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141 lines
4.2 KiB
C++
141 lines
4.2 KiB
C++
//==- DominatorInternals.cpp - Dominator Calculation -------------*- C++ -*-==//
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//
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// The LLVM Compiler Infrastructure
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//
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// This file was developed by Owen Anderson and is distributed under
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// the University of Illinois Open Source License. See LICENSE.TXT for details.
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//
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//===----------------------------------------------------------------------===//
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#include "llvm/Analysis/Dominators.h"
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#include "llvm/ADT/DenseMap.h"
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#include "llvm/ADT/SmallPtrSet.h"
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//===----------------------------------------------------------------------===//
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//
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// DominatorTree construction - This pass constructs immediate dominator
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// information for a flow-graph based on the algorithm described in this
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// document:
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//
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// A Fast Algorithm for Finding Dominators in a Flowgraph
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// T. Lengauer & R. Tarjan, ACM TOPLAS July 1979, pgs 121-141.
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//
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// This implements both the O(n*ack(n)) and the O(n*log(n)) versions of EVAL and
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// LINK, but it turns out that the theoretically slower O(n*log(n))
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// implementation is actually faster than the "efficient" algorithm (even for
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// large CFGs) because the constant overheads are substantially smaller. The
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// lower-complexity version can be enabled with the following #define:
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//
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#define BALANCE_IDOM_TREE 0
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//
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//===----------------------------------------------------------------------===//
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namespace llvm {
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void Compress(DominatorTreeBase& DT, BasicBlock *VIn) {
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std::vector<BasicBlock *> Work;
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SmallPtrSet<BasicBlock *, 32> Visited;
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BasicBlock *VInAncestor = DT.Info[VIn].Ancestor;
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DominatorTreeBase::InfoRec &VInVAInfo = DT.Info[VInAncestor];
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if (VInVAInfo.Ancestor != 0)
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Work.push_back(VIn);
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while (!Work.empty()) {
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BasicBlock *V = Work.back();
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DominatorTree::InfoRec &VInfo = DT.Info[V];
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BasicBlock *VAncestor = VInfo.Ancestor;
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DominatorTreeBase::InfoRec &VAInfo = DT.Info[VAncestor];
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// Process Ancestor first
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if (Visited.insert(VAncestor) &&
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VAInfo.Ancestor != 0) {
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Work.push_back(VAncestor);
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continue;
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}
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Work.pop_back();
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// Update VInfo based on Ancestor info
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if (VAInfo.Ancestor == 0)
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continue;
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BasicBlock *VAncestorLabel = VAInfo.Label;
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BasicBlock *VLabel = VInfo.Label;
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if (DT.Info[VAncestorLabel].Semi < DT.Info[VLabel].Semi)
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VInfo.Label = VAncestorLabel;
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VInfo.Ancestor = VAInfo.Ancestor;
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}
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}
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BasicBlock *Eval(DominatorTreeBase& DT, BasicBlock *V) {
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DominatorTreeBase::InfoRec &VInfo = DT.Info[V];
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#if !BALANCE_IDOM_TREE
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// Higher-complexity but faster implementation
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if (VInfo.Ancestor == 0)
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return V;
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Compress(DT, V);
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return VInfo.Label;
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#else
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// Lower-complexity but slower implementation
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if (VInfo.Ancestor == 0)
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return VInfo.Label;
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Compress(DT, V);
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BasicBlock *VLabel = VInfo.Label;
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BasicBlock *VAncestorLabel = DT.Info[VInfo.Ancestor].Label;
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if (DT.Info[VAncestorLabel].Semi >= DT.Info[VLabel].Semi)
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return VLabel;
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else
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return VAncestorLabel;
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#endif
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}
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void Link(DominatorTreeBase& DT, BasicBlock *V, BasicBlock *W,
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DominatorTreeBase::InfoRec &WInfo) {
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#if !BALANCE_IDOM_TREE
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// Higher-complexity but faster implementation
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WInfo.Ancestor = V;
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#else
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// Lower-complexity but slower implementation
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BasicBlock *WLabel = WInfo.Label;
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unsigned WLabelSemi = DT.Info[WLabel].Semi;
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BasicBlock *S = W;
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InfoRec *SInfo = &DT.Info[S];
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BasicBlock *SChild = SInfo->Child;
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InfoRec *SChildInfo = &DT.Info[SChild];
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while (WLabelSemi < DT.Info[SChildInfo->Label].Semi) {
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BasicBlock *SChildChild = SChildInfo->Child;
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if (SInfo->Size+DT.Info[SChildChild].Size >= 2*SChildInfo->Size) {
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SChildInfo->Ancestor = S;
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SInfo->Child = SChild = SChildChild;
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SChildInfo = &DT.Info[SChild];
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} else {
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SChildInfo->Size = SInfo->Size;
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S = SInfo->Ancestor = SChild;
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SInfo = SChildInfo;
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SChild = SChildChild;
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SChildInfo = &DT.Info[SChild];
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}
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}
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DominatorTreeBase::InfoRec &VInfo = DT.Info[V];
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SInfo->Label = WLabel;
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assert(V != W && "The optimization here will not work in this case!");
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unsigned WSize = WInfo.Size;
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unsigned VSize = (VInfo.Size += WSize);
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if (VSize < 2*WSize)
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std::swap(S, VInfo.Child);
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while (S) {
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SInfo = &DT.Info[S];
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SInfo->Ancestor = V;
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S = SInfo->Child;
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}
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#endif
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}
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}
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