert(v.type == AGX_INDEX_NORMAL && "only SSA gets registers allocated");
   rctx->ssa_to_reg[v.value] = reg;

   assert(!BITSET_TEST(rctx->visited, v.value) && "SSA violated");
   BITSET_SET(rctx->visited, v.value);

   assert(rctx->ncomps[v.value] >= 1);
   unsigned end = reg + rctx->ncomps[v.value] - 1;
   assert(!BITSET_TEST_RANGE(rctx->used_regs, reg, end) && "no interference");
   BITSET_SET_RANGE(rctx->used_regs, reg, end);
}

static unsigned
affinity_base_of_collect(struct ra_ctx *rctx, agx_instr *collect, unsigned src)
{
   unsigned src_reg = rctx->ssa_to_reg[collect->src[src].value];
   unsigned src_offset = src * agx_size_align_16(collect->src[src].size);

   if (src_reg >= src_offset)
      return src_reg - src_offset;
   else
      return ~0;
}

static unsigned
pick_regs(struct ra_ctx *rctx, agx_instr *I, unsigned d)
{
   agx_index idx = I->dest[d];
   assert(idx.type == AGX_INDEX_NORMAL);

   unsigned count = agx_write_registers(I, d);
   unsigned align = agx_size_align_16(idx.size);
   assert(count >= 1);

   /* Try to allocate collects compatibly with their sources */
   if (I->op == AGX_OPCODE_COLLECT) {
      agx_foreach_ssa_src(I, s) {
         assert(BITSET_TEST(rctx->visited, I->src[s].value) &&
                "registers assigned in an order compatible with dominance "
                "and this is not a phi node, so we have assigned a register");

         unsigned base = affinity_base_of_collect(rctx, I, s);
         if (base >= rctx->bound || (base + count) > rctx->bound)
            continue;

         /* Unaligned destinations can happen when dest size > src size */
         if (base % align)
            continue;

         if (!BITSET_TEST_RANGE(rctx->used_regs, base, base + count - 1))
            return base;
      }
   }

   /* Try to allocate sources of collects contiguously */
   if (rctx->src_to_collect[idx.value] != NULL) {
      agx_instr *collect = rctx->src_to_collect[idx.value];

      assert(count == align && "collect sources are scalar");

      /* Find our offset in the collect. If our source is repeated in the
       * collect, this may not be unique. We arbitrarily choose the first.
       */
      unsigned our_source = ~0;
      agx_foreach_ssa_src(collect, s) {
         if (agx_is_equiv(collect->src[s], idx)) {
            our_source = s;
            break;
         }
      }

      assert(our_source < collect->nr_srcs && "source must be in the collect");

      /* See if we can allocate compatibly with any source of the collect */
      agx_foreach_ssa_src(collect, s) {
         if (!BITSET_TEST(rctx->visited, collect->src[s].value))
            continue;

         /* Determine where the collect should start relative to the source */
         unsigned base = affinity_base_of_collect(rctx, collect, s);
         if (base >= rctx->bound)
            continue;

         unsigned our_reg = base + (our_source * align);

         /* Don't allocate past the end of the register file */
         if ((our_reg + align) > rctx->bound)
            continue;

         /* If those registers are free, then choose them */
         if (!BITSET_TEST_RANGE(rctx->used_regs, our_reg, our_reg + align - 1))
            return our_reg;
      }
   }

   /* Default to any contiguous sequence of registers */
   return find_regs(rctx->used_regs, count, align, rctx->bound);
}

/** Assign registers to SSA values in a block. */

static void
agx_ra_assign_local(struct ra_ctx *rctx)
{
   BITSET_DECLARE(used_regs, AGX_NUM_REGS) = { 0 };

   agx_block *block = rctx->block;
   uint8_t *ssa_to_reg = rctx->ssa_to_reg;
   uint8_t *ncomps = rctx->ncomps;
   rctx->used_regs = used_regs;

   reserve_live_in(rctx);

   /* Force the nesting counter r0l live throughout shaders using control flow.
    * This could be optimized (sync with agx_calc_register_demand).
    */
   if (rctx->shader->any_cf)
      BITSET_SET(used_regs, 0);

   agx_foreach_instr_in_block(block, I) {
      /* Optimization: if a split contains the last use of a vector, the split
       * can be removed by assigning the destinations overlapping the source.
       */
      if (I->op == AGX_OPCODE_SPLIT && I->src[0].kill) {
         unsigned reg = ssa_to_reg[I->src[0].value];
         unsigned width = agx_size_align_16(agx_split_width(I));

         agx_foreach_dest(I, d) {
            /* Free up the source */
            unsigned offset_reg = reg + (d * width);
            BITSET_CLEAR_RANGE(used_regs, offset_reg, offset_reg + width - 1);

