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[Opt] Identical store/load elimination by control-flow graph (#1741)
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@xumingkuan
xumingkuan authored Aug 20, 2020
1 parent 4047448 commit 5548c55
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Showing 3 changed files with 118 additions and 256 deletions.
122 changes: 113 additions & 9 deletions taichi/ir/control_flow_graph.cpp
View file
Original file line number Diff line number Diff line change
Expand Up @@ -179,8 +179,14 @@ Stmt *CFGNode::get_store_forwarding_data(Stmt *var, int position) const {
return result;
}
Stmt *result = nullptr;
bool result_visible = false;
auto visible = [&](Stmt *stmt) {
// Do we need to check if `stmt` is before `position` here?
// Check if `stmt` is before `position` here.
if (stmt->parent == block) {
return stmt->parent->locate(stmt) < position;
}
// TODO: What if `stmt` appears in an ancestor of `block` but after
// `position`?
return parent_blocks.find(stmt->parent) != parent_blocks.end();
};
auto update_result = [&](Stmt *stmt) {
Expand All @@ -190,16 +196,22 @@ Stmt *CFGNode::get_store_forwarding_data(Stmt *var, int position) const {
}
if (!result) {
result = data;
} else if (!irpass::analysis::same_statements(result, data)) {
result_visible = visible(data);
return true; // continue the following loops
}
if (!irpass::analysis::same_statements(result, data)) {
// check the special case of alloca (initialized to 0)
if (!(result->is<AllocaStmt>() && data->is<ConstStmt>() &&
data->width() == 1 &&
data->as<ConstStmt>()->val[0].equal_value(0))) {
return false; // return nullptr
}
}
if (visible(data))
if (!result_visible && visible(data)) {
// pick the visible one for store-to-load forwarding
result = data;
result_visible = true;
}
return true; // continue the following loops
};
for (auto stmt : reach_in) {
Expand All @@ -223,7 +235,7 @@ Stmt *CFGNode::get_store_forwarding_data(Stmt *var, int position) const {
block->statements[position]->id);
return nullptr;
}
if (!visible(result)) {
if (!result_visible) {
return nullptr;
}
return result;
Expand Down Expand Up @@ -264,6 +276,46 @@ bool CFGNode::store_to_load_forwarding(bool after_lower_access) {
i--; // to cancel i++ in the for loop
modified = true;
}
continue;
}
// Identical store elimination
if (auto local_store = stmt->cast<LocalStoreStmt>()) {
result = get_store_forwarding_data(local_store->ptr, i);
if (result) {
if (result->is<AllocaStmt>()) {
// special case of alloca (initialized to 0)
if (auto stored_data = local_store->data->cast<ConstStmt>()) {
bool all_zero = true;
for (auto &val : stored_data->val.data) {
if (!val.equal_value(0)) {
all_zero = false;
break;
}
}
if (all_zero) {
erase(i); // This causes end_location--
i--; // to cancel i++ in the for loop
modified = true;
}
}
} else {
// not alloca
if (irpass::analysis::same_statements(result, local_store->data)) {
erase(i); // This causes end_location--
i--; // to cancel i++ in the for loop
modified = true;
}
}
}
} else if (auto global_store = stmt->cast<GlobalStoreStmt>()) {
if (!after_lower_access) {
result = get_store_forwarding_data(global_store->ptr, i);
if (irpass::analysis::same_statements(result, global_store->data)) {
erase(i); // This causes end_location--
i--; // to cancel i++ in the for loop
modified = true;
}
}
}
}
return modified;
Expand Down Expand Up @@ -342,6 +394,7 @@ bool CFGNode::dead_store_elimination(bool after_lower_access) {
replace_with(i, std::move(local_load), true);
// Notice that we have a load here.
live_in_this_node.insert(atomic->dest);
killed_in_this_node.erase(atomic->dest);
modified = true;
continue;
} else if (!is_parallel_executed) {
Expand All @@ -354,6 +407,11 @@ bool CFGNode::dead_store_elimination(bool after_lower_access) {
