/* * 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_instr_set.h" /* * Implements Global Code Motion. A description of GCM can be found in * "Global Code Motion; Global Value Numbering" by Cliff Click. * Unfortunately, the algorithm presented in the paper is broken in a * number of ways. The algorithm used here differs substantially from the * one in the paper but it is, in my opinion, much easier to read and * verify correcness. */ struct gcm_block_info { /* Number of loops this block is inside */ unsigned loop_depth; /* The last instruction inserted into this block. This is used as we * traverse the instructions and insert them back into the program to * put them in the right order. */ nir_instr *last_instr; }; /* Flags used in the instr->pass_flags field for various instruction states */ enum { GCM_INSTR_PINNED = (1 << 0), GCM_INSTR_SCHEDULED_EARLY = (1 << 1), GCM_INSTR_SCHEDULED_LATE = (1 << 2), GCM_INSTR_PLACED = (1 << 3), }; struct gcm_state { nir_function_impl *impl; nir_instr *instr; /* The list of non-pinned instructions. As we do the late scheduling, * we pull non-pinned instructions out of their blocks and place them in * this list. This saves us from having linked-list problems when we go * to put instructions back in their blocks. */ struct exec_list instrs; struct gcm_block_info *blocks; }; /* Recursively walks the CFG and builds the block_info structure */ static void gcm_build_block_info(struct exec_list *cf_list, struct gcm_state *state, unsigned loop_depth) { foreach_list_typed(nir_cf_node, node, node, cf_list) { switch (node->type) { case nir_cf_node_block: { nir_block *block = nir_cf_node_as_block(node); state->blocks[block->index].loop_depth = loop_depth; break; } case nir_cf_node_if: { nir_if *if_stmt = nir_cf_node_as_if(node); gcm_build_block_info(&if_stmt->then_list, state, loop_depth); gcm_build_block_info(&if_stmt->else_list, state, loop_depth); break; } case nir_cf_node_loop: { nir_loop *loop = nir_cf_node_as_loop(node); gcm_build_block_info(&loop->body, state, loop_depth + 1); break; } default: unreachable("Invalid CF node type"); } } } /* Walks the instruction list and marks immovable instructions as pinned * * This function also serves to initialize the instr->pass_flags field. * After this is completed, all instructions' pass_flags fields will be set * to either GCM_INSTR_PINNED or 0. */ static bool gcm_pin_instructions_block(nir_block *block, struct gcm_state *state) { nir_foreach_instr_safe(instr, block) { switch (instr->type) { case nir_instr_type_alu: switch (nir_instr_as_alu(instr)->op) { case nir_op_fddx: case nir_op_fddy: case nir_op_fddx_fine: case nir_op_fddy_fine: case nir_op_fddx_coarse: case nir_op_fddy_coarse: /* These can only go in uniform control flow; pin them for now */ instr->pass_flags = GCM_INSTR_PINNED; break; default: instr->pass_flags = 0; break; } break; case nir_instr_type_deref: instr->pass_flags = 0; break; case nir_instr_type_tex: switch (nir_instr_as_tex(instr)->op) { case nir_texop_tex: case nir_texop_txb: case nir_texop_lod: /* These two take implicit derivatives so they need to be pinned */ instr->pass_flags = GCM_INSTR_PINNED; break; default: instr->pass_flags = 0; break; } break; case nir_instr_type_load_const: instr->pass_flags = 0; break; case nir_instr_type_intrinsic: { const nir_intrinsic_info *info = &nir_intrinsic_infos[nir_instr_as_intrinsic(instr)->intrinsic]; if ((info->flags & NIR_INTRINSIC_CAN_ELIMINATE) && (info->flags & NIR_INTRINSIC_CAN_REORDER)) { instr->pass_flags = 0; } else { instr->pass_flags = GCM_INSTR_PINNED; } break; } case nir_instr_type_jump: case nir_instr_type_ssa_undef: case nir_instr_type_phi: instr->pass_flags = GCM_INSTR_PINNED; break; default: unreachable("Invalid instruction type in GCM"); } if (!(instr->pass_flags & GCM_INSTR_PINNED)) { /* If this is an unpinned instruction, go ahead and pull it out of * the program and put it on the instrs list. This has a couple * of benifits. First, it makes the scheduling algorithm more * efficient because we can avoid walking over basic blocks and * pinned instructions. Second, it keeps us from causing linked * list confusion when we're trying to put everything in its * proper place at the end of the pass. * * Note that we don't use nir_instr_remove here because that also * cleans up uses and defs and we want to keep that information. */ exec_node_remove(&instr->node); exec_list_push_tail(&state->instrs, &instr->node); } } return true; } static void gcm_schedule_early_instr(nir_instr *instr, struct gcm_state *state); /** Update an instructions schedule for the given source * * This function is called iteratively as we walk the sources of an * instruction. It ensures that the given source instruction has been * scheduled and then update this instruction's