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1 //===- ListReducer.h - Trim down list while retaining property --*- C++ -*-===//
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 class is to be used as a base class for operations that want to zero in
11 // on a subset of the input which still causes the bug we are tracking.
12 //
13 //===----------------------------------------------------------------------===//
14 
15 #ifndef LLVM_TOOLS_BUGPOINT_LISTREDUCER_H
16 #define LLVM_TOOLS_BUGPOINT_LISTREDUCER_H
17 
18 #include "llvm/Support/ErrorHandling.h"
19 #include "llvm/Support/raw_ostream.h"
20 #include <algorithm>
21 #include <cstdlib>
22 #include <vector>
23 
24 namespace llvm {
25 
26   extern bool BugpointIsInterrupted;
27 
28 template<typename ElTy>
29 struct ListReducer {
30   enum TestResult {
31     NoFailure,         // No failure of the predicate was detected
32     KeepSuffix,        // The suffix alone satisfies the predicate
33     KeepPrefix,        // The prefix alone satisfies the predicate
34     InternalError      // Encountered an error trying to run the predicate
35   };
36 
~ListReducerListReducer37   virtual ~ListReducer() {}
38 
39   // doTest - This virtual function should be overriden by subclasses to
40   // implement the test desired.  The testcase is only required to test to see
41   // if the Kept list still satisfies the property, but if it is going to check
42   // the prefix anyway, it can.
43   //
44   virtual TestResult doTest(std::vector<ElTy> &Prefix,
45                             std::vector<ElTy> &Kept,
46                             std::string &Error) = 0;
47 
48   // reduceList - This function attempts to reduce the length of the specified
49   // list while still maintaining the "test" property.  This is the core of the
50   // "work" that bugpoint does.
51   //
reduceListListReducer52   bool reduceList(std::vector<ElTy> &TheList, std::string &Error) {
53     std::vector<ElTy> empty;
54     std::srand(0x6e5ea738); // Seed the random number generator
55     switch (doTest(TheList, empty, Error)) {
56     case KeepPrefix:
57       if (TheList.size() == 1) // we are done, it's the base case and it fails
58         return true;
59       else
60         break; // there's definitely an error, but we need to narrow it down
61 
62     case KeepSuffix:
63       // cannot be reached!
64       llvm_unreachable("bugpoint ListReducer internal error: "
65                        "selected empty set.");
66 
67     case NoFailure:
68       return false; // there is no failure with the full set of passes/funcs!
69 
70     case InternalError:
71       assert(!Error.empty());
72       return true;
73     }
74 
75     // Maximal number of allowed splitting iterations,
76     // before the elements are randomly shuffled.
77     const unsigned MaxIterationsWithoutProgress = 3;
78 
79     // Maximal number of allowed single-element trim iterations. We add a
80     // threshhold here as single-element reductions may otherwise take a
81     // very long time to complete.
82     const unsigned MaxTrimIterationsWithoutBackJump = 3;
83     bool ShufflingEnabled = true;
84 
85 Backjump:
86     unsigned MidTop = TheList.size();
87     unsigned MaxIterations = MaxIterationsWithoutProgress;
88     unsigned NumOfIterationsWithoutProgress = 0;
89     while (MidTop > 1) { // Binary split reduction loop
90       // Halt if the user presses ctrl-c.
91       if (BugpointIsInterrupted) {
92         errs() << "\n\n*** Reduction Interrupted, cleaning up...\n\n";
93         return true;
94       }
95 
96       // If the loop doesn't make satisfying progress, try shuffling.
97       // The purpose of shuffling is to avoid the heavy tails of the
98       // distribution (improving the speed of convergence).
99       if (ShufflingEnabled &&
100           NumOfIterationsWithoutProgress > MaxIterations) {
101         std::vector<ElTy> ShuffledList(TheList);
102         std::random_shuffle(ShuffledList.begin(), ShuffledList.end());
103         errs() << "\n\n*** Testing shuffled set...\n\n";
104         // Check that random shuffle doesn't loose the bug
105         if (doTest(ShuffledList, empty, Error) == KeepPrefix) {
106           // If the bug is still here, use the shuffled list.
