DebugCounter.h

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//===- llvm/Support/DebugCounter.h - Debug counter support ------*- 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
//
//===----------------------------------------------------------------------===//
/// \file
/// This file provides an implementation of debug counters.  Debug
/// counters are a tool that let you narrow down a miscompilation to a specific
/// thing happening.
///
/// To give a use case: Imagine you have a file, very large, and you
/// are trying to understand the minimal transformation that breaks it. Bugpoint
/// and bisection is often helpful here in narrowing it down to a specific pass,
/// but it's still a very large file, and a very complicated pass to try to
/// debug.  That is where debug counting steps in.  You can instrument the pass
/// with a debug counter before it does a certain thing, and depending on the
/// counts, it will either execute that thing or not.  The debug counter itself
/// consists of a skip and a count.  Skip is the number of times shouldExecute
/// needs to be called before it returns true.  Count is the number of times to
/// return true once Skip is 0.  So a skip=47, count=2 ,would skip the first 47
/// executions by returning false from shouldExecute, then execute twice, and
/// then return false again.
/// Note that a counter set to a negative number will always execute.
/// For a concrete example, during predicateinfo creation, the renaming pass
/// replaces each use with a renamed use.
////
/// If I use DEBUG_COUNTER to create a counter called "predicateinfo", and
/// variable name RenameCounter, and then instrument this renaming with a debug
/// counter, like so:
///
/// if (!DebugCounter::shouldExecute(RenameCounter)
/// <continue or return or whatever not executing looks like>
///
/// Now I can, from the command line, make it rename or not rename certain uses
/// by setting the skip and count.
/// So for example
/// bin/opt -debug-counter=predicateinfo-skip=47,predicateinfo-count=1
/// will skip renaming the first 47 uses, then rename one, then skip the rest.
//===----------------------------------------------------------------------===//
 
#ifndef LLVM_SUPPORT_DEBUGCOUNTER_H
#define LLVM_SUPPORT_DEBUGCOUNTER_H
 
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/StringRef.h"
#include "llvm/ADT/UniqueVector.h"
#include "llvm/Support/Debug.h"
#include <string>
 
namespace llvm {
 
class raw_ostream;
 
class DebugCounter {
public:
  ~DebugCounter();
 
  /// Returns a reference to the singleton instance.
  static DebugCounter &instance();
 
  // Used by the command line option parser to push a new value it parsed.
  void push_back(const std::string &);
 
  // Register a counter with the specified name.
  //
  // FIXME: Currently, counter registration is required to happen before command
  // line option parsing. The main reason to register counters is to produce a
  // nice list of them on the command line, but i'm not sure this is worth it.
  static unsigned registerCounter(StringRef Name, StringRef Desc) {
    return instance().addCounter(std::string(Name), std::string(Desc));
  }
  inline static bool shouldExecute(unsigned CounterName) {
    if (!isCountingEnabled())
      return true;
 
    auto &Us = instance();
    auto Result = Us.Counters.find(CounterName);
    if (Result != Us.Counters.end()) {
      auto &CounterInfo = Result->second;
      ++CounterInfo.Count;
 
      // We only execute while the Skip is not smaller than Count,
      // and the StopAfter + Skip is larger than Count.
      // Negative counters always execute.
      if (CounterInfo.Skip < 0)
        return true;
      if (CounterInfo.Skip >= CounterInfo.Count)
        return false;
      if (CounterInfo.StopAfter < 0)
        return true;
      return CounterInfo.StopAfter + CounterInfo.Skip >= CounterInfo.Count;
    }
    // Didn't find the counter, should we warn?
    return true;
  }
 
  // Return true if a given counter had values set (either programatically or on
  // the command line).  This will return true even if those values are
  // currently in a state where the counter will always execute.
  static bool isCounterSet(unsigned ID) {
    return instance().Counters[ID].IsSet;
  }
 
  // Return the Count for a counter. This only works for set counters.
  static int64_t getCounterValue(unsigned ID) {
    auto &Us = instance();
    auto Result = Us.Counters.find(ID);
    assert(Result != Us.Counters.end() && "Asking about a non-set counter");
    return Result->second.Count;
  }
 
  // Set a registered counter to a given Count value.
  static void setCounterValue(unsigned ID, int64_t Count) {
    auto &Us = instance();
    Us.Counters[ID].Count = Count;
  }
 
  // Dump or print the current counter set into llvm::dbgs().
  LLVM_DUMP_METHOD void dump() const;
 
  void print(raw_ostream &OS) const;
 
  // Get the counter ID for a given named counter, or return 0 if none is found.
  unsigned getCounterId(const std::string &Name) const {
    return RegisteredCounters.idFor(Name);
  }
 
  // Return the number of registered counters.
  unsigned int getNumCounters() const { return RegisteredCounters.size(); }
 
  // Return the name and description of the counter with the given ID.
  std::pair<std::string, std::string> getCounterInfo(unsigned ID) const {
    return std::make_pair(RegisteredCounters[ID], Counters.lookup(ID).Desc);
  }
 
  // Iterate through the registered counters
  typedef UniqueVector<std::string> CounterVector;
  CounterVector::const_iterator begin() const {
    return RegisteredCounters.begin();
  }
  CounterVector::const_iterator end() const { return RegisteredCounters.end(); }
 
  // Force-enables counting all DebugCounters.
  //
  // Since DebugCounters are incompatible with threading (not only do they not
  // make sense, but we'll also see data races), this should only be used in
  // contexts where we're certain we won't spawn threads.
  static void enableAllCounters() { instance().Enabled = true; }
 
private:
  static bool isCountingEnabled() {
// Compile to nothing when debugging is off
#ifdef NDEBUG
    return false;
#else
    return instance().Enabled;
#endif
  }
 
  unsigned addCounter(const std::string &Name, const std::string &Desc) {
    unsigned Result = RegisteredCounters.insert(Name);
    Counters[Result] = {};
    Counters[Result].Desc = Desc;
    return Result;
  }
  // Struct to store counter info.
  struct CounterInfo {
    int64_t Count = 0;
    int64_t Skip = 0;
    int64_t StopAfter = -1;
    bool IsSet = false;
    std::string Desc;
  };
  DenseMap<unsigned, CounterInfo> Counters;
  CounterVector RegisteredCounters;
 
  // Whether we should do DebugCounting at all. DebugCounters aren't
  // thread-safe, so this should always be false in multithreaded scenarios.
  bool Enabled = false;
};
 
#define DEBUG_COUNTER(VARNAME, COUNTERNAME, DESC)                              \
  static const unsigned VARNAME =                                              \
      DebugCounter::registerCounter(COUNTERNAME, DESC)
 
} // namespace llvm
#endif