tools/bugpoint/ListReducer.h

   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/Error.h"
  19 #include "llvm/Support/raw_ostream.h"
  20 #include <algorithm>
  21 #include <cstdlib>
  22 #include <random>
  23 #include <vector>
  24
  25 namespace llvm {
  26
  27 extern bool BugpointIsInterrupted;
  28
  29 template <typename ElTy> 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   };
  35
  36   virtual ~ListReducer() {}
  37
  38   /// This virtual function should be overriden by subclasses to implement the
  39   /// test desired.  The testcase is only required to test to see if the Kept
  40   /// list still satisfies the property, but if it is going to check the prefix
  41   /// anyway, it can.
  42   virtual Expected<TestResult> doTest(std::vector<ElTy> &Prefix,
  43                                       std::vector<ElTy> &Kept) = 0;
  44
  45   /// This function attempts to reduce the length of the specified list while
  46   /// still maintaining the "test" property.  This is the core of the "work"
  47   /// that bugpoint does.
  48   Expected<bool> reduceList(std::vector<ElTy> &TheList) {
  49     std::vector<ElTy> empty;
  50     std::mt19937 randomness(0x6e5ea738);  // Seed the random number generator
  51     Expected<TestResult> Result = doTest(TheList, empty);
  52     if (Error E = Result.takeError())
  53       return std::move(E);
  54     switch (*Result) {
  55     case KeepPrefix:
  56       if (TheList.size() == 1) // we are done, it's the base case and it fails
  57         return true;
  58       else
  59         break; // there's definitely an error, but we need to narrow it down
  60
  61     case KeepSuffix:
  62       // cannot be reached!
  63       llvm_unreachable("bugpoint ListReducer internal error: "
  64                        "selected empty set.");
  65
  66     case NoFailure:
  67       return false; // there is no failure with the full set of passes/funcs!
  68     }
  69
  70     // Maximal number of allowed splitting iterations,
  71     // before the elements are randomly shuffled.
  72     const unsigned MaxIterationsWithoutProgress = 3;
  73
  74     // Maximal number of allowed single-element trim iterations. We add a
  75     // threshold here as single-element reductions may otherwise take a
  76     // very long time to complete.
  77     const unsigned MaxTrimIterationsWithoutBackJump = 3;
  78     bool ShufflingEnabled = true;
  79
  80   Backjump:
  81     unsigned MidTop = TheList.size();
  82     unsigned MaxIterations = MaxIterationsWithoutProgress;
  83     unsigned NumOfIterationsWithoutProgress = 0;
  84     while (MidTop > 1) { // Binary split reduction loop
  85       // Halt if the user presses ctrl-c.
  86       if (BugpointIsInterrupted) {
  87         errs() << "\n\n*** Reduction Interrupted, cleaning up...\n\n";
  88         return true;
  89       }
  90
  91       // If the loop doesn't make satisfying progress, try shuffling.
  92       // The purpose of shuffling is to avoid the heavy tails of the
  93       // distribution (improving the speed of convergence).
  94       if (ShufflingEnabled && NumOfIterationsWithoutProgress > MaxIterations) {
  95         std::vector<ElTy> ShuffledList(TheList);
  96         std::shuffle(ShuffledList.begin(), ShuffledList.end(), randomness);
  97         errs() << "\n\n*** Testing shuffled set...\n\n";
  98         // Check that random shuffle doesn't lose the bug
  99         Expected<TestResult> Result = doTest(ShuffledList, empty);
 100         // TODO: Previously, this error was ignored and we treated it as if
 101         // shuffling hid the bug. This should really either be consumeError if
 102         // that behaviour was sensible, or we should propagate the error.
 103         assert(!Result.takeError() && "Shuffling caused internal error?");
 104
 105         if (*Result == 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 infinite 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       Expected<TestResult> Result = doTest(Prefix, Suffix);
 125       if (Error E = Result.takeError())
 126         return std::move(E);
 127       switch (*Result) {
 128       case KeepSuffix:
 129         // The property still holds.  We can just drop the prefix elements, and
 130         // shorten the list to the "kept" elements.
 131         TheList.swap(Suffix);
 132         MidTop = TheList.size();
 133         // Reset progress treshold and progress counter
 134         MaxIterations = MaxIterationsWithoutProgress;
 135         NumOfIterationsWithoutProgress = 0;
 136         break;
 137       case KeepPrefix:
 138         // The predicate still holds, shorten the list to the prefix elements.
 139         TheList.swap(Prefix);
 140         MidTop = TheList.size();
 141         // Reset progress treshold and progress counter
 142         MaxIterations = MaxIterationsWithoutProgress;
 143         NumOfIterationsWithoutProgress = 0;
 144         break;
 145       case NoFailure:
 146         // Otherwise the property doesn't hold.  Some of the elements we removed
 147         // must be necessary to maintain the property.
 148         MidTop = Mid;
 149         NumOfIterationsWithoutProgress++;
 150         break;
 151       }
 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) {
 178           // Check interior elts
 179           if (BugpointIsInterrupted) {
 180             errs() << "\n\n*** Reduction Interrupted, cleaning up...\n\n";
 181             return true;
 182           }
 183
 184           std::vector<ElTy> TestList(TheList);
 185           TestList.erase(TestList.begin() + i);
 186
 187           Expected<TestResult> Result = doTest(EmptyList, TestList);
 188           if (Error E = Result.takeError())
 189             return std::move(E);
 190           if (*Result == KeepSuffix) {
 191             // We can trim down the list!
 192             TheList.swap(TestList);
 193             --i; // Don't skip an element of the list
 194             Changed = true;
 195           }
 196         }
 197         if (TrimIterations >= MaxTrimIterationsWithoutBackJump)
 198           break;
 199         TrimIterations++;
 200       }
 201     }
 202
 203     return true; // there are some failure and we've narrowed them down
 204   }
 205 };
 206
 207 } // End llvm namespace
 208
 209 #endif