mesa/src/compiler/nir/nir_opt_peephole_select.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.
 */

#include "nir/nir_builder.h"
#include "nir.h"
#include "nir_builder.h"
#include "nir_control_flow.h"

/*
 * Implements a small peephole optimization that looks for
 *
 * if (cond) {
 *    <then SSA defs>
 * } else {
 *    <else SSA defs>
 * }
 * phi
 * ...
 * phi
 *
 * and replaces it with:
 *
 * <then SSA defs>
 * <else SSA defs>
 * bcsel
 * ...
 * bcsel
 *
 * where the SSA defs are ALU operations or other cheap instructions (not
 * texturing, for example).
 *
 * If the number of ALU operations in the branches is greater than the limit
 * parameter, then the optimization is skipped.  In limit=0 mode, the SSA defs
 * must only be MOVs which we expect to get copy-propagated away once they're
 * out of the inner blocks.
 */

static bool
block_check_for_allowed_instrs(nir_block *block, unsigned *count,
                               const nir_opt_peephole_select_options *options)
{
   bool alu_ok = options->limit != 0;

   /* Used on non-control-flow HW to flatten all IFs. */
   if (options->limit == ~0) {
      nir_foreach_instr(instr, block) {
         switch (instr->type) {
         case nir_instr_type_alu:
         case nir_instr_type_deref:
         case nir_instr_type_load_const:
         case nir_instr_type_phi:
         case nir_instr_type_undef:
         case nir_instr_type_tex:
            break;

         case nir_instr_type_intrinsic: {
            nir_intrinsic_instr *intr = nir_instr_as_intrinsic(instr);
            switch (intr->intrinsic) {
            case nir_intrinsic_terminate:
            case nir_intrinsic_terminate_if:
            case nir_intrinsic_demote:
            case nir_intrinsic_demote_if:
               /* For non-CF hardware, we need to be able to move discards up
                * and flatten, so let them pass.
                */
               break;
            default:
               if (!nir_intrinsic_can_reorder(intr))
                  return false;
            }
            break;
         }

         case nir_instr_type_call:
         case nir_instr_type_jump:
         case nir_instr_type_parallel_copy:
            return false;
         }
      }
      return true;
   }

   nir_foreach_instr(instr, block) {
      switch (instr->type) {
      case nir_instr_type_intrinsic: {
         nir_intrinsic_instr *intrin = nir_instr_as_intrinsic(instr);

         switch (intrin->intrinsic) {
         case nir_intrinsic_load_deref: {
            nir_deref_instr *const deref = nir_src_as_deref(intrin->src[0]);

            switch (deref->modes) {
            case nir_var_shader_in:
            case nir_var_uniform:
            case nir_var_image:
               /* Don't try to remove flow control around an indirect load
                * because that flow control may be trying to avoid invalid
                * loads.
                */
               if (!options->indirect_load_ok && nir_deref_instr_has_indirect(deref))
                  return false;

               break;

            default:
               return false;
            }
            break;
         }

         case nir_intrinsic_load_ubo:
         case nir_intrinsic_load_ubo_vec4:
            if (!options->indirect_load_ok && !nir_src_is_const(intrin->src[1]))
               return false;
            if (!(nir_intrinsic_access(intrin) & ACCESS_CAN_SPECULATE))
               return false;
            break;

         case nir_intrinsic_load_global_constant:
         case nir_intrinsic_load_constant_agx:
            if (!options->indirect_load_ok && !nir_src_is_const(intrin->src[0]))
               return false;
            if (!(nir_intrinsic_access(intrin) & ACCESS_CAN_SPECULATE))
               return false;
            break;