replace_with(i, std::move(global_load), true);
// Notice that we have a load here.
live_in_this_node.insert(atomic->dest);
// Note: It's possible that a global pointer is not erased from
// killed_in_this_node although it should be. This may harm the
// performance of identical load elimination but it's faster than
// checking the contents one by one.
killed_in_this_node.erase(atomic->dest);
modified = true;
continue;
}
Expand All @@ -375,11 +433,55 @@ bool CFGNode::dead_store_elimination(bool after_lower_access) {
}
}
auto load_ptrs = irpass::analysis::get_load_pointers(stmt);
if (load_ptrs.size() == 1 && store_ptrs.empty() && stmt->width() == 1) {
// Identical load elimination
auto load_ptr = load_ptrs.front();
if (!after_lower_access ||
(load_ptr->is<AllocaStmt>() || load_ptr->is<StackAllocaStmt>())) {
// After lower_access, we only analyze local variables and stacks.
if (!may_contain_variable(killed_in_this_node, load_ptr) &&
contain_variable(live_in_this_node, load_ptr)) {
// Only perform identical load elimination within a CFGNode.
for (int j = i + 1; j < end_location; j++) {
auto next_load_ptrs =
irpass::analysis::get_load_pointers(block->statements[j].get());
bool found = false;
for (auto &next_load_ptr : next_load_ptrs) {
if (irpass::analysis::maybe_same_address(load_ptr,
next_load_ptr)) {
found = true;
break;
}
}
if (found) {
if (irpass::analysis::same_statements(
stmt, block->statements[j].get())) {
block->statements[j]->replace_with(stmt);
erase(j);
modified = true;
break;
} else {
TI_WARN("Identical load elimination failed.");
}
}
}
}
}
}
for (auto &load_ptr : load_ptrs) {
if (!after_lower_access ||
(load_ptr->is<AllocaStmt>() || load_ptr->is<StackAllocaStmt>())) {
// After lower_access, we only analyze local variables and stacks.
live_in_this_node.insert(load_ptr);
if (store_ptrs.empty()) {
// Only allow identical load elimination (i.e. allow this statement
// to be eliminated) if this statement doesn't store any data.
// Note: It's possible that a global pointer is not erased from
// killed_in_this_node although it should be. This may harm the
// performance of identical load elimination but it's faster than
// checking the contents one by one.
killed_in_this_node.erase(load_ptr);
}
}
}
}
Expand Down Expand Up @@ -484,12 +586,14 @@ void ControlFlowGraph::reaching_definition_analysis(bool after_lower_access) {
if (!after_lower_access) {
for (int i = 0; i < num_nodes; i++) {
for (int j = nodes[i]->begin_location; j < nodes[i]->end_location; j++) {
if (auto global_load =
nodes[i]->block->statements[j]->cast<GlobalLoadStmt>()) {
nodes[start_node]->reach_gen.insert(global_load->ptr);
auto stmt = nodes[i]->block->statements[j].get();
if (stmt->is<GlobalPtrStmt>() || stmt->is<ExternalPtrStmt>() ||
stmt->is<BlockLocalPtrStmt>() || stmt->is<ThreadLocalPtrStmt>() ||
stmt->is<GlobalTemporaryStmt>()) {
// TODO: unify them
// A global pointer that may contain some data before this kernel.
nodes[start_node]->reach_gen.insert(stmt);
}
// Since we only do store-to-load forwarding, we don't need to mark
// other global pointers' data source at the start node.
}
}
}
Expand Down
5 changes: 5 additions & 0 deletions taichi/ir/control_flow_graph.h
View file
Original file line number Diff line number Diff line change
Expand Up @@ -88,9 +88,14 @@ class ControlFlowGraph {
void live_variable_analysis(bool after_lower_access);

void simplify_graph();

// This pass cannot eliminate container statements properly for now.
bool unreachable_code_elimination();

// Also performs identical store elimination.
bool store_to_load_forwarding(bool after_lower_access);

// Also performs identical load elimination.
bool dead_store_elimination(bool after_lower_access);
};

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