1.3.2.2.3.Lengauer-Tarjan算法第四步

在第四步，240行的DT.Vertex[i]得到的是序號為i的節點。如果DT.IDoms中的節點與Semi所指的節點不一致（即直接支配者節點與半支配者節點不一致），把DT.IDoms調整為DT.IDoms[DT.IDoms[W]]。因為推論1中，當sdom(w) != sdom(v)時，Idom(w) = Idom(v)，第三步并沒有計算該部分，這由第四步來完成，并且239行的循環從靠近樹根處開始，以確保DT.IDoms[DT.IDoms[W]]一定有意義。

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Calculate（續）

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238??? ? // Step #4:Explicitly define the immediate dominator of ea ch vertex

239??? ? for (unsignedi = 2; i <= N; ++i) {

240??? ??? typenameGraphT::NodeType* W = DT.Vertex[i];

241??? ??? typenameGraphT::NodeType*& WIDom = DT.IDoms[W];

242??? ??? if (WIDom != DT.Vertex[DT.Info[W].Semi])

243??? ????? WIDom = DT.IDoms[WIDom];

244??? ? }

245???

246??? ? if (DT.Roots.empty()) return;

247???

248??? ? // Add a node forthe root.? This node might be the actualroot, if there is

249??? ? // one exit block,or it may be the virtual exit (denoted by (BasicBlock *)0)

250??? ? // which postdominates all real exits ifthere are multiple exit blocks, or

251??? ? // an infiniteloop.

252??? ? typenameGraphT::NodeType* Root = !MultipleRoots ? DT.Roots[0] : 0;

253???

254??? ? DT.DomTreeNodes[Root] = DT.RootNode =

255??? ??????????????????????? new DomTreeNodeBase<typenameGraphT::NodeType>(Root, 0);

256???

257??? ? // Loop over all ofthe reachable blocks in the function...

258??? ? for (unsignedi = 2; i <= N; ++i) {

259??? ??? typenameGraphT::NodeType* W = DT.Vertex[i];

260???

261??? ??? DomTreeNodeBase<typenameGraphT::NodeType> *BBNode = DT.DomTreeNodes[W];

262??? ??? if (BBNode) continue;? // Haven'tcalculated this node yet?

263???

264??? ??? typenameGraphT::NodeType* ImmDom = DT.getIDom(W);

265???

266??? ??? assert(ImmDom|| DT.DomTreeNodes[NULL]);

267???

268??? ??? // Get orcalculate the node for the immediate dominator

269??? ??? DomTreeNodeBase<typenameGraphT::NodeType> *IDomNode =

270??? ????????????????????????????????????????????????????DT.getNodeForBlock(ImmDom);

271???

272??? ??? // Add a new treenode for this BasicBlock, and link it as a child of

273??? ??? // IDomNode

274??? ??? DomTreeNodeBase<typenameGraphT::NodeType> *C =

275??? ??????????????????? newDomTreeNodeBase<typenameGraphT::NodeType>(W, IDomNode);

276??? ??? DT.DomTreeNodes[W] = IDomNode->addChild(C);

277??? ? }

278???

279??? ? // Free temporarymemory used to construct idom's

280??? ? DT.IDoms.clear();

281??? ? DT.Info.clear();

282??? ? std::vector<typenameGraphT::NodeType*>().swap(DT.Vertex);

283???

284??? ? DT.updateDFSNumbers();

285??? }

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余下的代碼則是構建一棵支配樹（dominator tree）。我們例子的支配樹如下圖所示。

284行的updateDFSNumbers函數以深度優先序遍歷配置樹，對DomTreeNodeBase中的DFSNumIn及DFSNumOut域賦值。

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579??? ? void updateDFSNumbers(){

580??? ??? unsigned DFSNum = 0;

581???

582??? ??? SmallVector<std::pair<DomTreeNodeBase<NodeT>*,

583??? ??????????????? typenameDomTreeNodeBase<NodeT>::iterator>, 32> WorkStack;

584???

585??? ??? DomTreeNodeBase<NodeT> *ThisRoot =getRootNode();

586???

587??? ??? if (!ThisRoot)

588??? ????? return;

589???

590??? ??? // Even in thecase of multiple exits that form the post dominator root

591??? ??? // nodes, do notiterate over all exits, but start from the virtual root

592??? ??? // node.Otherwise bbs, that are not post dominated by any exit but by the

593??? ??? // virtual rootnode, will never be assigned a DFS number.

594??? ???WorkStack.push_back(std::make_pair(ThisRoot, ThisRoot->begin()));

595??? ??? ThisRoot->DFSNumIn = DFSNum++;

596???

597??? ??? while(!WorkStack.empty()) {

598??? ????? DomTreeNodeBase<NodeT> *Node =WorkStack.back().first;

599??? ????? typename DomTreeNodeBase<NodeT>::iteratorChildIt =

600??? ??????? WorkStack.back().second;

601???

602??? ????? // If wevisited all of the children of this node, "recurse" back up the

603??? ????? // stacksetting the DFOutNum.

604??? ????? if (ChildIt == Node->end()) {

605??? ??????? Node->DFSNumOut = DFSNum++;

606??? ??????? WorkStack.pop_back();

607??? ????? } else {

608??? ??????? // Otherwise,recursively visit this child.

609??? ??????? DomTreeNodeBase<NodeT> *Child =*ChildIt;

610??? ??????? ++WorkStack.back().second;

611???

612??? ??????? WorkStack.push_back(std::make_pair(Child,Child->begin()));

613??? ??????? Child->DFSNumIn = DFSNum++;

614??? ????? }

615??? ??? }

616???

617??? ??? SlowQueries = 0;

618??? ??? DFSInfoValid = true;

619??? ? }

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編號的結果如上圖所示。注意DT是DominatorTree中的成員，因此這些內容可以帶到下一個遍使用。

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