spirv_cfg.cpp - third_party/spirv-cross - Git at Google

 /*
  * Copyright 2016-2021 Arm Limited
  * SPDX-License-Identifier: Apache-2.0 OR MIT
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
  * You may obtain a copy of the License at
  *
  *     http://www.apache.org/licenses/LICENSE-2.0
  *
  * Unless required by applicable law or agreed to in writing, software
  * distributed under the License is distributed on an "AS IS" BASIS,
  * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */

 /*
  * At your option, you may choose to accept this material under either:
  *  1. The Apache License, Version 2.0, found at <http://www.apache.org/licenses/LICENSE-2.0>, or
  *  2. The MIT License, found at <http://opensource.org/licenses/MIT>.
  */

 #include "spirv_cfg.hpp"
 #include "spirv_cross.hpp"
 #include <algorithm>
 #include <assert.h>

 using namespace std;

 namespace SPIRV_CROSS_NAMESPACE
 {
 CFG::CFG(Compiler &compiler_, const SPIRFunction &func_)
     : compiler(compiler_)
     , func(func_)
 {
 	build_post_order_visit_order();
 	build_immediate_dominators();
 }

 uint32_t CFG::find_common_dominator(uint32_t a, uint32_t b) const
 {
 	while (a != b)
 	{
 		if (get_visit_order(a) < get_visit_order(b))
 			a = get_immediate_dominator(a);
 		else
 			b = get_immediate_dominator(b);
 	}
 	return a;
 }

 void CFG::build_immediate_dominators()
 {
 	// Traverse the post-order in reverse and build up the immediate dominator tree.
 	immediate_dominators.clear();
 	immediate_dominators[func.entry_block] = func.entry_block;

 	for (auto i = post_order.size(); i; i--)
 	{
 		uint32_t block = post_order[i - 1];
 		auto &pred = preceding_edges[block];
 		if (pred.empty()) // This is for the entry block, but we've already set up the dominators.
 			continue;

 		for (auto &edge : pred)
 		{
 			if (immediate_dominators[block])
 			{
 				assert(immediate_dominators[edge]);
 				immediate_dominators[block] = find_common_dominator(immediate_dominators[block], edge);
 			}
 			else
 				immediate_dominators[block] = edge;
 		}
 	}
 }

 bool CFG::is_back_edge(uint32_t to) const
 {
 	// We have a back edge if the visit order is set with the temporary magic value 0.
 	// Crossing edges will have already been recorded with a visit order.
 	auto itr = visit_order.find(to);
 	return itr != end(visit_order) && itr->second.get() == 0;
 }

 bool CFG::has_visited_forward_edge(uint32_t to) const
 {
 	// If > 0, we have visited the edge already, and this is not a back edge branch.
 	auto itr = visit_order.find(to);
 	return itr != end(visit_order) && itr->second.get() > 0;
 }

 bool CFG::post_order_visit(uint32_t block_id)
 {
 	// If we have already branched to this block (back edge), stop recursion.
 	// If our branches are back-edges, we do not record them.
 	// We have to record crossing edges however.
 	if (has_visited_forward_edge(block_id))
 		return true;
 	else if (is_back_edge(block_id))
 		return false;

 	// Block back-edges from recursively revisiting ourselves.
 	visit_order[block_id].get() = 0;

 	auto &block = compiler.get<SPIRBlock>(block_id);

 	// If this is a loop header, add an implied branch to the merge target.
 	// This is needed to avoid annoying cases with do { ... } while(false) loops often generated by inliners.
 	// To the CFG, this is linear control flow, but we risk picking the do/while scope as our dominating block.
 	// This makes sure that if we are accessing a variable outside the do/while, we choose the loop header as dominator.
 	// We could use has_visited_forward_edge, but this break code-gen where the merge block is unreachable in the CFG.