            /* Assign the destination where the source was */
            if (!agx_is_null(I->dest[d]))
               assign_regs(rctx, I->dest[d], offset_reg);
         }

         continue;
      } else if (I->op == AGX_OPCODE_PRELOAD) {
         /* We must coalesce all preload moves */
         assert(I->dest[0].size == I->src[0].size);
         assert(I->src[0].type == AGX_INDEX_REGISTER);

         assign_regs(rctx, I->dest[0], I->src[0].value);
         continue;
      }

      /* First, free killed sources */
      agx_foreach_ssa_src(I, s) {
         if (I->src[s].kill) {
            unsigned reg = ssa_to_reg[I->src[s].value];
            unsigned count = ncomps[I->src[s].value];

            assert(count >= 1);
            BITSET_CLEAR_RANGE(used_regs, reg, reg + count - 1);
         }
      }

      /* Next, assign destinations one at a time. This is always legal
       * because of the SSA form.
       */
      agx_foreach_ssa_dest(I, d) {
         assign_regs(rctx, I->dest[d], pick_regs(rctx, I, d));
      }
   }

   STATIC_ASSERT(sizeof(block->regs_out) == sizeof(used_regs));
   memcpy(block->regs_out, used_regs, sizeof(used_regs));
}

/*
 * Lower phis to parallel copies at the logical end of a given block. If a block
 * needs parallel copies inserted, a successor of the block has a phi node. To
 * have a (nontrivial) phi node, a block must have multiple predecessors. So the
 * edge from the block to the successor (with phi) is not the only edge entering
 * the successor. Because the control flow graph has no critical edges, this
 * edge must therefore be the only edge leaving the block, so the block must
 * have only a single successor.
 */
static void
agx_insert_parallel_copies(agx_context *ctx, agx_block *block)
{
   bool any_succ = false;
   unsigned nr_phi = 0;

   /* Phi nodes logically happen on the control flow edge, so parallel copies
    * are added at the end of the predecessor */
   agx_builder b = agx_init_builder(ctx, agx_after_block_logical(block));

   agx_foreach_successor(block, succ) {
      assert(nr_phi == 0 && "control flow graph has a critical edge");

      agx_foreach_phi_in_block(succ, phi) {
         assert(!any_succ && "control flow graph has a critical edge");
         nr_phi++;
      }

      any_succ = true;

      /* Nothing to do if there are no phi nodes */
      if (nr_phi == 0)
         continue;

      unsigned pred_index = agx_predecessor_index(succ, block);

      /* Create a parallel copy lowering all the phi nodes */
      struct agx_copy *copies = calloc(sizeof(*copies), nr_phi);

      unsigned i = 0;

      agx_foreach_phi_in_block(succ, phi) {
         agx_index dest = phi->dest[0];
         agx_index src = phi->src[pred_index];

         assert(dest.type == AGX_INDEX_REGISTER);
         assert(src.type == AGX_INDEX_REGISTER);
         assert(dest.size == src.size);

         copies[i++] = (struct agx_copy) {
            .dest = dest.value,
            .src = src,
         };
      }

      agx_emit_parallel_copies(&b, copies, nr_phi);

      free(copies);
   }
}

void
agx_ra(agx_context *ctx)
{
   unsigned *alloc = calloc(ctx->alloc, sizeof(unsigned));

   agx_compute_liveness(ctx);
   uint8_t *ssa_to_reg = calloc(ctx->alloc, sizeof(uint8_t));
   uint8_t *ncomps = calloc(ctx->alloc, sizeof(uint8_t));
   agx_instr **src_to_collect = calloc(ctx->alloc, sizeof(agx_instr *));
   BITSET_WORD *visited = calloc(BITSET_WORDS(ctx->alloc), sizeof(BITSET_WORD));

   agx_foreach_instr_global(ctx, I) {
      /* Record collects so we can coalesce when assigning */
      if (I->op == AGX_OPCODE_COLLECT) {
         agx_foreach_ssa_src(I, s) {
            src_to_collect[I->src[s].value] = I;
         }
      }

      agx_foreach_ssa_dest(I, d) {
         unsigned v = I->dest[d].value;
         assert(ncomps[v] == 0 && "broken SSA");
         ncomps[v] = agx_write_registers(I, d);
      }
   }

