llvm-6502/include/llvm/Support/GenericDomTreeConstruction.h
David Blaikie 4ade012dd0 Add comment about a gotcha I ran across while touching this code.
I haven't looked closely at exactly why the side effect is required, but
this seems better than not mentioning it at all.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@226030 91177308-0d34-0410-b5e6-96231b3b80d8
2015-01-14 19:59:18 +00:00

294 lines
10 KiB
C++

//===- GenericDomTreeConstruction.h - Dominator Calculation ------*- C++ -*-==//
//
// The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
/// \file
///
/// Generic dominator tree construction - This file provides routines to
/// construct immediate dominator information for a flow-graph based on the
/// algorithm described in this document:
///
/// A Fast Algorithm for Finding Dominators in a Flowgraph
/// T. Lengauer & R. Tarjan, ACM TOPLAS July 1979, pgs 121-141.
///
/// This implements the O(n*log(n)) versions of EVAL and LINK, because it turns
/// out that the theoretically slower O(n*log(n)) implementation is actually
/// faster than the almost-linear O(n*alpha(n)) version, even for large CFGs.
///
//===----------------------------------------------------------------------===//
#ifndef LLVM_SUPPORT_GENERICDOMTREECONSTRUCTION_H
#define LLVM_SUPPORT_GENERICDOMTREECONSTRUCTION_H
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/Support/GenericDomTree.h"
namespace llvm {
template<class GraphT>
unsigned DFSPass(DominatorTreeBase<typename GraphT::NodeType>& DT,
typename GraphT::NodeType* V, unsigned N) {
// This is more understandable as a recursive algorithm, but we can't use the
// recursive algorithm due to stack depth issues. Keep it here for
// documentation purposes.
#if 0
InfoRec &VInfo = DT.Info[DT.Roots[i]];
VInfo.DFSNum = VInfo.Semi = ++N;
VInfo.Label = V;
Vertex.push_back(V); // Vertex[n] = V;
for (succ_iterator SI = succ_begin(V), E = succ_end(V); SI != E; ++SI) {
InfoRec &SuccVInfo = DT.Info[*SI];
if (SuccVInfo.Semi == 0) {
SuccVInfo.Parent = V;
N = DTDFSPass(DT, *SI, N);
}
}
#else
bool IsChildOfArtificialExit = (N != 0);
SmallVector<std::pair<typename GraphT::NodeType*,
typename GraphT::ChildIteratorType>, 32> Worklist;
Worklist.push_back(std::make_pair(V, GraphT::child_begin(V)));
while (!Worklist.empty()) {
typename GraphT::NodeType* BB = Worklist.back().first;
typename GraphT::ChildIteratorType NextSucc = Worklist.back().second;
typename DominatorTreeBase<typename GraphT::NodeType>::InfoRec &BBInfo =
DT.Info[BB];
// First time we visited this BB?
if (NextSucc == GraphT::child_begin(BB)) {
BBInfo.DFSNum = BBInfo.Semi = ++N;
BBInfo.Label = BB;
DT.Vertex.push_back(BB); // Vertex[n] = V;
if (IsChildOfArtificialExit)
BBInfo.Parent = 1;
IsChildOfArtificialExit = false;
}
// store the DFS number of the current BB - the reference to BBInfo might
// get invalidated when processing the successors.
unsigned BBDFSNum = BBInfo.DFSNum;
// If we are done with this block, remove it from the worklist.
if (NextSucc == GraphT::child_end(BB)) {
Worklist.pop_back();
continue;
}
// Increment the successor number for the next time we get to it.