 	// Make a point out of visiting merge target first. This is to make sure that post visit order outside the loop
 	// is lower than inside the loop, which is going to be key for some traversal algorithms like post-dominance analysis.
 	// For selection constructs true/false blocks will end up visiting the merge block directly and it works out fine,
 	// but for loops, only the header might end up actually branching to merge block.
 	if (block.merge == SPIRBlock::MergeLoop && post_order_visit(block.merge_block))
 		add_branch(block_id, block.merge_block);

 	// First visit our branch targets.
 	switch (block.terminator)
 	{
 	case SPIRBlock::Direct:
 		if (post_order_visit(block.next_block))
 			add_branch(block_id, block.next_block);
 		break;

 	case SPIRBlock::Select:
 		if (post_order_visit(block.true_block))
 			add_branch(block_id, block.true_block);
 		if (post_order_visit(block.false_block))
 			add_branch(block_id, block.false_block);
 		break;

 	case SPIRBlock::MultiSelect:
 	{
 		const auto &cases = compiler.get_case_list(block);
 		for (const auto &target : cases)
 		{
 			if (post_order_visit(target.block))
 				add_branch(block_id, target.block);
 		}
 		if (block.default_block && post_order_visit(block.default_block))
 			add_branch(block_id, block.default_block);
 		break;
 	}
 	default:
 		break;
 	}

 	// If this is a selection merge, add an implied branch to the merge target.
 	// This is needed to avoid cases where an inner branch dominates the outer branch.
 	// This can happen if one of the branches exit early, e.g.:
 	// if (cond) { ...; break; } else { var = 100 } use_var(var);
 	// We can use the variable without a Phi since there is only one possible parent here.
 	// However, in this case, we need to hoist out the inner variable to outside the branch.
 	// Use same strategy as loops.
 	if (block.merge == SPIRBlock::MergeSelection && post_order_visit(block.next_block))
 	{
 		// If there is only one preceding edge to the merge block and it's not ourselves, we need a fixup.
 		// Add a fake branch so any dominator in either the if (), or else () block, or a lone case statement
 		// will be hoisted out to outside the selection merge.
 		// If size > 1, the variable will be automatically hoisted, so we should not mess with it.
 		// The exception here is switch blocks, where we can have multiple edges to merge block,
 		// all coming from same scope, so be more conservative in this case.
 		// Adding fake branches unconditionally breaks parameter preservation analysis,
 		// which looks at how variables are accessed through the CFG.
 		auto pred_itr = preceding_edges.find(block.next_block);
 		if (pred_itr != end(preceding_edges))
 		{
 			auto &pred = pred_itr->second;
 			auto succ_itr = succeeding_edges.find(block_id);
 			size_t num_succeeding_edges = 0;
 			if (succ_itr != end(succeeding_edges))
 				num_succeeding_edges = succ_itr->second.size();

 			if (block.terminator == SPIRBlock::MultiSelect && num_succeeding_edges == 1)
 			{
 				// Multiple branches can come from the same scope due to "break;", so we need to assume that all branches
 				// come from same case scope in worst case, even if there are multiple preceding edges.
 				// If we have more than one succeeding edge from the block header, it should be impossible
 				// to have a dominator be inside the block.
 				// Only case this can go wrong is if we have 2 or more edges from block header and
 				// 2 or more edges to merge block, and still have dominator be inside a case label.
 				if (!pred.empty())
 					add_branch(block_id, block.next_block);
 			}
 			else
 			{
 				if (pred.size() == 1 && *pred.begin() != block_id)
 					add_branch(block_id, block.next_block);
 			}
 		}
 		else
 		{
 			// If the merge block does not have any preceding edges, i.e. unreachable, hallucinate it.
 			// We're going to do code-gen for it, and domination analysis requires that we have at least one preceding edge.
 			add_branch(block_id, block.next_block);
 		}
 	}

