/*
* 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, 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 = 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
cf_node_has_side_effects(nir_cf_node *node)
{
nir_foreach_block_in_cf_node(block, node) {
nir_foreach_instr(instr, block) {
if (instr->type == nir_instr_type_call)
return true;
/* 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 true;
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 true;
}
}
return false;
}
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 (cf_node_has_side_effects(&loop->cf_node))
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 && cur != before;
cur = cur->imm_dom) {
nir_foreach_instr(instr, cur) {
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->u32[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(function, shader)
if (function->impl)
progress |= opt_dead_cf_impl(function->impl);
return progress;
}