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// Copyright (c) 2021 Google LLC.
//
// 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.
#include "source/opt/control_dependence.h"
#include <cassert>
#include <tuple>
#include "source/opt/basic_block.h"
#include "source/opt/cfg.h"
#include "source/opt/dominator_analysis.h"
#include "source/opt/function.h"
#include "source/opt/instruction.h"
// Computes the control dependence graph (CDG) using the algorithm in Cytron
// 1991, "Efficiently Computing Static Single Assignment Form and the Control
// Dependence Graph." It relies on the fact that the control dependence sources
// (blocks on which a block is control dependent) are exactly the post-dominance
// frontier for that block. The explanation and proofs are given in Section 6 of
// that paper.
// Link: https://www.cs.utexas.edu/~pingali/CS380C/2010/papers/ssaCytron.pdf
//
// The algorithm in Section 4.2 of the same paper is used to construct the
// dominance frontier. It uses the post-dominance tree, which is available in
// the IR context.
namespace spvtools {
namespace opt {
constexpr uint32_t ControlDependenceAnalysis::kPseudoEntryBlock;
uint32_t ControlDependence::GetConditionID(const CFG& cfg) const {
if (source_bb_id() == 0) {
// Entry dependence; return 0.
return 0;
}
const BasicBlock* source_bb = cfg.block(source_bb_id());
const Instruction* branch = source_bb->terminator();
assert((branch->opcode() == spv::Op::OpBranchConditional ||
branch->opcode() == spv::Op::OpSwitch) &&
"invalid control dependence; last instruction must be conditional "
"branch or switch");
return branch->GetSingleWordInOperand(0);
}
bool ControlDependence::operator<(const ControlDependence& other) const {
return std::tie(source_bb_id_, target_bb_id_, branch_target_bb_id_) <
std::tie(other.source_bb_id_, other.target_bb_id_,
other.branch_target_bb_id_);
}
bool ControlDependence::operator==(const ControlDependence& other) const {
return std::tie(source_bb_id_, target_bb_id_, branch_target_bb_id_) ==
std::tie(other.source_bb_id_, other.target_bb_id_,
other.branch_target_bb_id_);
}
std::ostream& operator<<(std::ostream& os, const ControlDependence& dep) {
os << dep.source_bb_id() << "->" << dep.target_bb_id();
if (dep.branch_target_bb_id() != dep.target_bb_id()) {
os << " through " << dep.branch_target_bb_id();
}
return os;
}
void ControlDependenceAnalysis::ComputePostDominanceFrontiers(
const CFG& cfg, const PostDominatorAnalysis& pdom) {
// Compute post-dominance frontiers (reverse graph).
// The dominance frontier for a block X is equal to (Equation 4)
// DF_local(X) U { B in DF_up(Z) | X = ipdom(Z) }
// (ipdom(Z) is the immediate post-dominator of Z.)
// where
// DF_local(X) = { Y | X -> Y in CFG, X does not strictly post-dominate Y }
// represents the contribution of X's predecessors to the DF, and
// DF_up(Z) = { Y | Y in DF(Z), ipdom(Z) does not strictly post-dominate Y }
// (note: ipdom(Z) = X.)
// represents the contribution of a block to its immediate post-
// dominator's DF.
// This is computed in one pass through a post-order traversal of the
// post-dominator tree.
// Assert that there is a block other than the pseudo exit in the pdom tree,
// as we need one to get the function entry point (as the pseudo exit is not
// actually part of the function.)
assert(!cfg.IsPseudoExitBlock(pdom.GetDomTree().post_begin()->bb_));
Function* function = pdom.GetDomTree().post_begin()->bb_->GetParent();
uint32_t function_entry = function->entry()->id();
// Explicitly initialize pseudo-entry block, as it doesn't depend on anything,
// so it won't be initialized in the following loop.
reverse_nodes_[kPseudoEntryBlock] = {};
for (auto it = pdom.GetDomTree().post_cbegin();
it != pdom.GetDomTree().post_cend(); ++it) {
ComputePostDominanceFrontierForNode(cfg, pdom, function_entry, *it);
}
}
void ControlDependenceAnalysis::ComputePostDominanceFrontierForNode(
const CFG& cfg, const PostDominatorAnalysis& pdom, uint32_t function_entry,
const DominatorTreeNode& pdom_node) {
const uint32_t label = pdom_node.id();
ControlDependenceList& edges = reverse_nodes_[label];
for (uint32_t pred : cfg.preds(label)) {
if (!pdom.StrictlyDominates(label, pred)) {
edges.push_back(ControlDependence(pred, label));
}
}
if (label == function_entry) {
// Add edge from pseudo-entry to entry.
// In CDG construction, an edge is added from entry to exit, so only the
// exit node can post-dominate entry.
edges.push_back(ControlDependence(kPseudoEntryBlock, label));
}
for (DominatorTreeNode* child : pdom_node) {
// Note: iterate dependences by value, as we need a copy.
for (const ControlDependence& dep : reverse_nodes_[child->id()]) {
// Special-case pseudo-entry, as above.
if (dep.source_bb_id() == kPseudoEntryBlock ||
!pdom.StrictlyDominates(label, dep.source_bb_id())) {
edges.push_back(ControlDependence(dep.source_bb_id(), label,
dep.branch_target_bb_id()));
}
}
}
}
void ControlDependenceAnalysis::ComputeControlDependenceGraph(
const CFG& cfg, const PostDominatorAnalysis& pdom) {
ComputePostDominanceFrontiers(cfg, pdom);
ComputeForwardGraphFromReverse();
}
void ControlDependenceAnalysis::ComputeForwardGraphFromReverse() {
for (const auto& entry : reverse_nodes_) {
// Ensure an entry is created for each node.
forward_nodes_[entry.first];
for (const ControlDependence& dep : entry.second) {
forward_nodes_[dep.source_bb_id()].push_back(dep);
}
}
}
} // namespace opt
} // namespace spvtools
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