         case nir_intrinsic_masked_swizzle_amd:
         case nir_intrinsic_quad_swizzle_amd:
            if (!nir_intrinsic_fetch_inactive(intrin))
               return false;
            FALLTHROUGH;
         case nir_intrinsic_load_uniform:
         case nir_intrinsic_load_preamble:
         case nir_intrinsic_load_scalar_arg_amd:
         case nir_intrinsic_load_vector_arg_amd:
         case nir_intrinsic_load_helper_invocation:
         case nir_intrinsic_is_helper_invocation:
         case nir_intrinsic_load_front_face:
         case nir_intrinsic_load_view_index:
         case nir_intrinsic_load_layer_id:
         case nir_intrinsic_load_frag_coord:
         case nir_intrinsic_load_pixel_coord:
         case nir_intrinsic_load_frag_coord_z:
         case nir_intrinsic_load_frag_coord_w:
         case nir_intrinsic_load_sample_pos:
         case nir_intrinsic_load_sample_pos_or_center:
         case nir_intrinsic_load_sample_id:
         case nir_intrinsic_load_sample_mask_in:
         case nir_intrinsic_load_vertex_id_zero_base:
         case nir_intrinsic_load_first_vertex:
         case nir_intrinsic_load_base_instance:
         case nir_intrinsic_load_instance_id:
         case nir_intrinsic_load_draw_id:
         case nir_intrinsic_load_num_workgroups:
         case nir_intrinsic_load_workgroup_id:
         case nir_intrinsic_load_local_invocation_id:
         case nir_intrinsic_load_local_invocation_index:
         case nir_intrinsic_load_subgroup_id:
         case nir_intrinsic_load_subgroup_invocation:
         case nir_intrinsic_load_num_subgroups:
         case nir_intrinsic_load_frag_shading_rate:
         case nir_intrinsic_is_sparse_texels_resident:
         case nir_intrinsic_sparse_residency_code_and:
         case nir_intrinsic_read_invocation:
         case nir_intrinsic_quad_broadcast:
         case nir_intrinsic_quad_swap_horizontal:
         case nir_intrinsic_quad_swap_vertical:
         case nir_intrinsic_quad_swap_diagonal:
         case nir_intrinsic_lane_permute_16_amd:
         case nir_intrinsic_ddx:
         case nir_intrinsic_ddx_fine:
         case nir_intrinsic_ddx_coarse:
         case nir_intrinsic_ddy:
         case nir_intrinsic_ddy_fine:
         case nir_intrinsic_ddy_coarse:
         case nir_intrinsic_load_const_ir3:
            if (!alu_ok)
               return false;
            break;

         case nir_intrinsic_terminate:
         case nir_intrinsic_terminate_if:
         case nir_intrinsic_demote:
         case nir_intrinsic_demote_if:
            if (!options->discard_ok)
               return false;
            break;

         default:
            return false;
         }

         break;
      }

      case nir_instr_type_deref:
      case nir_instr_type_load_const:
      case nir_instr_type_undef:
         break;

      case nir_instr_type_alu: {
         nir_alu_instr *mov = nir_instr_as_alu(instr);
         bool movelike = false;

         switch (mov->op) {
         case nir_op_mov:
         case nir_op_fneg:
         case nir_op_ineg:
         case nir_op_fabs:
         case nir_op_iabs:
         case nir_op_vec2:
         case nir_op_vec3:
         case nir_op_vec4:
         case nir_op_vec5:
         case nir_op_vec8:
         case nir_op_vec16:
            movelike = true;
            break;

         case nir_op_fcos:
         case nir_op_fdiv:
         case nir_op_fexp2:
         case nir_op_flog2:
         case nir_op_fmod:
         case nir_op_fpow:
         case nir_op_frcp:
         case nir_op_frem:
         case nir_op_frsq:
         case nir_op_fsin:
         case nir_op_idiv:
         case nir_op_irem:
         case nir_op_udiv:
            if (!alu_ok || !options->expensive_alu_ok)
               return false;

            break;

         default:
            if (!alu_ok) {
               /* It must be a move-like operation. */
               return false;
            }
            break;
         }

         if (alu_ok) {
            /* If the ALU operation is an fsat or a move-like operation, do
             * not count it.  The expectation is that it will eventually be
             * merged as a destination modifier or source modifier on some
             * other instruction.
             */
            if (mov->op != nir_op_fsat && !movelike)
               (*count)++;
         } else {
            /* The only uses of this definition must be phis in the successor */
            nir_foreach_use_including_if(use, &mov->def) {
               if (nir_src_is_if(use) ||
                   nir_src_parent_instr(use)->type != nir_instr_type_phi ||
                   nir_src_parent_instr(use)->block != block->successors[0])
                  return false;
            }
         }
         break;
      }

      default:
         return false;
      }
   }

   return true;
}

static nir_opt_peephole_select_options
get_options_for_if(nir_if *if_stmt,
                   const nir_opt_peephole_select_options *options)
{
   nir_opt_peephole_select_options if_options = *options;

   if (if_stmt->control == nir_selection_control_flatten) {
      /* Override driver defaults */
      if_options.limit = UINT_MAX - 1; /* Maximum without unsafe flattening. */
      if_options.indirect_load_ok = true;
      if_options.expensive_alu_ok = true;
   } else if (if_stmt->control == nir_selection_control_dont_flatten) {
      if_options.limit = 0;
      if_options.indirect_load_ok = false;
   }

   return if_options;
}

/* If we're moving discards out of the if we need to add the if's condition to it */
static void
rewrite_discard_conds(nir_instr *instr, nir_def *if_cond, bool is_else)
{
   if (instr->type != nir_instr_type_intrinsic)
      return;
   nir_intrinsic_instr *intr = nir_instr_as_intrinsic(instr);

