/* * Copyright © 2014 Connor Abbott * * 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: * Connor Abbott (cwabbott0@gmail.com) * */ #include "nir.h" #include "nir_control_flow.h" /* * This file implements an optimization that deletes statically * unreachable/dead code. In NIR, one way this can happen is when an if * statement has a constant condition: * * if (true) { * ... * } * * We delete the if statement and paste the contents of the always-executed * branch into the surrounding control flow, possibly removing more code if * the branch had a jump at the end. * * Another way is that control flow can end in a jump so that code after it * never gets executed. In particular, this can happen after optimizing * something like: * * if (true) { * ... * break; * } * ... * * We also consider the case where both branches of an if end in a jump, e.g.: * * if (...) { * break; * } else { * continue; * } * ... * * Finally, we also handle removing useless loops and ifs, i.e. loops and ifs * with no side effects and without any definitions that are used * elsewhere. This case is a little different from the first two in that the * code is actually run (it just never does anything), but there are similar * issues with needing to be careful with restarting after deleting the * cf_node (see dead_cf_list()) so this is a convenient place to remove them. */ static void remove_after_cf_node(nir_cf_node *node) { nir_cf_node *end = node; while (!nir_cf_node_is_last(end)) end = nir_cf_node_next(end); nir_cf_list list; nir_cf_extract(&list, nir_after_cf_node(node), nir_after_cf_node(end)); nir_cf_delete(&list); } static void opt_constant_if(nir_if *if_stmt, bool condition) { /* First, we need to remove any phi nodes after the if by rewriting uses to * point to the correct source. */ nir_block *after = nir_cf_node_as_block(nir_cf_node_next(&if_stmt->cf_node)); nir_block *last_block = condition ? nir_if_last_then_block(if_stmt) : nir_if_last_else_block(if_stmt); nir_foreach_instr_safe(instr, after) { if (instr->type != nir_instr_type_phi) break; nir_phi_instr *phi = nir_instr_as_phi(instr); nir_ssa_def *def = NULL; nir_foreach_phi_src(phi_src, phi) { if (phi_src->pred != last_block) continue; assert(phi_src->src.is_ssa); def = phi_src->src.ssa; } assert(def); assert(phi->dest.is_ssa); nir_ssa_def_rewrite_uses(&phi->dest.ssa, def); nir_instr_remove(instr); } /* The control flow list we're about to paste in may include a jump at the * end, and in that case we have to delete the rest of the control flow * list after the if since it's unreachable and the validator will balk if * we don't. */ if (!exec_list_is_empty(&last_block->instr_list)) { nir_instr *last_instr = nir_block_last_instr(last_block); if (last_instr->type == nir_instr_type_jump) remove_after_cf_node(&if_stmt->cf_node); } /* Finally, actually paste in the then or else branch and delete the if. */ struct exec_list *cf_list = condition ? &if_stmt->then_list : &if_stmt->else_list; nir_cf_list list; nir_cf_list_extract(&list, cf_list); nir_cf_reinsert(&list, nir_after_cf_node(&if_stmt->cf_node)); nir_cf_node_remove(&if_stmt->cf_node); } static bool def_only_used_in_cf_node(nir_ssa_def *def, void *_node) { nir_cf_node *node = _node; assert(node->type == nir_cf_node_loop || node->type == nir_cf_node_if); nir_block *before = nir_cf_node_as_block(nir_cf_node_prev(node)); nir_block *after = nir_cf_node_as_block(nir_cf_node_next(node)); nir_foreach_use(use, def) { /* Because NIR is structured, we can easily determine whether or not a * value escapes a CF node by looking at the block indices of its uses * to see if they lie outside the bounds of the CF node. * * Note: Normally, the uses of a phi instruction are considered to be * used in the block that is the predecessor of the phi corresponding to * that use. If we were computing liveness or something similar, that * would mean a special case here for phis. However, we're trying here * to determine if the SSA def ever escapes the loop. If it's used by a * phi that lives outside the loop then it doesn't matter if the * corresponding predecessor is inside the loop or not because the value * can go through the phi into the outside world and escape the loop. */ if (use->parent_instr->block->index <= before->index || use->parent_instr->block->index >= after->index) return false; } /* Same check for if-condition uses */ nir_foreach_if_use(use, def) { nir_block *use_block = nir_cf_node_as_block(nir_cf_node_prev(&use->parent_if->cf_node)); if (use_block->index <= before->index || use_block->index >= after->index) return false; } return true; } /* * Test if a loop node is dead. Such nodes are dead if: * * 1) It has no side effects (i.e. intrinsics which could possibly affect the * state of the program aside from producing an SSA value, indicated by a lack * of NIR_INTRINSIC_CAN_ELIMINATE). * * 2) It has no phi instructions after it, since those indicate values inside * the node being used after the node. * * 3) None of the values defined inside the node is used outside the node, * i.e. none of the definitions that dominate the node exit are used outside. * * If those conditions hold, then the node is dead and can be deleted. */ static bool node_is_dead(nir_cf_node *node) { assert(node->type == nir_cf_node_loop); nir_block *after = nir_cf_node_as_block(nir_cf_node_next(node)); /* Quick check if there are any phis that follow this CF node. If there * are, then we automatically know it isn't dead. */ if (!exec_list_is_empty(&after->instr_list) && nir_block_first_instr(after)->type == nir_instr_type_phi) return false; nir_function_impl *impl = nir_cf_node_get_function(node); nir_metadata_require(impl, nir_metadata_block_index); nir_foreach_block_in_cf_node(block, node) { bool inside_loop = node->type == nir_cf_node_loop; for (nir_cf_node *n = &block->cf_node; !inside_loop && n != node; n = n->parent) { if (n->type == nir_cf_node_loop) inside_loop = true; } nir_foreach_instr(instr, block) { if (instr->type == nir_instr_type_call) return false; /* Return and halt instructions can cause us to skip over other * side-effecting instructions after the loop, so consider them to * have side effects here. * * When the block is not inside a loop, break and continue might also * cause a skip. */ if (instr->type == nir_instr_type_jump && (!inside_loop || nir_instr_as_jump(instr)->type == nir_jump_return || nir_instr_as_jump(instr)->type == nir_jump_halt)) return false; if (instr->type == nir_instr_type_intrinsic) { nir_intrinsic_instr *intrin = nir_instr_as_intrinsic(instr); if (!(nir_intrinsic_infos[intrin->intrinsic].flags & NIR_INTRINSIC_CAN_ELIMINATE)) return false; switch (intrin->intrinsic) { case nir_intrinsic_load_deref: case nir_intrinsic_load_ssbo: case nir_intrinsic_load_global: /* If there's a memory barrier after the loop, a load might be * required to happen before some other instruction after the * barrier, so it is not valid to eliminate it -- unless we * know we can reorder it. * * Consider only loads that the result can be affected by other * invocations. */ if (intrin->intrinsic == nir_intrinsic_load_deref) { nir_deref_instr *deref = nir_src_as_deref(intrin->src[0]); if (!nir_deref_mode_may_be(deref, nir_var_mem_ssbo | nir_var_mem_shared | nir_var_mem_global | nir_var_shader_out)) break; } if (nir_intrinsic_access(intrin) & ACCESS_CAN_REORDER) break; return false; case nir_intrinsic_load_shared: case nir_intrinsic_load_output: case nir_intrinsic_load_per_vertex_output: /* Same as above loads. */ return false; default: /* Do nothing. */ break; } } if (!nir_foreach_ssa_def(instr, def_only_used_in_cf_node, node)) return false; } } return true; } static bool dead_cf_block(nir_block *block) { nir_if *following_if = nir_block_get_following_if(block); if (following_if) { if (!nir_src_is_const(following_if->condition)) return false; opt_constant_if(following_if, nir_src_as_bool(following_if->condition)); return true; } nir_loop *following_loop = nir_block_get_following_loop(block); if (!following_loop) return false; if (!node_is_dead(&following_loop->cf_node)) return false; nir_cf_node_remove(&following_loop->cf_node); return true; } static bool dead_cf_list(struct exec_list *list, bool *list_ends_in_jump) { bool progress = false; *list_ends_in_jump = false; nir_cf_node *prev = NULL; foreach_list_typed(nir_cf_node, cur, node, list) { switch (cur->type) { case nir_cf_node_block: { nir_block *block = nir_cf_node_as_block(cur); if (dead_cf_block(block)) { /* We just deleted the if or loop after this block, so we may have * deleted the block before or after it -- which one is an * implementation detail. Therefore, to recover the place we were * at, we have to use the previous cf_node. */ if (prev) { cur = nir_cf_node_next(prev); } else { cur = exec_node_data(nir_cf_node, exec_list_get_head(list), node); } block = nir_cf_node_as_block(cur); progress = true; } if (nir_block_ends_in_jump(block)) { *list_ends_in_jump = true; if (!exec_node_is_tail_sentinel(cur->node.next)) { remove_after_cf_node(cur); return true; } } break; } case nir_cf_node_if: { nir_if *if_stmt = nir_cf_node_as_if(cur); bool then_ends_in_jump, else_ends_in_jump; progress |= dead_cf_list(&if_stmt->then_list, &then_ends_in_jump); progress |= dead_cf_list(&if_stmt->else_list, &else_ends_in_jump); if (then_ends_in_jump && else_ends_in_jump) { *list_ends_in_jump = true; nir_block *next = nir_cf_node_as_block(nir_cf_node_next(cur)); if (!exec_list_is_empty(&next->instr_list) || !exec_node_is_tail_sentinel(next->cf_node.node.next)) { remove_after_cf_node(cur); return true; } } break; } case nir_cf_node_loop: { nir_loop *loop = nir_cf_node_as_loop(cur); bool dummy; progress |= dead_cf_list(&loop->body, &dummy); nir_block *next = nir_cf_node_as_block(nir_cf_node_next(cur)); if (next->predecessors->entries == 0 && (!exec_list_is_empty(&next->instr_list) || !exec_node_is_tail_sentinel(next->cf_node.node.next))) { remove_after_cf_node(cur); return true; } break; } default: unreachable("unknown cf node type"); } prev = cur; } return progress; } static bool opt_dead_cf_impl(nir_function_impl *impl) { bool dummy; bool progress = dead_cf_list(&impl->body, &dummy); if (progress) { nir_metadata_preserve(impl, nir_metadata_none); /* The CF manipulation code called by this pass is smart enough to keep * from breaking any SSA use/def chains by replacing any uses of removed * instructions with SSA undefs. However, it's not quite smart enough * to always preserve the dominance properties. In particular, if you * remove the one break from a loop, stuff in the loop may still be used * outside the loop even though there's no path between the two. We can * easily fix these issues by calling nir_repair_ssa which will ensure * that the dominance properties hold. */ nir_repair_ssa_impl(impl); } else { nir_metadata_preserve(impl, nir_metadata_all); } return progress; } bool nir_opt_dead_cf(nir_shader *shader) { bool progress = false; nir_foreach_function(function, shader) if (function->impl) progress |= opt_dead_cf_impl(function->impl); return progress; }