 	// Then visit ourselves. Start counting at one, to let 0 be a magic value for testing back vs. crossing edges.
 	visit_order[block_id].get() = ++visit_count;
 	post_order.push_back(block_id);
 	return true;
 }

 void CFG::build_post_order_visit_order()
 {
 	uint32_t block = func.entry_block;
 	visit_count = 0;
 	visit_order.clear();
 	post_order.clear();
 	post_order_visit(block);
 }

 void CFG::add_branch(uint32_t from, uint32_t to)
 {
 	const auto add_unique = [](SmallVector<uint32_t> &l, uint32_t value) {
 		auto itr = find(begin(l), end(l), value);
 		if (itr == end(l))
 			l.push_back(value);
 	};
 	add_unique(preceding_edges[to], from);
 	add_unique(succeeding_edges[from], to);
 }

 uint32_t CFG::find_loop_dominator(uint32_t block_id) const
 {
 	while (block_id != SPIRBlock::NoDominator)
 	{
 		auto itr = preceding_edges.find(block_id);
 		if (itr == end(preceding_edges))
 			return SPIRBlock::NoDominator;
 		if (itr->second.empty())
 			return SPIRBlock::NoDominator;

 		uint32_t pred_block_id = SPIRBlock::NoDominator;
 		bool ignore_loop_header = false;

 		// If we are a merge block, go directly to the header block.
 		// Only consider a loop dominator if we are branching from inside a block to a loop header.
 		// NOTE: In the CFG we forced an edge from header to merge block always to support variable scopes properly.
 		for (auto &pred : itr->second)
 		{
 			auto &pred_block = compiler.get<SPIRBlock>(pred);
 			if (pred_block.merge == SPIRBlock::MergeLoop && pred_block.merge_block == ID(block_id))
 			{
 				pred_block_id = pred;
 				ignore_loop_header = true;
 				break;
 			}
 			else if (pred_block.merge == SPIRBlock::MergeSelection && pred_block.next_block == ID(block_id))
 			{
 				pred_block_id = pred;
 				break;
 			}
 		}

 		// No merge block means we can just pick any edge. Loop headers dominate the inner loop, so any path we
 		// take will lead there.
 		if (pred_block_id == SPIRBlock::NoDominator)
 			pred_block_id = itr->second.front();

 		block_id = pred_block_id;

 		if (!ignore_loop_header && block_id)
 		{
 			auto &block = compiler.get<SPIRBlock>(block_id);
 			if (block.merge == SPIRBlock::MergeLoop)
 				return block_id;
 		}
 	}

 	return block_id;
 }

 bool CFG::node_terminates_control_flow_in_sub_graph(BlockID from, BlockID to) const
 {
 	// Walk backwards, starting from "to" block.
 	// Only follow pred edges if they have a 1:1 relationship, or a merge relationship.
 	// If we cannot find a path to "from", we must assume that to is inside control flow in some way.

 	auto &from_block = compiler.get<SPIRBlock>(from);
 	BlockID ignore_block_id = 0;
 	if (from_block.merge == SPIRBlock::MergeLoop)
 		ignore_block_id = from_block.merge_block;

 	while (to != from)
 	{
 		auto pred_itr = preceding_edges.find(to);
 		if (pred_itr == end(preceding_edges))
 			return false;

 		DominatorBuilder builder(*this);
 		for (auto &edge : pred_itr->second)
 			builder.add_block(edge);

 		uint32_t dominator = builder.get_dominator();
 		if (dominator == 0)
 			return false;

 		auto &dom = compiler.get<SPIRBlock>(dominator);

 		bool true_path_ignore = false;
 		bool false_path_ignore = false;

 		bool merges_to_nothing = dom.merge == SPIRBlock::MergeNone ||
 		                         (dom.merge == SPIRBlock::MergeSelection && dom.next_block &&
 		                          compiler.get<SPIRBlock>(dom.next_block).terminator == SPIRBlock::Unreachable) ||
 		                         (dom.merge == SPIRBlock::MergeLoop && dom.merge_block &&
 		                          compiler.get<SPIRBlock>(dom.merge_block).terminator == SPIRBlock::Unreachable);

