| /* |
| * 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, nir_src_for_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 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)) |
| 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; |
| } |
| |
| 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; |
| } |