107           TheList.swap(ShuffledList);
108           MidTop = TheList.size();
109           // Must increase the shuffling treshold to avoid the small
110           // probability of inifinite looping without making progress.
111           MaxIterations += 2;
112           errs() << "\n\n*** Shuffling does not hide the bug...\n\n";
113         } else {
114           ShufflingEnabled = false; // Disable shuffling further on
115           errs() << "\n\n*** Shuffling hides the bug...\n\n";
116         }
117         NumOfIterationsWithoutProgress = 0;
118       }
119 
120       unsigned Mid = MidTop / 2;
121       std::vector<ElTy> Prefix(TheList.begin(), TheList.begin()+Mid);
122       std::vector<ElTy> Suffix(TheList.begin()+Mid, TheList.end());
123 
124       switch (doTest(Prefix, Suffix, Error)) {
125       case KeepSuffix:
126         // The property still holds.  We can just drop the prefix elements, and
127         // shorten the list to the "kept" elements.
128         TheList.swap(Suffix);
129         MidTop = TheList.size();
130         // Reset progress treshold and progress counter
131         MaxIterations = MaxIterationsWithoutProgress;
132         NumOfIterationsWithoutProgress = 0;
133         break;
134       case KeepPrefix:
135         // The predicate still holds, shorten the list to the prefix elements.
136         TheList.swap(Prefix);
137         MidTop = TheList.size();
138         // Reset progress treshold and progress counter
139         MaxIterations = MaxIterationsWithoutProgress;
140         NumOfIterationsWithoutProgress = 0;
141         break;
142       case NoFailure:
143         // Otherwise the property doesn't hold.  Some of the elements we removed
144         // must be necessary to maintain the property.
145         MidTop = Mid;
146         NumOfIterationsWithoutProgress++;
147         break;
148       case InternalError:
149         return true;  // Error was set by doTest.
150       }
151       assert(Error.empty() && "doTest did not return InternalError for error");
152     }
153 
154     // Probability of backjumping from the trimming loop back to the binary
155     // split reduction loop.
156     const int BackjumpProbability = 10;
157 
158     // Okay, we trimmed as much off the top and the bottom of the list as we
159     // could.  If there is more than two elements in the list, try deleting
160     // interior elements and testing that.
161     //
162     if (TheList.size() > 2) {
163       bool Changed = true;
164       std::vector<ElTy> EmptyList;
165       unsigned TrimIterations = 0;
166       while (Changed) {  // Trimming loop.
167         Changed = false;
168 
169         // If the binary split reduction loop made an unfortunate sequence of
170         // splits, the trimming loop might be left off with a huge number of
171         // remaining elements (large search space). Backjumping out of that
172         // search space and attempting a different split can significantly
173         // improve the convergence speed.
174         if (std::rand() % 100 < BackjumpProbability)
175           goto Backjump;
176 
177         for (unsigned i = 1; i < TheList.size()-1; ++i) { // Check interior elts
178           if (BugpointIsInterrupted) {
179             errs() << "\n\n*** Reduction Interrupted, cleaning up...\n\n";
180             return true;
181           }
182 
183           std::vector<ElTy> TestList(TheList);
184           TestList.erase(TestList.begin()+i);
185 
186           if (doTest(EmptyList, TestList, Error) == KeepSuffix) {
187             // We can trim down the list!
188             TheList.swap(TestList);
189             --i;  // Don't skip an element of the list
190             Changed = true;
191           }
192           if (!Error.empty())
193             return true;
194         }
195         if (TrimIterations >= MaxTrimIterationsWithoutBackJump)
196           break;
197         TrimIterations++;
198       }
199     }
200 
201     return true; // there are some failure and we've narrowed them down
202   }
203 };
204 
205 } // End llvm namespace
206 
207 #endif
208