block if the source * instruction is lower down the tree. */ static bool gcm_schedule_early_src(nir_src *src, void *void_state) { struct gcm_state *state = void_state; nir_instr *instr = state->instr; assert(src->is_ssa); gcm_schedule_early_instr(src->ssa->parent_instr, void_state); /* While the index isn't a proper dominance depth, it does have the * property that if A dominates B then A->index <= B->index. Since we * know that this instruction must have been dominated by all of its * sources at some point (even if it's gone through value-numbering), * all of the sources must lie on the same branch of the dominance tree. * Therefore, we can just go ahead and just compare indices. */ if (instr->block->index < src->ssa->parent_instr->block->index) instr->block = src->ssa->parent_instr->block; /* We need to restore the state instruction because it may have been * changed through the gcm_schedule_early_instr call above. Since we * may still be iterating through sources and future calls to * gcm_schedule_early_src for the same instruction will still need it. */ state->instr = instr; return true; } /** Schedules an instruction early * * This function performs a recursive depth-first search starting at the * given instruction and proceeding through the sources to schedule * instructions as early as they can possibly go in the dominance tree. * The instructions are "scheduled" by updating their instr->block field. */ static void gcm_schedule_early_instr(nir_instr *instr, struct gcm_state *state) { if (instr->pass_flags & GCM_INSTR_SCHEDULED_EARLY) return; instr->pass_flags |= GCM_INSTR_SCHEDULED_EARLY; /* Pinned instructions are already scheduled so we don't need to do * anything. Also, bailing here keeps us from ever following the * sources of phi nodes which can be back-edges. */ if (instr->pass_flags & GCM_INSTR_PINNED) return; /* Start with the instruction at the top. As we iterate over the * sources, it will get moved down as needed. */ instr->block = nir_start_block(state->impl); state->instr = instr; nir_foreach_src(instr, gcm_schedule_early_src, state); } static void gcm_schedule_late_instr(nir_instr *instr, struct gcm_state *state); /** Schedules the instruction associated with the given SSA def late * * This function works by first walking all of the uses of the given SSA * definition, ensuring that they are scheduled, and then computing the LCA * (least common ancestor) of its uses. It then schedules this instruction * as close to the LCA as possible while trying to stay out of loops. */ static bool gcm_schedule_late_def(nir_ssa_def *def, void *void_state) { struct gcm_state *state = void_state; nir_block *lca = NULL; nir_foreach_use(use_src, def) { nir_instr *use_instr = use_src->parent_instr; gcm_schedule_late_instr(use_instr, state); /* Phi instructions are a bit special. SSA definitions don't have to * dominate the sources of the phi nodes that use them; instead, they * have to dominate the predecessor block corresponding to the phi * source. We handle this by looking through the sources, finding * any that are usingg this SSA def, and using those blocks instead * of the one the phi lives in. */ if (use_instr->type == nir_instr_type_phi) { nir_phi_instr *phi = nir_instr_as_phi(use_instr); nir_foreach_phi_src(phi_src, phi) { if (phi_src->src.ssa == def) lca = nir_dominance_lca(lca, phi_src->pred); } } else { lca = nir_dominance_lca(lca, use_instr->block); } } nir_foreach_if_use(use_src, def) { nir_if *if_stmt = use_src->parent_if; /* For if statements, we consider the block to be the one immediately * preceding the if CF node. */ nir_block *pred_block = nir_cf_node_as_block(nir_cf_node_prev(&if_stmt->cf_node)); lca = nir_dominance_lca(lca, pred_block); } /* Some instructions may never be used. We'll just leave them scheduled * early and let dead code clean them up. */ if (lca == NULL) return true; /* We now have the LCA of all of the uses. If our invariants hold, * this is dominated by the block that we chose when scheduling early. * We now walk up the dominance tree and pick the lowest block that is * as far outside loops as we can get. */ nir_block *best = lca; for (nir_block *block = lca; block != NULL; block = block->imm_dom) { if (state->blocks[block->index].loop_depth < state->blocks[best->index].loop_depth) best = block; if (block == def->parent_instr->block) break; } def->parent_instr->block = best; return true; } /** Schedules an instruction late * * This function performs a depth-first search starting at the given * instruction and proceeding through its uses to schedule instructions as * late as they can reasonably go in the dominance tree. The instructions * are "scheduled" by updating their instr->block field. * * The name of this function is actually a bit of a