 		if (dom.self == from || merges_to_nothing)
 		{
 			// We can only ignore inner branchy paths if there is no merge,
 			// i.e. no code is generated afterwards. E.g. this allows us to elide continue:
 			// for (;;) { if (cond) { continue; } else { break; } }.
 			// Codegen here in SPIR-V will be something like either no merge if one path directly breaks, or
 			// we merge to Unreachable.
 			if (ignore_block_id && dom.terminator == SPIRBlock::Select)
 			{
 				auto &true_block = compiler.get<SPIRBlock>(dom.true_block);
 				auto &false_block = compiler.get<SPIRBlock>(dom.false_block);
 				auto &ignore_block = compiler.get<SPIRBlock>(ignore_block_id);
 				true_path_ignore = compiler.execution_is_branchless(true_block, ignore_block);
 				false_path_ignore = compiler.execution_is_branchless(false_block, ignore_block);
 			}
 		}

 		// Cases where we allow traversal. This serves as a proxy for post-dominance in a loop body.
 		// TODO: Might want to do full post-dominance analysis, but it's a lot of churn for something like this ...
 		// - We're the merge block of a selection construct. Jump to header.
 		// - We're the merge block of a loop. Jump to header.
 		// - Direct branch. Trivial.
 		// - Allow cases inside a branch if the header cannot merge execution before loop exit.
 		if ((dom.merge == SPIRBlock::MergeSelection && dom.next_block == to) ||
 		    (dom.merge == SPIRBlock::MergeLoop && dom.merge_block == to) ||
 		    (dom.terminator == SPIRBlock::Direct && dom.next_block == to) ||
 		    (dom.terminator == SPIRBlock::Select && dom.true_block == to && false_path_ignore) ||
 		    (dom.terminator == SPIRBlock::Select && dom.false_block == to && true_path_ignore))
 		{
 			// Allow walking selection constructs if the other branch reaches out of a loop construct.
 			// It cannot be in-scope anymore.
 			to = dominator;
 		}
 		else
 			return false;
 	}

 	return true;
 }

 DominatorBuilder::DominatorBuilder(const CFG &cfg_)
     : cfg(cfg_)
 {
 }

 void DominatorBuilder::add_block(uint32_t block)
 {
 	if (!cfg.get_immediate_dominator(block))
 	{
 		// Unreachable block via the CFG, we will never emit this code anyways.
 		return;
 	}

 	if (!dominator)
 	{
 		dominator = block;
 		return;
 	}

 	if (block != dominator)
 		dominator = cfg.find_common_dominator(block, dominator);
 }

 void DominatorBuilder::lift_continue_block_dominator()
 {
 	// It is possible for a continue block to be the dominator of a variable is only accessed inside the while block of a do-while loop.
 	// We cannot safely declare variables inside a continue block, so move any variable declared
 	// in a continue block to the entry block to simplify.
 	// It makes very little sense for a continue block to ever be a dominator, so fall back to the simplest
 	// solution.

 	if (!dominator)
 		return;

 	auto &block = cfg.get_compiler().get<SPIRBlock>(dominator);
 	auto post_order = cfg.get_visit_order(dominator);

 	// If we are branching to a block with a higher post-order traversal index (continue blocks), we have a problem
 	// since we cannot create sensible GLSL code for this, fallback to entry block.
 	bool back_edge_dominator = false;
 	switch (block.terminator)
 	{
 	case SPIRBlock::Direct:
 		if (cfg.get_visit_order(block.next_block) > post_order)
 			back_edge_dominator = true;
 		break;

 	case SPIRBlock::Select:
 		if (cfg.get_visit_order(block.true_block) > post_order)
 			back_edge_dominator = true;
 		if (cfg.get_visit_order(block.false_block) > post_order)
 			back_edge_dominator = true;
 		break;

 	case SPIRBlock::MultiSelect:
 	{
 		auto &cases = cfg.get_compiler().get_case_list(block);
 		for (auto &target : cases)
 		{
 			if (cfg.get_visit_order(target.block) > post_order)
 				back_edge_dominator = true;
 		}
 		if (block.default_block && cfg.get_visit_order(block.default_block) > post_order)
 			back_edge_dominator = true;
 		break;
 	}