   /* Assign registers in dominance-order. This coincides with source-order due
    * to a NIR invariant, so we do not need special handling for this.
    */
   agx_foreach_block(ctx, block) {
      agx_ra_assign_local(&(struct ra_ctx) {
         .shader = ctx,
         .block = block,
         .ssa_to_reg = ssa_to_reg,
         .src_to_collect = src_to_collect,
         .ncomps = ncomps,
         .visited = visited,
         .bound = AGX_NUM_REGS
      });
   }

   for (unsigned i = 0; i < ctx->alloc; ++i) {
      if (ncomps[i])
         ctx->max_reg = MAX2(ctx->max_reg, ssa_to_reg[i] + ncomps[i] - 1);
   }

   agx_foreach_instr_global(ctx, ins) {
      agx_foreach_ssa_src(ins, s) {
         unsigned v = ssa_to_reg[ins->src[s].value];
         agx_replace_src(ins, s, agx_register(v, ins->src[s].size));
      }

      agx_foreach_ssa_dest(ins, d) {
         unsigned v = ssa_to_reg[ins->dest[d].value];
         ins->dest[d] = agx_replace_index(ins->dest[d], agx_register(v, ins->dest[d].size));
      }
   }

   agx_foreach_instr_global_safe(ctx, ins) {
      /* Lower away RA pseudo-instructions */
      agx_builder b = agx_init_builder(ctx, agx_after_instr(ins));

      if (ins->op == AGX_OPCODE_COLLECT) {
         assert(ins->dest[0].type == AGX_INDEX_REGISTER);
         unsigned base = ins->dest[0].value;
         unsigned width = agx_size_align_16(ins->src[0].size);

         struct agx_copy *copies = alloca(sizeof(copies[0]) * ins->nr_srcs);
         unsigned n = 0;

         /* Move the sources */
         agx_foreach_src(ins, i) {
            if (agx_is_null(ins->src[i])) continue;
            assert(ins->src[i].size == ins->src[0].size);

            copies[n++] = (struct agx_copy) {
               .dest = base + (i * width),
               .src = ins->src[i]
            };
         }

         agx_emit_parallel_copies(&b, copies, n);
         agx_remove_instruction(ins);
         continue;
      } else if (ins->op == AGX_OPCODE_SPLIT) {
         assert(ins->src[0].type == AGX_INDEX_REGISTER);
         unsigned base = ins->src[0].value;
         unsigned width = agx_size_align_16(agx_split_width(ins));

         struct agx_copy copies[4];
         assert(ins->nr_dests <= ARRAY_SIZE(copies));

         unsigned n = 0;

         /* Move the sources */
         agx_foreach_dest(ins, i) {
            if (ins->dest[i].type != AGX_INDEX_REGISTER)
               continue;

            copies[n++] = (struct agx_copy) {
               .dest = ins->dest[i].value,
               .src = agx_register(base + (i * width), ins->dest[i].size)
            };
         }

         /* Lower away */
         agx_builder b = agx_init_builder(ctx, agx_after_instr(ins));
         agx_emit_parallel_copies(&b, copies, n);
         agx_remove_instruction(ins);
         continue;
      }


   }

   /* Insert parallel copies lowering phi nodes */
   agx_foreach_block(ctx, block) {
      agx_insert_parallel_copies(ctx, block);
   }

   agx_foreach_instr_global_safe(ctx, I) {
      switch (I->op) {
      /* Pseudoinstructions for RA must be removed now */
      case AGX_OPCODE_PHI:
      case AGX_OPCODE_LOGICAL_END:
      case AGX_OPCODE_PRELOAD:
         agx_remove_instruction(I);
         break;

      /* Coalesced moves can be removed */
      case AGX_OPCODE_MOV:
         if (I->src[0].type == AGX_INDEX_REGISTER &&
             I->dest[0].size == I->src[0].size &&
             I->src[0].value == I->dest[0].value) {

            assert(I->dest[0].type == AGX_INDEX_REGISTER);
            agx_remove_instruction(I);
         }
         break;

      /* Writes to the nesting counter lowered to the real register */
      case AGX_OPCODE_NEST: {
         agx_builder b = agx_init_builder(ctx, agx_before_instr(I));
         agx_mov_to(&b, agx_register(0, AGX_SIZE_16), I->src[0]);
         agx_remove_instruction(I);
         break;
      }

      default:
         break;
      }
   }

   free(src_to_collect);
   free(ssa_to_reg);
   free(ncomps);
   free(visited);
   free(alloc);
}