++Worklist.back().second;
// Visit the successor next, if it isn't already visited.
typename GraphT::NodeType* Succ = *NextSucc;
typename DominatorTreeBase<typename GraphT::NodeType>::InfoRec &SuccVInfo =
DT.Info[Succ];
if (SuccVInfo.Semi == 0) {
SuccVInfo.Parent = BBDFSNum;
Worklist.push_back(std::make_pair(Succ, GraphT::child_begin(Succ)));
}
}
#endif
return N;
}
template<class GraphT>
typename GraphT::NodeType*
Eval(DominatorTreeBase<typename GraphT::NodeType>& DT,
typename GraphT::NodeType *VIn, unsigned LastLinked) {
typename DominatorTreeBase<typename GraphT::NodeType>::InfoRec &VInInfo =
DT.Info[VIn];
if (VInInfo.DFSNum < LastLinked)
return VIn;
SmallVector<typename GraphT::NodeType*, 32> Work;
SmallPtrSet<typename GraphT::NodeType*, 32> Visited;
if (VInInfo.Parent >= LastLinked)
Work.push_back(VIn);
while (!Work.empty()) {
typename GraphT::NodeType* V = Work.back();
typename DominatorTreeBase<typename GraphT::NodeType>::InfoRec &VInfo =
DT.Info[V];
typename GraphT::NodeType* VAncestor = DT.Vertex[VInfo.Parent];
// Process Ancestor first
if (Visited.insert(VAncestor).second && VInfo.Parent >= LastLinked) {
Work.push_back(VAncestor);
continue;
}
Work.pop_back();
// Update VInfo based on Ancestor info
if (VInfo.Parent < LastLinked)
continue;
typename DominatorTreeBase<typename GraphT::NodeType>::InfoRec &VAInfo =
DT.Info[VAncestor];
typename GraphT::NodeType* VAncestorLabel = VAInfo.Label;
typename GraphT::NodeType* VLabel = VInfo.Label;
if (DT.Info[VAncestorLabel].Semi < DT.Info[VLabel].Semi)
VInfo.Label = VAncestorLabel;
VInfo.Parent = VAInfo.Parent;
}
return VInInfo.Label;
}
template<class FuncT, class NodeT>
void Calculate(DominatorTreeBase<typename GraphTraits<NodeT>::NodeType>& DT,
FuncT& F) {
typedef GraphTraits<NodeT> GraphT;
unsigned N = 0;
bool MultipleRoots = (DT.Roots.size() > 1);
if (MultipleRoots) {
typename DominatorTreeBase<typename GraphT::NodeType>::InfoRec &BBInfo =
DT.Info[nullptr];
BBInfo.DFSNum = BBInfo.Semi = ++N;
BBInfo.Label = nullptr;
DT.Vertex.push_back(nullptr); // Vertex[n] = V;
}
// Step #1: Number blocks in depth-first order and initialize variables used
// in later stages of the algorithm.
for (unsigned i = 0, e = static_cast<unsigned>(DT.Roots.size());
i != e; ++i)
N = DFSPass<GraphT>(DT, DT.Roots[i], N);
// it might be that some blocks did not get a DFS number (e.g., blocks of
// infinite loops). In these cases an artificial exit node is required.
MultipleRoots |= (DT.isPostDominator() && N != GraphTraits<FuncT*>::size(&F));
// When naively implemented, the Lengauer-Tarjan algorithm requires a separate
// bucket for each vertex. However, this is unnecessary, because each vertex
// is only placed into a single bucket (that of its semidominator), and each
// vertex's bucket is processed before it is added to any bucket itself.
//
// Instead of using a bucket per vertex, we use a single array Buckets that
// has two purposes. Before the vertex V with preorder number i is processed,
// Buckets[i] stores the index of the first element in V's bucket. After V's
// bucket is processed, Buckets[i] stores the index of the next element in the
// bucket containing V, if any.