 	default:
 		break;
 	}

 	if (back_edge_dominator)
 		dominator = cfg.get_function().entry_block;
 }
 } // namespace SPIRV_CROSS_NAMESPACE
	/*
	* Copyright 2016-2021 Arm Limited
	* SPDX-License-Identifier: Apache-2.0 OR MIT
	*
	* Licensed under the Apache License, Version 2.0 (the "License");
	* you may not use this file except in compliance with the License.
	* You may obtain a copy of the License at
	*
	* http://www.apache.org/licenses/LICENSE-2.0
	*
	* Unless required by applicable law or agreed to in writing, software
	* distributed under the License is distributed on an "AS IS" BASIS,
	* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
	* See the License for the specific language governing permissions and
	* limitations under the License.
	*/

	/*
	* At your option, you may choose to accept this material under either:
	* 1. The Apache License, Version 2.0, found at <http://www.apache.org/licenses/LICENSE-2.0>, or
	* 2. The MIT License, found at <http://opensource.org/licenses/MIT>.
	*/

	#include "spirv_cfg.hpp"
	#include "spirv_cross.hpp"
	#include <algorithm>
	#include <assert.h>

	using namespace std;

	namespace SPIRV_CROSS_NAMESPACE
	{
	CFG::CFG(Compiler &compiler_, const SPIRFunction &func_)
	: compiler(compiler_)
	, func(func_)
	{
	build_post_order_visit_order();
	build_immediate_dominators();
	}

	uint32_t CFG::find_common_dominator(uint32_t a, uint32_t b) const
	{
	while (a != b)
	{
	if (get_visit_order(a) < get_visit_order(b))
	a = get_immediate_dominator(a);
	else
	b = get_immediate_dominator(b);
	}
	return a;
	}

	void CFG::build_immediate_dominators()
	{
	// Traverse the post-order in reverse and build up the immediate dominator tree.
	immediate_dominators.clear();
	immediate_dominators[func.entry_block] = func.entry_block;

	for (auto i = post_order.size(); i; i--)
	{
	uint32_t block = post_order[i - 1];
	auto &pred = preceding_edges[block];
	if (pred.empty()) // This is for the entry block, but we've already set up the dominators.
	continue;

	for (auto &edge : pred)
	{
	if (immediate_dominators[block])
	{
	assert(immediate_dominators[edge]);
	immediate_dominators[block] = find_common_dominator(immediate_dominators[block], edge);
	}
	else
	immediate_dominators[block] = edge;
	}
	}
	}

	bool CFG::is_back_edge(uint32_t to) const
	{
	// We have a back edge if the visit order is set with the temporary magic value 0.
	// Crossing edges will have already been recorded with a visit order.
	auto itr = visit_order.find(to);
	return itr != end(visit_order) && itr->second.get() == 0;
	}

	bool CFG::has_visited_forward_edge(uint32_t to) const
	{
	// If > 0, we have visited the edge already, and this is not a back edge branch.
	auto itr = visit_order.find(to);
	return itr != end(visit_order) && itr->second.get() > 0;
	}

	bool CFG::post_order_visit(uint32_t block_id)
	{
	// If we have already branched to this block (back edge), stop recursion.
	// If our branches are back-edges, we do not record them.
	// We have to record crossing edges however.
	if (has_visited_forward_edge(block_id))
	return true;
	else if (is_back_edge(block_id))
	return false;

	// Block back-edges from recursively revisiting ourselves.
	visit_order[block_id].get() = 0;

	auto &block = compiler.get<SPIRBlock>(block_id);

	// If this is a loop header, add an implied branch to the merge target.
	// This is needed to avoid annoying cases with do { ... } while(false) loops often generated by inliners.
	// To the CFG, this is linear control flow, but we risk picking the do/while scope as our dominating block.
	// This makes sure that if we are accessing a variable outside the do/while, we choose the loop header as dominator.
	// We could use has_visited_forward_edge, but this break code-gen where the merge block is unreachable in the CFG.