   if (intr->intrinsic != nir_intrinsic_terminate_if &&
       intr->intrinsic != nir_intrinsic_terminate &&
       intr->intrinsic != nir_intrinsic_demote_if &&
       intr->intrinsic != nir_intrinsic_demote)
      return;

   nir_builder b = nir_builder_at(nir_before_instr(instr));

   if (is_else)
      if_cond = nir_inot(&b, if_cond);

   if (intr->intrinsic == nir_intrinsic_terminate_if ||
       intr->intrinsic == nir_intrinsic_demote_if) {
      nir_src_rewrite(&intr->src[0], nir_iand(&b, intr->src[0].ssa, if_cond));
   } else {
      if (intr->intrinsic == nir_intrinsic_terminate)
         nir_terminate_if(&b, if_cond);
      else
         nir_demote_if(&b, if_cond);
      nir_instr_remove(instr);
   }
}

/**
 * Try to collapse nested ifs:
 * This optimization turns
 *
 * if (cond1) {
 *   <allowed instruction>
 *   if (cond2) {
 *     <any code>
 *   } else {
 *   }
 * } else {
 * }
 *
 * into
 *
 * <allowed instruction>
 * if (cond1 && cond2) {
 *   <any code>
 * } else {
 * }
 *
 */
static bool
nir_opt_collapse_if(nir_if *if_stmt, nir_shader *shader,
                    const nir_opt_peephole_select_options *options)
{
   /* the if has to be nested */
   if (if_stmt->cf_node.parent->type != nir_cf_node_if)
      return false;

   nir_if *parent_if = nir_cf_node_as_if(if_stmt->cf_node.parent);

   /* check if the else block is empty */
   if (!nir_cf_list_is_empty_block(&if_stmt->else_list))
      return false;

   /* this opt doesn't make much sense if the branch is empty */
   if (nir_cf_list_is_empty_block(&if_stmt->then_list))
      return false;

   /* the nested if has to be the only cf_node:
    * i.e. <block> <if_stmt> <block> */
   if (exec_list_length(&parent_if->then_list) != 3)
      return false;

   /* check if the else block of the parent if is empty */
   if (!nir_cf_list_is_empty_block(&parent_if->else_list))
      return false;

   /* check if the block after the nested if is empty except for phis */
   nir_block *last = nir_if_last_then_block(parent_if);
   nir_instr *last_instr = nir_block_last_instr(last);
   if (last_instr && last_instr->type != nir_instr_type_phi)
      return false;

   /* check if all outer phis become trivial after merging the ifs */
   nir_foreach_instr(instr, last) {
      if (parent_if->control == nir_selection_control_flatten)
         break;

      nir_phi_instr *phi = nir_instr_as_phi(instr);
      nir_phi_src *else_src =
         nir_phi_get_src_from_block(phi, nir_if_first_else_block(if_stmt));

      nir_foreach_use(src, &phi->def) {
         assert(nir_src_parent_instr(src)->type == nir_instr_type_phi);
         nir_phi_src *phi_src =
            nir_phi_get_src_from_block(nir_instr_as_phi(nir_src_parent_instr(src)),
                                       nir_if_first_else_block(parent_if));
         if (phi_src->src.ssa != else_src->src.ssa)
            return false;
      }
   }

   /* check if the block before the nested if matches the requirements */
   nir_block *first = nir_if_first_then_block(parent_if);
   nir_opt_peephole_select_options if_options = get_options_for_if(parent_if, options);
   unsigned count = 0;
   if (!block_check_for_allowed_instrs(first, &count, &if_options))
      return false;

   if (count > if_options.limit)
      return false;

   /* trivialize succeeding phis */
   nir_foreach_instr(instr, last) {
      nir_phi_instr *phi = nir_instr_as_phi(instr);
      nir_phi_src *else_src =
         nir_phi_get_src_from_block(phi, nir_if_first_else_block(if_stmt));
      nir_foreach_use_safe(src, &phi->def) {
         nir_phi_src *phi_src =
            nir_phi_get_src_from_block(nir_instr_as_phi(nir_src_parent_instr(src)),
                                       nir_if_first_else_block(parent_if));
         if (phi_src->src.ssa == else_src->src.ssa)
            nir_src_rewrite(&phi_src->src, &phi->def);
      }
   }

   /* combine condition with potential demote/terminate */
   nir_foreach_instr_safe(instr, first)
      rewrite_discard_conds(instr, parent_if->condition.ssa, false);

   /* combine the if conditions */
   struct nir_builder b = nir_builder_at(nir_before_cf_node(&if_stmt->cf_node));
   nir_def *cond = nir_iand(&b, if_stmt->condition.ssa,
                            parent_if->condition.ssa);
   nir_src_rewrite(&if_stmt->condition, cond);

