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//===- ListReducer.h - Trim down list while retaining property --*- C++ -*-===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
//
// This class is to be used as a base class for operations that want to zero in
// on a subset of the input which still causes the bug we are tracking.
//
//===----------------------------------------------------------------------===//
#ifndef LLVM_TOOLS_BUGPOINT_LISTREDUCER_H
#define LLVM_TOOLS_BUGPOINT_LISTREDUCER_H
#include "llvm/Support/Error.h"
#include "llvm/Support/raw_ostream.h"
#include <algorithm>
#include <cstdlib>
#include <random>
#include <vector>
namespace llvm {
extern bool BugpointIsInterrupted;
template <typename ElTy> struct ListReducer {
enum TestResult {
NoFailure, // No failure of the predicate was detected
KeepSuffix, // The suffix alone satisfies the predicate
KeepPrefix // The prefix alone satisfies the predicate
};
virtual ~ListReducer() {}
/// This virtual function should be overriden by subclasses to implement the
/// test desired. The testcase is only required to test to see if the Kept
/// list still satisfies the property, but if it is going to check the prefix
/// anyway, it can.
virtual Expected<TestResult> doTest(std::vector<ElTy> &Prefix,
std::vector<ElTy> &Kept) = 0;
/// This function attempts to reduce the length of the specified list while
/// still maintaining the "test" property. This is the core of the "work"
/// that bugpoint does.
Expected<bool> reduceList(std::vector<ElTy> &TheList) {
std::vector<ElTy> empty;
std::mt19937 randomness(0x6e5ea738); // Seed the random number generator
Expected<TestResult> Result = doTest(TheList, empty);
if (Error E = Result.takeError())
return std::move(E);
switch (*Result) {
case KeepPrefix:
if (TheList.size() == 1) // we are done, it's the base case and it fails
return true;
else
break; // there's definitely an error, but we need to narrow it down
case KeepSuffix:
// cannot be reached!
llvm_unreachable("bugpoint ListReducer internal error: "
"selected empty set.");
case NoFailure:
return false; // there is no failure with the full set of passes/funcs!
}
// Maximal number of allowed splitting iterations,
// before the elements are randomly shuffled.
const unsigned MaxIterationsWithoutProgress = 3;
// Maximal number of allowed single-element trim iterations. We add a
// threshold here as single-element reductions may otherwise take a
// very long time to complete.
const unsigned MaxTrimIterationsWithoutBackJump = 3;
bool ShufflingEnabled = true;
Backjump:
unsigned MidTop = TheList.size();
unsigned MaxIterations = MaxIterationsWithoutProgress;
unsigned NumOfIterationsWithoutProgress = 0;
while (MidTop > 1) { // Binary split reduction loop
// Halt if the user presses ctrl-c.
if (BugpointIsInterrupted) {
errs() << "\n\n*** Reduction Interrupted, cleaning up...\n\n";
return true;
}
// If the loop doesn't make satisfying progress, try shuffling.
// The purpose of shuffling is to avoid the heavy tails of the
// distribution (improving the speed of convergence).
if (ShufflingEnabled && NumOfIterationsWithoutProgress > MaxIterations) {
std::vector<ElTy> ShuffledList(TheList);
std::shuffle(ShuffledList.begin(), ShuffledList.end(), randomness);
errs() << "\n\n*** Testing shuffled set...\n\n";
// Check that random shuffle doesn't lose the bug
Expected<TestResult> Result = doTest(ShuffledList, empty);
// TODO: Previously, this error was ignored and we treated it as if
// shuffling hid the bug. This should really either be consumeError if
// that behaviour was sensible, or we should propagate the error.
assert(!Result.takeError() && "Shuffling caused internal error?");
if (*Result == KeepPrefix) {
// If the bug is still here, use the shuffled list.
TheList.swap(ShuffledList);
MidTop = TheList.size();
// Must increase the shuffling treshold to avoid the small
// probability of infinite looping without making progress.
MaxIterations += 2;
errs() << "\n\n*** Shuffling does not hide the bug...\n\n";
} else {
ShufflingEnabled = false; // Disable shuffling further on
errs() << "\n\n*** Shuffling hides the bug...\n\n";
}
NumOfIterationsWithoutProgress = 0;
}
unsigned Mid = MidTop / 2;
std::vector<ElTy> Prefix(TheList.begin(), TheList.begin() + Mid);
std::vector<ElTy> Suffix(TheList.begin() + Mid, TheList.end());
Expected<TestResult> Result = doTest(Prefix, Suffix);
if (Error E = Result.takeError())
return std::move(E);
switch (*Result) {
case KeepSuffix:
// The property still holds. We can just drop the prefix elements, and
// shorten the list to the "kept" elements.
TheList.swap(Suffix);
MidTop = TheList.size();
// Reset progress treshold and progress counter
MaxIterations = MaxIterationsWithoutProgress;
NumOfIterationsWithoutProgress = 0;
break;
case KeepPrefix:
// The predicate still holds, shorten the list to the prefix elements.
TheList.swap(Prefix);
MidTop = TheList.size();
// Reset progress treshold and progress counter
MaxIterations = MaxIterationsWithoutProgress;
NumOfIterationsWithoutProgress = 0;
break;
case NoFailure:
// Otherwise the property doesn't hold. Some of the elements we removed
// must be necessary to maintain the property.
MidTop = Mid;
NumOfIterationsWithoutProgress++;
break;
}
}
// Probability of backjumping from the trimming loop back to the binary
// split reduction loop.
const int BackjumpProbability = 10;
// Okay, we trimmed as much off the top and the bottom of the list as we
// could. If there is more than two elements in the list, try deleting
// interior elements and testing that.
//
if (TheList.size() > 2) {
bool Changed = true;
std::vector<ElTy> EmptyList;
unsigned TrimIterations = 0;
while (Changed) { // Trimming loop.
Changed = false;
// If the binary split reduction loop made an unfortunate sequence of
// splits, the trimming loop might be left off with a huge number of
// remaining elements (large search space). Backjumping out of that
// search space and attempting a different split can significantly
// improve the convergence speed.
if (std::rand() % 100 < BackjumpProbability)
goto Backjump;
for (unsigned i = 1; i < TheList.size() - 1; ++i) {
// Check interior elts
if (BugpointIsInterrupted) {
errs() << "\n\n*** Reduction Interrupted, cleaning up...\n\n";
return true;
}
std::vector<ElTy> TestList(TheList);
TestList.erase(TestList.begin() + i);
Expected<TestResult> Result = doTest(EmptyList, TestList);
if (Error E = Result.takeError())
return std::move(E);
if (*Result == KeepSuffix) {
// We can trim down the list!
TheList.swap(TestList);
--i; // Don't skip an element of the list
Changed = true;
}
}
if (TrimIterations >= MaxTrimIterationsWithoutBackJump)
break;
TrimIterations++;
}
}
return true; // there are some failure and we've narrowed them down
}
};
} // End llvm namespace
#endif
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