	// Make a point out of visiting merge target first. This is to make sure that post visit order outside the loop
	// is lower than inside the loop, which is going to be key for some traversal algorithms like post-dominance analysis.
	// For selection constructs true/false blocks will end up visiting the merge block directly and it works out fine,
	// but for loops, only the header might end up actually branching to merge block.
	if (block.merge == SPIRBlock::MergeLoop && post_order_visit(block.merge_block))
	add_branch(block_id, block.merge_block);

	// First visit our branch targets.
	switch (block.terminator)
	{
	case SPIRBlock::Direct:
	if (post_order_visit(block.next_block))
	add_branch(block_id, block.next_block);
	break;

	case SPIRBlock::Select:
	if (post_order_visit(block.true_block))
	add_branch(block_id, block.true_block);
	if (post_order_visit(block.false_block))
	add_branch(block_id, block.false_block);
	break;

	case SPIRBlock::MultiSelect:
	{
	const auto &cases = compiler.get_case_list(block);
	for (const auto &target : cases)
	{
	if (post_order_visit(target.block))
	add_branch(block_id, target.block);
	}
	if (block.default_block && post_order_visit(block.default_block))
	add_branch(block_id, block.default_block);
	break;
	}
	default:
	break;
	}

	// If this is a selection merge, add an implied branch to the merge target.
	// This is needed to avoid cases where an inner branch dominates the outer branch.
	// This can happen if one of the branches exit early, e.g.:
	// if (cond) { ...; break; } else { var = 100 } use_var(var);
	// We can use the variable without a Phi since there is only one possible parent here.
	// However, in this case, we need to hoist out the inner variable to outside the branch.
	// Use same strategy as loops.
	if (block.merge == SPIRBlock::MergeSelection && post_order_visit(block.next_block))
	{
	// If there is only one preceding edge to the merge block and it's not ourselves, we need a fixup.
	// Add a fake branch so any dominator in either the if (), or else () block, or a lone case statement
	// will be hoisted out to outside the selection merge.
	// If size > 1, the variable will be automatically hoisted, so we should not mess with it.
	// The exception here is switch blocks, where we can have multiple edges to merge block,
	// all coming from same scope, so be more conservative in this case.
	// Adding fake branches unconditionally breaks parameter preservation analysis,
	// which looks at how variables are accessed through the CFG.
	auto pred_itr = preceding_edges.find(block.next_block);
	if (pred_itr != end(preceding_edges))
	{
	auto &pred = pred_itr->second;
	auto succ_itr = succeeding_edges.find(block_id);
	size_t num_succeeding_edges = 0;
	if (succ_itr != end(succeeding_edges))
	num_succeeding_edges = succ_itr->second.size();

	if (block.terminator == SPIRBlock::MultiSelect && num_succeeding_edges == 1)
	{
	// Multiple branches can come from the same scope due to "break;", so we need to assume that all branches
	// come from same case scope in worst case, even if there are multiple preceding edges.
	// If we have more than one succeeding edge from the block header, it should be impossible
	// to have a dominator be inside the block.
	// Only case this can go wrong is if we have 2 or more edges from block header and
	// 2 or more edges to merge block, and still have dominator be inside a case label.
	if (!pred.empty())
	add_branch(block_id, block.next_block);
	}
	else
	{
	if (pred.size() == 1 && *pred.begin() != block_id)
	add_branch(block_id, block.next_block);
	}
	}
	else
	{
	// If the merge block does not have any preceding edges, i.e. unreachable, hallucinate it.
	// We're going to do code-gen for it, and domination analysis requires that we have at least one preceding edge.
	add_branch(block_id, block.next_block);
	}
	}

