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