+++ /dev/null
-/*
- * 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 if if 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, i.e. loops 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 =
- nir_cf_node_as_block(condition ? nir_if_last_then_node(if_stmt)
- : nir_if_last_else_node(if_stmt));
-
- nir_foreach_instr_safe(after, instr) {
- 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, phi_src) {
- 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_extract(&list, nir_before_cf_list(cf_list),
- nir_after_cf_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
-block_has_no_side_effects(nir_block *block, void *state)
-{
- (void) state;
-
- nir_foreach_instr(block, instr) {
- 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.
- */
-
- if (instr->type == nir_instr_type_jump &&
- nir_instr_as_jump(instr)->type == nir_jump_return)
- return false;
-
- if (instr->type != nir_instr_type_intrinsic)
- continue;
-
- nir_intrinsic_instr *intrin = nir_instr_as_intrinsic(instr);
- if (!nir_intrinsic_infos[intrin->intrinsic].flags &
- NIR_INTRINSIC_CAN_ELIMINATE)
- return false;
- }
-
- return true;
-}
-
-static bool
-def_not_live_out(nir_ssa_def *def, void *state)
-{
- nir_block *after = state;
-
- return !BITSET_TEST(after->live_in, def->live_index);
-}
-
-/*
- * Test if a loop is dead. A loop is 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 nodes after it, since those indicate values inside the
- * loop being used after the loop.
- *
- * 3) If there are no phi nodes after the loop, then the only way a value
- * defined inside the loop can be used outside the loop is if its definition
- * dominates the block after the loop. If none of the definitions that
- * dominate the loop exit are used outside the loop, then the loop is dead
- * and it can be deleted.
- */
-
-static bool
-loop_is_dead(nir_loop *loop)
-{
- nir_block *before = nir_cf_node_as_block(nir_cf_node_prev(&loop->cf_node));
- nir_block *after = nir_cf_node_as_block(nir_cf_node_next(&loop->cf_node));
-
- if (!exec_list_is_empty(&after->instr_list) &&
- nir_block_first_instr(after)->type == nir_instr_type_phi)
- return false;
-
- if (!nir_foreach_block_in_cf_node(&loop->cf_node, block_has_no_side_effects,
- NULL))
- return false;
-
- nir_function_impl *impl = nir_cf_node_get_function(&loop->cf_node);
- nir_metadata_require(impl, nir_metadata_live_ssa_defs |
- nir_metadata_dominance);
-
- for (nir_block *cur = after->imm_dom; cur != before; cur = cur->imm_dom) {
- nir_foreach_instr(cur, instr) {
- if (!nir_foreach_ssa_def(instr, def_not_live_out, after))
- 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) {
- nir_const_value *const_value =
- nir_src_as_const_value(following_if->condition);
-
- if (!const_value)
- return false;
-
- opt_constant_if(following_if, const_value->u[0] != 0);
- return true;
- }
-
- nir_loop *following_loop = nir_block_get_following_loop(block);
- if (!following_loop)
- return false;
-
- if (!loop_is_dead(following_loop))
- return false;
-
- nir_cf_node_remove(&following_loop->cf_node);
- return true;
-}
-
-static bool
-ends_in_jump(nir_block *block)
-{
- if (exec_list_is_empty(&block->instr_list))
- return false;
-
- nir_instr *instr = nir_block_last_instr(block);
- return instr->type == nir_instr_type_jump;
-}
-
-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 (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);
-
- 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);
-
- return progress;
-}
-
-bool
-nir_opt_dead_cf(nir_shader *shader)
-{
- bool progress = false;
-
- nir_foreach_function(shader, function)
- if (function->impl)
- progress |= opt_dead_cf_impl(function->impl);
-
- return progress;
-}
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