	// Then visit ourselves. Start counting at one, to let 0 be a magic value for testing back vs. crossing edges.
	visit_order[block_id].get() = ++visit_count;
	post_order.push_back(block_id);
	return true;
	}

	void CFG::build_post_order_visit_order()
	{
	uint32_t block = func.entry_block;
	visit_count = 0;
	visit_order.clear();
	post_order.clear();
	post_order_visit(block);
	}

	void CFG::add_branch(uint32_t from, uint32_t to)
	{
	const auto add_unique = [](SmallVector<uint32_t> &l, uint32_t value) {
	auto itr = find(begin(l), end(l), value);
	if (itr == end(l))
	l.push_back(value);
	};
	add_unique(preceding_edges[to], from);
	add_unique(succeeding_edges[from], to);
	}

	uint32_t CFG::find_loop_dominator(uint32_t block_id) const
	{
	while (block_id != SPIRBlock::NoDominator)
	{
	auto itr = preceding_edges.find(block_id);
	if (itr == end(preceding_edges))
	return SPIRBlock::NoDominator;
	if (itr->second.empty())
	return SPIRBlock::NoDominator;

	uint32_t pred_block_id = SPIRBlock::NoDominator;
	bool ignore_loop_header = false;

	// If we are a merge block, go directly to the header block.
	// Only consider a loop dominator if we are branching from inside a block to a loop header.
	// NOTE: In the CFG we forced an edge from header to merge block always to support variable scopes properly.
	for (auto &pred : itr->second)
	{
	auto &pred_block = compiler.get<SPIRBlock>(pred);
	if (pred_block.merge == SPIRBlock::MergeLoop && pred_block.merge_block == ID(block_id))
	{
	pred_block_id = pred;
	ignore_loop_header = true;
	break;
	}
	else if (pred_block.merge == SPIRBlock::MergeSelection && pred_block.next_block == ID(block_id))
	{
	pred_block_id = pred;
	break;
	}
	}

	// No merge block means we can just pick any edge. Loop headers dominate the inner loop, so any path we
	// take will lead there.
	if (pred_block_id == SPIRBlock::NoDominator)
	pred_block_id = itr->second.front();

	block_id = pred_block_id;

	if (!ignore_loop_header && block_id)
	{
	auto &block = compiler.get<SPIRBlock>(block_id);
	if (block.merge == SPIRBlock::MergeLoop)
	return block_id;
	}
	}

	return block_id;
	}

	bool CFG::node_terminates_control_flow_in_sub_graph(BlockID from, BlockID to) const
	{
	// Walk backwards, starting from "to" block.
	// Only follow pred edges if they have a 1:1 relationship, or a merge relationship.
	// If we cannot find a path to "from", we must assume that to is inside control flow in some way.

	auto &from_block = compiler.get<SPIRBlock>(from);
	BlockID ignore_block_id = 0;
	if (from_block.merge == SPIRBlock::MergeLoop)
	ignore_block_id = from_block.merge_block;

	while (to != from)
	{
	auto pred_itr = preceding_edges.find(to);
	if (pred_itr == end(preceding_edges))
	return false;

	DominatorBuilder builder(*this);
	for (auto &edge : pred_itr->second)
	builder.add_block(edge);

	uint32_t dominator = builder.get_dominator();
	if (dominator == 0)
	return false;

	auto &dom = compiler.get<SPIRBlock>(dominator);

	bool true_path_ignore = false;
	bool false_path_ignore = false;

	bool merges_to_nothing = dom.merge == SPIRBlock::MergeNone \|\|
	(dom.merge == SPIRBlock::MergeSelection && dom.next_block &&
	compiler.get<SPIRBlock>(dom.next_block).terminator == SPIRBlock::Unreachable) \|\|
	(dom.merge == SPIRBlock::MergeLoop && dom.merge_block &&
	compiler.get<SPIRBlock>(dom.merge_block).terminator == SPIRBlock::Unreachable);

