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//===- IteratedDominanceFrontier.h - Calculate IDF --------------*- C++ -*-===//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
/// \file
/// Compute iterated dominance frontiers using a linear time algorithm.
/// The algorithm used here is based on:
/// Sreedhar and Gao. A linear time algorithm for placing phi-nodes.
/// In Proceedings of the 22nd ACM SIGPLAN-SIGACT Symposium on Principles of
/// Programming Languages
/// POPL '95. ACM, New York, NY, 62-73.
/// It has been modified to not explicitly use the DJ graph data structure and
/// to directly compute pruned SSA using per-variable liveness information.
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/Support/GenericDomTree.h"
#include <queue>
namespace llvm {
namespace IDFCalculatorDetail {
/// Generic utility class used for getting the children of a basic block.
/// May be specialized if, for example, one wouldn't like to return nullpointer
/// successors.
template <class NodeTy, bool IsPostDom> struct ChildrenGetterTy {
using NodeRef = typename GraphTraits<NodeTy>::NodeRef;
using ChildrenTy = SmallVector<NodeRef, 8>;
ChildrenTy get(const NodeRef &N);
} // end of namespace IDFCalculatorDetail
/// Determine the iterated dominance frontier, given a set of defining
/// blocks, and optionally, a set of live-in blocks.
/// In turn, the results can be used to place phi nodes.
/// This algorithm is a linear time computation of Iterated Dominance Frontiers,
/// pruned using the live-in set.
/// By default, liveness is not used to prune the IDF computation.
/// The template parameters should be of a CFG block type.
template <class NodeTy, bool IsPostDom> class IDFCalculatorBase {
using OrderedNodeTy =
typename std::conditional<IsPostDom, Inverse<NodeTy *>, NodeTy *>::type;
using ChildrenGetterTy =
IDFCalculatorDetail::ChildrenGetterTy<NodeTy, IsPostDom>;
IDFCalculatorBase(DominatorTreeBase<NodeTy, IsPostDom> &DT) : DT(DT) {}
IDFCalculatorBase(DominatorTreeBase<NodeTy, IsPostDom> &DT,
const ChildrenGetterTy &C)
: DT(DT), ChildrenGetter(C) {}
/// Give the IDF calculator the set of blocks in which the value is
/// defined. This is equivalent to the set of starting blocks it should be
/// calculating the IDF for (though later gets pruned based on liveness).
/// Note: This set *must* live for the entire lifetime of the IDF calculator.
void setDefiningBlocks(const SmallPtrSetImpl<NodeTy *> &Blocks) {
DefBlocks = &Blocks;
/// Give the IDF calculator the set of blocks in which the value is
/// live on entry to the block. This is used to prune the IDF calculation to
/// not include blocks where any phi insertion would be dead.
/// Note: This set *must* live for the entire lifetime of the IDF calculator.
void setLiveInBlocks(const SmallPtrSetImpl<NodeTy *> &Blocks) {
LiveInBlocks = &Blocks;
useLiveIn = true;
/// Reset the live-in block set to be empty, and tell the IDF
/// calculator to not use liveness anymore.
void resetLiveInBlocks() {
LiveInBlocks = nullptr;
useLiveIn = false;
/// Calculate iterated dominance frontiers
/// This uses the linear-time phi algorithm based on DJ-graphs mentioned in
/// the file-level comment. It performs DF->IDF pruning using the live-in
/// set, to avoid computing the IDF for blocks where an inserted PHI node
/// would be dead.
void calculate(SmallVectorImpl<NodeTy *> &IDFBlocks);
DominatorTreeBase<NodeTy, IsPostDom> &DT;
ChildrenGetterTy ChildrenGetter;
bool useLiveIn = false;
const SmallPtrSetImpl<NodeTy *> *LiveInBlocks;
const SmallPtrSetImpl<NodeTy *> *DefBlocks;
// Implementation.
namespace IDFCalculatorDetail {
template <class NodeTy, bool IsPostDom>
typename ChildrenGetterTy<NodeTy, IsPostDom>::ChildrenTy
ChildrenGetterTy<NodeTy, IsPostDom>::get(const NodeRef &N) {
using OrderedNodeTy =
typename IDFCalculatorBase<NodeTy, IsPostDom>::OrderedNodeTy;
auto Children = children<OrderedNodeTy>(N);
return {Children.begin(), Children.end()};
} // end of namespace IDFCalculatorDetail
template <class NodeTy, bool IsPostDom>
void IDFCalculatorBase<NodeTy, IsPostDom>::calculate(
SmallVectorImpl<NodeTy *> &PHIBlocks) {
// Use a priority queue keyed on dominator tree level so that inserted nodes
// are handled from the bottom of the dominator tree upwards. We also augment
// the level with a DFS number to ensure that the blocks are ordered in a
// deterministic way.
using DomTreeNodePair =
std::pair<DomTreeNodeBase<NodeTy> *, std::pair<unsigned, unsigned>>;
using IDFPriorityQueue =
std::priority_queue<DomTreeNodePair, SmallVector<DomTreeNodePair, 32>,
IDFPriorityQueue PQ;
for (NodeTy *BB : *DefBlocks) {
if (DomTreeNodeBase<NodeTy> *Node = DT.getNode(BB))
PQ.push({Node, std::make_pair(Node->getLevel(), Node->getDFSNumIn())});
SmallVector<DomTreeNodeBase<NodeTy> *, 32> Worklist;
SmallPtrSet<DomTreeNodeBase<NodeTy> *, 32> VisitedPQ;
SmallPtrSet<DomTreeNodeBase<NodeTy> *, 32> VisitedWorklist;
while (!PQ.empty()) {
DomTreeNodePair RootPair =;
DomTreeNodeBase<NodeTy> *Root = RootPair.first;
unsigned RootLevel = RootPair.second.first;
// Walk all dominator tree children of Root, inspecting their CFG edges with
// targets elsewhere on the dominator tree. Only targets whose level is at
// most Root's level are added to the iterated dominance frontier of the
// definition set.
while (!Worklist.empty()) {
DomTreeNodeBase<NodeTy> *Node = Worklist.pop_back_val();
NodeTy *BB = Node->getBlock();
// Succ is the successor in the direction we are calculating IDF, so it is
// successor for IDF, and predecessor for Reverse IDF.
auto DoWork = [&](NodeTy *Succ) {
DomTreeNodeBase<NodeTy> *SuccNode = DT.getNode(Succ);
const unsigned SuccLevel = SuccNode->getLevel();
if (SuccLevel > RootLevel)
if (!VisitedPQ.insert(SuccNode).second)
NodeTy *SuccBB = SuccNode->getBlock();
if (useLiveIn && !LiveInBlocks->count(SuccBB))
if (!DefBlocks->count(SuccBB))
SuccNode, std::make_pair(SuccLevel, SuccNode->getDFSNumIn())));
for (auto Succ : ChildrenGetter.get(BB))
for (auto DomChild : *Node) {
if (VisitedWorklist.insert(DomChild).second)
} // end of namespace llvm