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Improve time to build dominators (#4916)

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Changed a couple small parts of the algorithm to reduce time to build
the dominator trees.  There should be no visible changes.

Add a depth first search algorithm that does not run a function on
backedges.  The check if an edge is a back edge is time consuming, and
pointless if the function run on it is a nop.
This commit is contained in:
Steven Perron 2022-09-02 12:27:10 -04:00 committed by GitHub
parent 8eb8509834
commit 529955e03d
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GPG Key ID: 4AEE18F83AFDEB23
4 changed files with 56 additions and 36 deletions
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53
source/cfa.h
View File

@ -52,12 +52,37 @@ class CFA {
uint32_t id);
public:
/// @brief Depth first traversal starting from the \p entry BasicBlock
///
/// This function performs a depth first traversal from the \p entry
/// BasicBlock and calls the pre/postorder functions when it needs to process
/// the node in pre order, post order.
///
/// @param[in] entry The root BasicBlock of a CFG
/// @param[in] successor_func A function which will return a pointer to the
/// successor nodes
/// @param[in] preorder A function that will be called for every block in a
/// CFG following preorder traversal semantics
/// @param[in] postorder A function that will be called for every block in a
/// CFG following postorder traversal semantics
/// @param[in] terminal A function that will be called to determine if the
/// search should stop at the given node.
/// NOTE: The @p successor_func and predecessor_func each return a pointer to
/// a collection such that iterators to that collection remain valid for the
/// lifetime of the algorithm.
static void DepthFirstTraversal(const BB* entry,
get_blocks_func successor_func,
std::function<void(cbb_ptr)> preorder,
std::function<void(cbb_ptr)> postorder,
std::function<bool(cbb_ptr)> terminal);
/// @brief Depth first traversal starting from the \p entry BasicBlock
///
/// This function performs a depth first traversal from the \p entry
/// BasicBlock and calls the pre/postorder functions when it needs to process
/// the node in pre order, post order. It also calls the backedge function
/// when a back edge is encountered.
/// when a back edge is encountered. The backedge function can be empty. The
/// runtime of the algorithm is improved if backedge is empty.
///
/// @param[in] entry The root BasicBlock of a CFG
/// @param[in] successor_func A function which will return a pointer to the
@ -67,12 +92,11 @@ class CFA {
/// @param[in] postorder A function that will be called for every block in a
/// CFG following postorder traversal semantics
/// @param[in] backedge A function that will be called when a backedge is
/// encountered during a traversal
/// encountered during a traversal.
/// @param[in] terminal A function that will be called to determine if the
/// search should stop at the given node.
/// NOTE: The @p successor_func and predecessor_func each return a pointer to
/// a
/// collection such that iterators to that collection remain valid for the
/// a collection such that iterators to that collection remain valid for the
/// lifetime of the algorithm.
static void DepthFirstTraversal(
const BB* entry, get_blocks_func successor_func,
@ -136,6 +160,16 @@ bool CFA<BB>::FindInWorkList(const std::vector<block_info>& work_list,
return false;
}
template <class BB>
void CFA<BB>::DepthFirstTraversal(const BB* entry,
get_blocks_func successor_func,
std::function<void(cbb_ptr)> preorder,
std::function<void(cbb_ptr)> postorder,
std::function<bool(cbb_ptr)> terminal) {
DepthFirstTraversal(entry, successor_func, preorder, postorder,
/* backedge = */ {}, terminal);
}
template <class BB>
void CFA<BB>::DepthFirstTraversal(
const BB* entry, get_blocks_func successor_func,
@ -143,6 +177,11 @@ void CFA<BB>::DepthFirstTraversal(
std::function<void(cbb_ptr)> postorder,
std::function<void(cbb_ptr, cbb_ptr)> backedge,
std::function<bool(cbb_ptr)> terminal) {
assert(successor_func && "The successor function cannot be empty.");
assert(preorder && "The preorder function cannot be empty.");
assert(postorder && "The postorder function cannot be empty.");
assert(terminal && "The terminal function cannot be empty.");
std::unordered_set<uint32_t> processed;
/// NOTE: work_list is the sequence of nodes from the root node to the node
@ -162,7 +201,7 @@ void CFA<BB>::DepthFirstTraversal(
} else {
BB* child = *top.iter;
top.iter++;
if (FindInWorkList(work_list, child->id())) {
if (backedge && FindInWorkList(work_list, child->id())) {
backedge(top.block, child);
}
if (processed.count(child->id()) == 0) {
@ -269,14 +308,12 @@ std::vector<BB*> CFA<BB>::TraversalRoots(const std::vector<BB*>& blocks,
auto mark_visited = [&visited](const BB* b) { visited.insert(b); };
auto ignore_block = [](const BB*) {};
auto ignore_blocks = [](const BB*, const BB*) {};
auto no_terminal_blocks = [](const BB*) { return false; };
auto traverse_from_root = [&mark_visited, &succ_func, &ignore_block,
&ignore_blocks,
&no_terminal_blocks](const BB* entry) {
DepthFirstTraversal(entry, succ_func, mark_visited, ignore_block,
ignore_blocks, no_terminal_blocks);
no_terminal_blocks);
};
std::vector<BB*> result;

4
source/opt/cfg.cpp
View File

@ -87,7 +87,6 @@ void CFG::ComputeStructuredOrder(Function* func, BasicBlock* root,
// Compute structured successors and do DFS.
ComputeStructuredSuccessors(func);
auto ignore_block = [](cbb_ptr) {};
auto ignore_edge = [](cbb_ptr, cbb_ptr) {};
auto terminal = [end](cbb_ptr bb) { return bb == end; };
auto get_structured_successors = [this](const BasicBlock* b) {
@ -100,8 +99,7 @@ void CFG::ComputeStructuredOrder(Function* func, BasicBlock* root,
order->push_front(const_cast<BasicBlock*>(b));
};
CFA<BasicBlock>::DepthFirstTraversal(root, get_structured_successors,
ignore_block, post_order, ignore_edge,
terminal);
ignore_block, post_order, terminal);
}
void CFG::ForEachBlockInPostOrder(BasicBlock* bb,

