1 //===- CFGReachabilityAnalysis.cpp - Basic reachability analysis ----------===//
2 //
3 //                     The LLVM Compiler Infrastructure
4 //
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
7 //
8 //===----------------------------------------------------------------------===//
9 //
10 // This file defines a flow-sensitive, (mostly) path-insensitive reachability
11 // analysis based on Clang's CFGs.  Clients can query if a given basic block
12 // is reachable within the CFG.
13 //
14 //===----------------------------------------------------------------------===//
15 
16 #include "clang/Analysis/Analyses/CFGReachabilityAnalysis.h"
17 #include "clang/Analysis/CFG.h"
18 #include "llvm/ADT/BitVector.h"
19 #include "llvm/ADT/SmallVector.h"
20 
21 using namespace clang;
22 
23 CFGReverseBlockReachabilityAnalysis::CFGReverseBlockReachabilityAnalysis(
24     const CFG &cfg)
25     : analyzed(cfg.getNumBlockIDs(), false) {}
26 
27 bool CFGReverseBlockReachabilityAnalysis::isReachable(const CFGBlock *Src,
28                                           const CFGBlock *Dst) {
29   const unsigned DstBlockID = Dst->getBlockID();
30 
31   // If we haven't analyzed the destination node, run the analysis now
32   if (!analyzed[DstBlockID]) {
33     mapReachability(Dst);
34     analyzed[DstBlockID] = true;
35   }
36 
37   // Return the cached result
38   return reachable[DstBlockID][Src->getBlockID()];
39 }
40 
41 // Maps reachability to a common node by walking the predecessors of the
42 // destination node.
43 void CFGReverseBlockReachabilityAnalysis::mapReachability(const CFGBlock *Dst) {
44   SmallVector<const CFGBlock *, 11> worklist;
45   llvm::BitVector visited(analyzed.size());
46 
47   ReachableSet &DstReachability = reachable[Dst->getBlockID()];
48   DstReachability.resize(analyzed.size(), false);
49 
50   // Start searching from the destination node, since we commonly will perform
51   // multiple queries relating to a destination node.
52   worklist.push_back(Dst);
53   bool firstRun = true;
54 
55   while (!worklist.empty()) {
56     const CFGBlock *block = worklist.pop_back_val();
57 
58     if (visited[block->getBlockID()])
59       continue;
60     visited[block->getBlockID()] = true;
61 
62     // Update reachability information for this node -> Dst
63     if (!firstRun) {
64       // Don't insert Dst -> Dst unless it was a predecessor of itself
65       DstReachability[block->getBlockID()] = true;
66     }
67     else
68       firstRun = false;
69 
70     // Add the predecessors to the worklist.
71     for (CFGBlock::const_pred_iterator i = block->pred_begin(),
72          e = block->pred_end(); i != e; ++i) {
73       if (*i)
74         worklist.push_back(*i);
75     }
76   }
77 }
78