src/compiler/nir/nir_opt_dead_cf.c - platform/external/mesa3d - Git at Google

 /*
  * 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;
 }
	/*
	* 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;
	}