misnomer as it doesn't * schedule them "as late as possible" as the paper implies. Instead, it * first finds the lates possible place it can schedule the instruction and * then possibly schedules it earlier than that. The actual location is as * far down the tree as we can go while trying to stay out of loops. */ static void gcm_schedule_late_instr(nir_instr *instr, struct gcm_state *state) { if (instr->pass_flags & GCM_INSTR_SCHEDULED_LATE) return; instr->pass_flags |= GCM_INSTR_SCHEDULED_LATE; /* Pinned instructions are already scheduled so we don't need to do * anything. Also, bailing here keeps us from ever following phi nodes * which can be back-edges. */ if (instr->pass_flags & GCM_INSTR_PINNED) return; nir_foreach_ssa_def(instr, gcm_schedule_late_def, state); } static void gcm_place_instr(nir_instr *instr, struct gcm_state *state); static bool gcm_place_instr_def(nir_ssa_def *def, void *state) { nir_foreach_use(use_src, def) gcm_place_instr(use_src->parent_instr, state); return false; } /** Places an instrution back into the program * * The earlier passes of GCM simply choose blocks for each instruction and * otherwise leave them alone. This pass actually places the instructions * into their chosen blocks. * * To do so, we use a standard post-order depth-first search linearization * algorithm. We walk over the uses of the given instruction and ensure * that they are placed and then place this instruction. Because we are * working on multiple blocks at a time, we keep track of the last inserted * instruction per-block in the state structure's block_info array. When * we insert an instruction in a block we insert it before the last * instruction inserted in that block rather than the last instruction * inserted globally. */ static void gcm_place_instr(nir_instr *instr, struct gcm_state *state) { if (instr->pass_flags & GCM_INSTR_PLACED) return; instr->pass_flags |= GCM_INSTR_PLACED; /* Phi nodes are our once source of back-edges. Since right now we are * only doing scheduling within blocks, we don't need to worry about * them since they are always at the top. Just skip them completely. */ if (instr->type == nir_instr_type_phi) { assert(instr->pass_flags & GCM_INSTR_PINNED); return; } nir_foreach_ssa_def(instr, gcm_place_instr_def, state); if (instr->pass_flags & GCM_INSTR_PINNED) { /* Pinned instructions have an implicit dependence on the pinned * instructions that come after them in the block. Since the pinned * instructions will naturally "chain" together, we only need to * explicitly visit one of them. */ for (nir_instr *after = nir_instr_next(instr); after; after = nir_instr_next(after)) { if (after->pass_flags & GCM_INSTR_PINNED) { gcm_place_instr(after, state); break; } } } struct gcm_block_info *block_info = &state->blocks[instr->block->index]; if (!(instr->pass_flags & GCM_INSTR_PINNED)) { exec_node_remove(&instr->node); if (block_info->last_instr) { exec_node_insert_node_before(&block_info->last_instr->node, &instr->node); } else { /* Schedule it at the end of the block */ nir_instr *jump_instr = nir_block_last_instr(instr->block); if (jump_instr && jump_instr->type == nir_instr_type_jump) { exec_node_insert_node_before(&jump_instr->node, &instr->node); } else { exec_list_push_tail(&instr->block->instr_list, &instr->node); } } } block_info->last_instr = instr; } static bool opt_gcm_impl(nir_function_impl *impl, bool value_number) { nir_metadata_require(impl, nir_metadata_block_index | nir_metadata_dominance); struct gcm_state state; state.impl = impl; state.instr = NULL; exec_list_make_empty(&state.instrs); state.blocks = rzalloc_array(NULL, struct gcm_block_info, impl->num_blocks); gcm_build_block_info(&impl->body, &state, 0); nir_foreach_block(block, impl) { gcm_pin_instructions_block(block, &state); } bool progress = false; if (value_number) { struct set *gvn_set = nir_instr_set_create(NULL); foreach_list_typed_safe(nir_instr, instr, node, &state.instrs) { if (nir_instr_set_add_or_rewrite(gvn_set, instr)) { nir_instr_remove(instr); progress = true; } } nir_instr_set_destroy(gvn_set); } foreach_list_typed(nir_instr, instr, node, &state.instrs) gcm_schedule_early_instr(instr, &state); foreach_list_typed(nir_instr, instr, node, &state.instrs) gcm_schedule_late_instr(instr, &state); while (!exec_list_is_empty(&state.instrs)) { nir_instr *instr = exec_node_data(nir_instr, state.instrs.tail_sentinel.prev, node); gcm_place_instr(instr, &state); } ralloc_free(state.blocks); nir_metadata_preserve(impl, nir_metadata_block_index | nir_metadata_dominance); return progress; } bool nir_opt_gcm(nir_shader *shader, bool value_number) { bool progress = false; nir_foreach_function(function, shader) { if (function->impl) progress |= opt_gcm_impl(function->impl, value_number); } return progress; }