SmallVector<unsigned, 32> Buckets;
Buckets.resize(N + 1);
for (unsigned i = 1; i <= N; ++i)
Buckets[i] = i;
for (unsigned i = N; i >= 2; --i) {
typename GraphT::NodeType* W = DT.Vertex[i];
typename DominatorTreeBase<typename GraphT::NodeType>::InfoRec &WInfo =
DT.Info[W];
// Step #2: Implicitly define the immediate dominator of vertices
for (unsigned j = i; Buckets[j] != i; j = Buckets[j]) {
typename GraphT::NodeType* V = DT.Vertex[Buckets[j]];
typename GraphT::NodeType* U = Eval<GraphT>(DT, V, i + 1);
DT.IDoms[V] = DT.Info[U].Semi < i ? U : W;
}
// Step #3: Calculate the semidominators of all vertices
// initialize the semi dominator to point to the parent node
WInfo.Semi = WInfo.Parent;
typedef GraphTraits<Inverse<NodeT> > InvTraits;
for (typename InvTraits::ChildIteratorType CI =
InvTraits::child_begin(W),
E = InvTraits::child_end(W); CI != E; ++CI) {
typename InvTraits::NodeType *N = *CI;
if (DT.Info.count(N)) { // Only if this predecessor is reachable!
unsigned SemiU = DT.Info[Eval<GraphT>(DT, N, i + 1)].Semi;
if (SemiU < WInfo.Semi)
WInfo.Semi = SemiU;
}
}
// If V is a non-root vertex and sdom(V) = parent(V), then idom(V) is
// necessarily parent(V). In this case, set idom(V) here and avoid placing
// V into a bucket.
if (WInfo.Semi == WInfo.Parent) {
DT.IDoms[W] = DT.Vertex[WInfo.Parent];
} else {
Buckets[i] = Buckets[WInfo.Semi];
Buckets[WInfo.Semi] = i;
}
}
if (N >= 1) {
typename GraphT::NodeType* Root = DT.Vertex[1];
for (unsigned j = 1; Buckets[j] != 1; j = Buckets[j]) {
typename GraphT::NodeType* V = DT.Vertex[Buckets[j]];
DT.IDoms[V] = Root;
}
}
// Step #4: Explicitly define the immediate dominator of each vertex
for (unsigned i = 2; i <= N; ++i) {
typename GraphT::NodeType* W = DT.Vertex[i];
typename GraphT::NodeType*& WIDom = DT.IDoms[W];
if (WIDom != DT.Vertex[DT.Info[W].Semi])
WIDom = DT.IDoms[WIDom];
}
if (DT.Roots.empty()) return;
// Add a node for the root. This node might be the actual root, if there is
// one exit block, or it may be the virtual exit (denoted by (BasicBlock *)0)
// which postdominates all real exits if there are multiple exit blocks, or
// an infinite loop.
typename GraphT::NodeType* Root = !MultipleRoots ? DT.Roots[0] : nullptr;
DT.RootNode =
(DT.DomTreeNodes[Root] =
llvm::make_unique<DomTreeNodeBase<typename GraphT::NodeType>>(
Root, nullptr)).get();
// Loop over all of the reachable blocks in the function...
for (unsigned i = 2; i <= N; ++i) {
typename GraphT::NodeType* W = DT.Vertex[i];
// Don't replace this with 'count', the insertion side effect is important
if (DT.DomTreeNodes[W])
continue; // Haven't calculated this node yet?
typename GraphT::NodeType* ImmDom = DT.getIDom(W);
assert(ImmDom || DT.DomTreeNodes[nullptr]);
// Get or calculate the node for the immediate dominator
DomTreeNodeBase<typename GraphT::NodeType> *IDomNode =
DT.getNodeForBlock(ImmDom);
// Add a new tree node for this BasicBlock, and link it as a child of
// IDomNode
DT.DomTreeNodes[W] = IDomNode->addChild(
llvm::make_unique<DomTreeNodeBase<typename GraphT::NodeType>>(
W, IDomNode));
}
// Free temporary memory used to construct idom's
DT.IDoms.clear();
DT.Info.clear();
DT.Vertex.clear();
DT.Vertex.shrink_to_fit();
DT.updateDFSNumbers();
}
}
#endif