ListReducer.h revision 126840f49e8d49156a342e836d4b2adca46dc3ba 
1//===- ListReducer.h - Trim down list while retaining property --*- C++ -*-===//
2//
3// This class is to be used as a base class for operations that want to zero in
4// on a subset of the input which still causes the bug we are tracking.
5//
6//===----------------------------------------------------------------------===//
7
8#ifndef BUGPOINT_LIST_REDUCER_H
9#define BUGPOINT_LIST_REDUCER_H
10
11#include <vector>
12
13template<typename ElTy>
14struct ListReducer {
15  enum TestResult {
16    NoFailure,         // No failure of the predicate was detected
17    KeepSuffix,        // The suffix alone satisfies the predicate
18    KeepPrefix,        // The prefix alone satisfies the predicate
19  };
20
21  // doTest - This virtual function should be overriden by subclasses to
22  // implement the test desired.  The testcase is only required to test to see
23  // if the Kept list still satisfies the property, but if it is going to check
24  // the prefix anyway, it can.
25  //
26  virtual TestResult doTest(const std::vector<ElTy> &Prefix,
27                            const std::vector<ElTy> &Kept) = 0;
28
29  // reduceList - This function attempts to reduce the length of the specified
30  // list while still maintaining the "test" property.  This is the core of the
31  // "work" that bugpoint does.
32  //
33  void reduceList(std::vector<ElTy> &TheList) {
34    unsigned MidTop = TheList.size();
35    while (MidTop > 1) {
36      unsigned Mid = MidTop / 2;
37      std::vector<ElTy> Prefix(TheList.begin()+Mid, TheList.end());
38      std::vector<ElTy> Kept  (TheList.begin(), TheList.begin()+Mid);
39
40      switch (doTest(Prefix, Kept)) {
41      case KeepSuffix:
42        // The property still holds.  We can just drop the prefix elements, and
43        // shorten the list to the "kept" elements.
44        TheList.swap(Kept);
45        MidTop = TheList.size();
46        break;
47      case KeepPrefix:
48        // The predicate still holds, shorten the list to the prefix elements.
49        TheList.swap(Prefix);
50        MidTop = TheList.size();
51        break;
52      case NoFailure:
53        // Otherwise the property doesn't hold.  Some of the elements we removed
54        // must be neccesary to maintain the property.
55        MidTop = Mid;
56        break;
57      }
58    }
59
60    // Okay, we trimmed as much off the top and the bottom of the list as we
61    // could.  If there is more two elements in the list, try deleting interior
62    // elements and testing that.
63    //
64    if (TheList.size() > 2) {
65      bool Changed = true;
66      std::vector<ElTy> EmptyList;
67      while (Changed) {
68        Changed = false;
69        std::vector<ElTy> TrimmedList;
70        for (unsigned i = 1; i < TheList.size()-1; ++i) { // Check interior elts
71          std::vector<ElTy> TestList(TheList);
72          TestList.erase(TestList.begin()+i);
73
74          if (doTest(EmptyList, TestList) == KeepSuffix) {
75            // We can trim down the list!
76            TheList.swap(TestList);
77            --i;  // Don't skip an element of the list
78            Changed = true;
79          }
80        }
81      }
82    }
83  }
84};
85
86#endif
87