	if (dom.self == from \|\| merges_to_nothing)
	{
	// We can only ignore inner branchy paths if there is no merge,
	// i.e. no code is generated afterwards. E.g. this allows us to elide continue:
	// for (;;) { if (cond) { continue; } else { break; } }.
	// Codegen here in SPIR-V will be something like either no merge if one path directly breaks, or
	// we merge to Unreachable.
	if (ignore_block_id && dom.terminator == SPIRBlock::Select)
	{
	auto &true_block = compiler.get<SPIRBlock>(dom.true_block);
	auto &false_block = compiler.get<SPIRBlock>(dom.false_block);
	auto &ignore_block = compiler.get<SPIRBlock>(ignore_block_id);
	true_path_ignore = compiler.execution_is_branchless(true_block, ignore_block);
	false_path_ignore = compiler.execution_is_branchless(false_block, ignore_block);
	}
	}

	// Cases where we allow traversal. This serves as a proxy for post-dominance in a loop body.
	// TODO: Might want to do full post-dominance analysis, but it's a lot of churn for something like this ...
	// - We're the merge block of a selection construct. Jump to header.
	// - We're the merge block of a loop. Jump to header.
	// - Direct branch. Trivial.
	// - Allow cases inside a branch if the header cannot merge execution before loop exit.
	if ((dom.merge == SPIRBlock::MergeSelection && dom.next_block == to) \|\|
	(dom.merge == SPIRBlock::MergeLoop && dom.merge_block == to) \|\|
	(dom.terminator == SPIRBlock::Direct && dom.next_block == to) \|\|
	(dom.terminator == SPIRBlock::Select && dom.true_block == to && false_path_ignore) \|\|
	(dom.terminator == SPIRBlock::Select && dom.false_block == to && true_path_ignore))
	{
	// Allow walking selection constructs if the other branch reaches out of a loop construct.
	// It cannot be in-scope anymore.
	to = dominator;
	}
	else
	return false;
	}

	return true;
	}

	DominatorBuilder::DominatorBuilder(const CFG &cfg_)
	: cfg(cfg_)
	{
	}

	void DominatorBuilder::add_block(uint32_t block)
	{
	if (!cfg.get_immediate_dominator(block))
	{
	// Unreachable block via the CFG, we will never emit this code anyways.
	return;
	}

	if (!dominator)
	{
	dominator = block;
	return;
	}

	if (block != dominator)
	dominator = cfg.find_common_dominator(block, dominator);
	}

	void DominatorBuilder::lift_continue_block_dominator()
	{
	// It is possible for a continue block to be the dominator of a variable is only accessed inside the while block of a do-while loop.
	// We cannot safely declare variables inside a continue block, so move any variable declared
	// in a continue block to the entry block to simplify.
	// It makes very little sense for a continue block to ever be a dominator, so fall back to the simplest
	// solution.

	if (!dominator)
	return;

	auto &block = cfg.get_compiler().get<SPIRBlock>(dominator);
	auto post_order = cfg.get_visit_order(dominator);

	// If we are branching to a block with a higher post-order traversal index (continue blocks), we have a problem
	// since we cannot create sensible GLSL code for this, fallback to entry block.
	bool back_edge_dominator = false;
	switch (block.terminator)
	{
	case SPIRBlock::Direct:
	if (cfg.get_visit_order(block.next_block) > post_order)
	back_edge_dominator = true;
	break;

	case SPIRBlock::Select:
	if (cfg.get_visit_order(block.true_block) > post_order)
	back_edge_dominator = true;
	if (cfg.get_visit_order(block.false_block) > post_order)
	back_edge_dominator = true;
	break;

	case SPIRBlock::MultiSelect:
	{
	auto &cases = cfg.get_compiler().get_case_list(block);
	for (auto &target : cases)
	{
	if (cfg.get_visit_order(target.block) > post_order)
	back_edge_dominator = true;
	}
	if (block.default_block && cfg.get_visit_order(block.default_block) > post_order)
	back_edge_dominator = true;
	break;
	}

	default:
	break;
	}

	if (back_edge_dominator)
	dominator = cfg.get_function().entry_block;
	}
	} // namespace SPIRV_CROSS_NAMESPACE