28
source/opt/dominator_tree.cpp
View File

@ -57,10 +57,8 @@ template <typename BBType, typename SuccessorLambda, typename PreLambda,
typename PostLambda>
static void DepthFirstSearch(const BBType* bb, SuccessorLambda successors,
PreLambda pre, PostLambda post) {
// Ignore backedge operation.
auto nop_backedge = [](const BBType*, const BBType*) {};
auto no_terminal_blocks = [](const BBType*) { return false; };
CFA<BBType>::DepthFirstTraversal(bb, successors, pre, post, nop_backedge,
CFA<BBType>::DepthFirstTraversal(bb, successors, pre, post,
no_terminal_blocks);
}
@ -105,7 +103,8 @@ class BasicBlockSuccessorHelper {
using Function = typename GetFunctionClass<BBType>::FunctionType;
using BasicBlockListTy = std::vector<BasicBlock*>;
using BasicBlockMapTy = std::map<const BasicBlock*, BasicBlockListTy>;
using BasicBlockMapTy =
std::unordered_map<const BasicBlock*, BasicBlockListTy>;
public:
// For compliance with the dominance tree computation, entry nodes are
@ -160,19 +159,7 @@ BasicBlockSuccessorHelper<BBType>::BasicBlockSuccessorHelper(
template <typename BBType>
void BasicBlockSuccessorHelper<BBType>::CreateSuccessorMap(
Function& f, const BasicBlock* placeholder_start_node) {
std::map<uint32_t, BasicBlock*> id_to_BB_map;
auto GetSuccessorBasicBlock = [&f, &id_to_BB_map](uint32_t successor_id) {
BasicBlock*& Succ = id_to_BB_map[successor_id];
if (!Succ) {
for (BasicBlock& BBIt : f) {
if (successor_id == BBIt.id()) {
Succ = &BBIt;
break;
}
}
}
return Succ;
};
IRContext* context = f.DefInst().context();
if (invert_graph_) {
// For the post dominator tree, we see the inverted graph.
@ -186,9 +173,8 @@ void BasicBlockSuccessorHelper<BBType>::CreateSuccessorMap(
BasicBlockListTy& pred_list = predecessors_[&bb];
const auto& const_bb = bb;
const_bb.ForEachSuccessorLabel(
[this, &pred_list, &bb,
&GetSuccessorBasicBlock](const uint32_t successor_id) {
BasicBlock* succ = GetSuccessorBasicBlock(successor_id);
[this, &pred_list, &bb, context](const uint32_t successor_id) {
BasicBlock* succ = context->get_instr_block(successor_id);
// Inverted graph: our successors in the CFG
// are our predecessors in the inverted graph.
this->successors_[succ].push_back(&bb);
@ -209,7 +195,7 @@ void BasicBlockSuccessorHelper<BBType>::CreateSuccessorMap(
const auto& const_bb = bb;
const_bb.ForEachSuccessorLabel([&](const uint32_t successor_id) {
BasicBlock* succ = GetSuccessorBasicBlock(successor_id);
BasicBlock* succ = context->get_instr_block(successor_id);
succ_list.push_back(succ);
predecessors_[succ].push_back(&bb);
});

7
source/val/validate_cfg.cpp
View File

@ -896,14 +896,13 @@ spv_result_t PerformCfgChecks(ValidationState_t& _) {
// CFG to ensure we cover all the blocks.
std::vector<const BasicBlock*> postorder;
auto ignore_block = [](const BasicBlock*) {};
auto ignore_edge = [](const BasicBlock*, const BasicBlock*) {};
auto no_terminal_blocks = [](const BasicBlock*) { return false; };
if (!function.ordered_blocks().empty()) {
/// calculate dominators
CFA<BasicBlock>::DepthFirstTraversal(
function.first_block(), function.AugmentedCFGSuccessorsFunction(),
ignore_block, [&](const BasicBlock* b) { postorder.push_back(b); },
ignore_edge, no_terminal_blocks);
no_terminal_blocks);
auto edges = CFA<BasicBlock>::CalculateDominators(
postorder, function.AugmentedCFGPredecessorsFunction());
for (auto edge : edges) {
@ -953,7 +952,7 @@ spv_result_t PerformCfgChecks(ValidationState_t& _) {
CFA<BasicBlock>::DepthFirstTraversal(
function.first_block(),
function.AugmentedStructuralCFGSuccessorsFunction(), ignore_block,
[&](const BasicBlock* b) { postorder.push_back(b); }, ignore_edge,
[&](const BasicBlock* b) { postorder.push_back(b); },
no_terminal_blocks);
auto edges = CFA<BasicBlock>::CalculateDominators(
postorder, function.AugmentedStructuralCFGPredecessorsFunction());
@ -967,7 +966,7 @@ spv_result_t PerformCfgChecks(ValidationState_t& _) {
function.pseudo_exit_block(),
function.AugmentedStructuralCFGPredecessorsFunction(), ignore_block,
[&](const BasicBlock* b) { postdom_postorder.push_back(b); },
ignore_edge, no_terminal_blocks);
no_terminal_blocks);
auto postdom_edges = CFA<BasicBlock>::CalculateDominators(
postdom_postorder,
function.AugmentedStructuralCFGSuccessorsFunction());