   /* move the whole inner if before the parent if */
   nir_cf_list tmp;
   nir_cf_extract(&tmp, nir_before_block(first),
                  nir_after_block(last));
   nir_cf_reinsert(&tmp, nir_before_cf_node(&parent_if->cf_node));

   /* The now empty parent if will be cleaned up by other passes */
   return true;
}

static bool
nir_opt_peephole_select_block(nir_block *block, nir_shader *shader,
                              const nir_opt_peephole_select_options *options)
{
   if (nir_cf_node_is_first(&block->cf_node))
      return false;

   nir_cf_node *prev_node = nir_cf_node_prev(&block->cf_node);
   if (prev_node->type != nir_cf_node_if)
      return false;

   nir_block *prev_block = nir_cf_node_as_block(nir_cf_node_prev(prev_node));

   /* If the last instruction before this if/else block is a jump, we can't
    * append stuff after it because it would break a bunch of assumption about
    * control flow (nir_validate expects the successor of a return/halt jump
    * to be the end of the function, which might not match the successor of
    * the if/else blocks).
    */
   if (nir_block_ends_in_return_or_halt(prev_block))
      return false;

   nir_if *if_stmt = nir_cf_node_as_if(prev_node);

   /* first, try to collapse the if */
   if (nir_opt_collapse_if(if_stmt, shader, options))
      return true;

   nir_block *then_block = nir_if_first_then_block(if_stmt);
   nir_block *else_block = nir_if_first_else_block(if_stmt);

   /* We can only have one block in each side ... */
   if (nir_if_last_then_block(if_stmt) != then_block ||
       nir_if_last_else_block(if_stmt) != else_block)
      return false;

   nir_opt_peephole_select_options if_options = get_options_for_if(if_stmt, options);

   /* ... and those blocks must only contain "allowed" instructions. */
   unsigned count = 0;
   if (!block_check_for_allowed_instrs(then_block, &count, &if_options) ||
       !block_check_for_allowed_instrs(else_block, &count, &if_options))
      return false;

   if (count > if_options.limit)
      return false;

   /* At this point, we know that the previous CFG node is an if-then
    * statement containing only moves to phi nodes in this block.  We can
    * just remove that entire CF node and replace all of the phi nodes with
    * selects.
    */

   /* First, we move the remaining instructions from the blocks to the
    * block before.  We have already guaranteed that this is safe by
    * calling block_check_for_allowed_instrs()
    */
   nir_foreach_instr_safe(instr, then_block) {
      exec_node_remove(&instr->node);
      instr->block = prev_block;
      exec_list_push_tail(&prev_block->instr_list, &instr->node);
      rewrite_discard_conds(instr, if_stmt->condition.ssa, false);
   }

   nir_foreach_instr_safe(instr, else_block) {
      exec_node_remove(&instr->node);
      instr->block = prev_block;
      exec_list_push_tail(&prev_block->instr_list, &instr->node);
      rewrite_discard_conds(instr, if_stmt->condition.ssa, true);
   }

   nir_foreach_phi_safe(phi, block) {
      nir_alu_instr *sel = nir_alu_instr_create(shader, nir_op_bcsel);
      sel->src[0].src = nir_src_for_ssa(if_stmt->condition.ssa);
      /* Splat the condition to all channels */
      memset(sel->src[0].swizzle, 0, sizeof sel->src[0].swizzle);

      assert(exec_list_length(&phi->srcs) == 2);
      nir_foreach_phi_src(src, phi) {
         assert(src->pred == then_block || src->pred == else_block);

         unsigned idx = src->pred == then_block ? 1 : 2;
         sel->src[idx].src = nir_src_for_ssa(src->src.ssa);
      }

      nir_def_init(&sel->instr, &sel->def,
                   phi->def.num_components, phi->def.bit_size);

      nir_def_rewrite_uses(&phi->def,
                           &sel->def);

      nir_instr_insert_before(&phi->instr, &sel->instr);
      nir_instr_remove(&phi->instr);
   }

   nir_cf_node_remove(&if_stmt->cf_node);
   return true;
}

static bool
nir_opt_peephole_select_impl(nir_function_impl *impl,
                             const nir_opt_peephole_select_options *options)
{
   nir_shader *shader = impl->function->shader;
   bool progress = false;

   nir_foreach_block_safe(block, impl) {
      progress |= nir_opt_peephole_select_block(block, shader, options);
   }

   return nir_progress(progress, impl, nir_metadata_none);
}

bool
nir_opt_peephole_select(nir_shader *shader,
                        const nir_opt_peephole_select_options *options)
{
   bool progress = false;

   nir_foreach_function_impl(impl, shader) {
      progress |= nir_opt_peephole_select_impl(impl, options);
   }

   return progress;
}