tools/llvm-dwarfdump/Statistics.cpp

#include "llvm/ADT/DenseMap.h"
#include "llvm/DebugInfo/DIContext.h"
#include "llvm/DebugInfo/DWARF/DWARFContext.h"
#include "llvm/DebugInfo/DWARF/DWARFDebugLoc.h"
#include "llvm/Object/ObjectFile.h"

#define DEBUG_TYPE "dwarfdump"
using namespace llvm;
using namespace object;

/// Holds statistics for one function (or other entity that has a PC range and
/// contains variables, such as a compile unit).
struct PerFunctionStats {
  /// Number of inlined instances of this function.
  unsigned NumFnInlined = 0;
  /// Number of variables with location across all inlined instances.
  unsigned TotalVarWithLoc = 0;
  /// Number of constants with location across all inlined instances.
  unsigned ConstantMembers = 0;
  /// List of all Variables in this function.
  SmallDenseSet<uint32_t, 4> VarsInFunction;
  /// Compile units also cover a PC range, but have this flag set to false.
  bool IsFunction = false;
};

/// Holds accumulated global statistics about local variables.
struct GlobalStats {
  /// Total number of PC range bytes covered by DW_AT_locations.
  unsigned ScopeBytesCovered = 0;
  /// Total number of PC range bytes in each variable's enclosing scope,
  /// starting from the first definition of the variable.
  unsigned ScopeBytesFromFirstDefinition = 0;
};

/// Extract the low pc from a Die.
static uint64_t getLowPC(DWARFDie Die) {
  auto RangesOrError = Die.getAddressRanges();
  DWARFAddressRangesVector Ranges;
  if (RangesOrError)
    Ranges = RangesOrError.get();
  else
    llvm::consumeError(RangesOrError.takeError());
  if (Ranges.size())
    return Ranges[0].LowPC;
  return dwarf::toAddress(Die.find(dwarf::DW_AT_low_pc), 0);
}

/// Collect debug info quality metrics for one DIE.
static void collectStatsForDie(DWARFDie Die, std::string Prefix,
                               uint64_t ScopeLowPC, uint64_t BytesInScope,
                               StringMap<PerFunctionStats> &FnStatMap,
                               GlobalStats &GlobalStats) {
  bool HasLoc = false;
  uint64_t BytesCovered = 0;
  uint64_t OffsetToFirstDefinition = 0;
  if (Die.find(dwarf::DW_AT_const_value)) {
    // This catches constant members *and* variables.
    HasLoc = true;
    BytesCovered = BytesInScope;
  } else if (Die.getTag() == dwarf::DW_TAG_variable ||
             Die.getTag() == dwarf::DW_TAG_formal_parameter) {
    // Handle variables and function arguments.
    auto FormValue = Die.find(dwarf::DW_AT_location);
    HasLoc = FormValue.hasValue();
    if (HasLoc) {
      // Get PC coverage.
      if (auto DebugLocOffset = FormValue->getAsSectionOffset()) {
        auto *DebugLoc = Die.getDwarfUnit()->getContext().getDebugLoc();
        if (auto List = DebugLoc->getLocationListAtOffset(*DebugLocOffset)) {
          for (auto Entry : List->Entries)
            BytesCovered += Entry.End - Entry.Begin;
          if (List->Entries.size()) {
            uint64_t FirstDef = List->Entries[0].Begin;
            uint64_t UnitOfs = getLowPC(Die.getDwarfUnit()->getUnitDIE());
            // Ranges sometimes start before the lexical scope.
            if (UnitOfs + FirstDef >= ScopeLowPC)
              OffsetToFirstDefinition = UnitOfs + FirstDef - ScopeLowPC;
            // Or even after it. Count that as a failure.
            if (OffsetToFirstDefinition > BytesInScope)
              OffsetToFirstDefinition = 0;
          }
        }
        assert(BytesInScope);
      } else {
        // Assume the entire range is covered by a single location.
        BytesCovered = BytesInScope;
      }
    }
  } else {
    // Not a variable or constant member.
    return;
  }

  // Collect PC range coverage data.
  auto &FnStats = FnStatMap[Prefix];
  if (DWARFDie D =
          Die.getAttributeValueAsReferencedDie(dwarf::DW_AT_abstract_origin))
    Die = D;
  // This is a unique ID for the variable inside the current object file.
  unsigned CanonicalDieOffset = Die.getOffset();
  FnStats.VarsInFunction.insert(CanonicalDieOffset);
  if (BytesInScope) {
    FnStats.TotalVarWithLoc += (unsigned)HasLoc;
    // Adjust for the fact the variables often start their lifetime in the
    // middle of the scope.
    BytesInScope -= OffsetToFirstDefinition;
    // Turns out we have a lot of ranges that extend past the lexical scope.
    GlobalStats.ScopeBytesCovered += std::min(BytesInScope, BytesCovered);
    GlobalStats.ScopeBytesFromFirstDefinition += BytesInScope;
    assert(GlobalStats.ScopeBytesCovered <=
           GlobalStats.ScopeBytesFromFirstDefinition);
  } else {
    FnStats.ConstantMembers++;
  }
}

/// Recursively collect debug info quality metrics.
static void collectStatsRecursive(DWARFDie Die, std::string Prefix,
                                  uint64_t ScopeLowPC, uint64_t BytesInScope,
                                  StringMap<PerFunctionStats> &FnStatMap,
                                  GlobalStats &GlobalStats) {
  // Handle any kind of lexical scope.
  if (Die.getTag() == dwarf::DW_TAG_subprogram ||
      Die.getTag() == dwarf::DW_TAG_inlined_subroutine ||
      Die.getTag() == dwarf::DW_TAG_lexical_block) {
    // Ignore forward declarations.
    if (Die.find(dwarf::DW_AT_declaration))
      return;

    // Count the function.
    if (Die.getTag() != dwarf::DW_TAG_lexical_block) {
      StringRef Name = Die.getName(DINameKind::LinkageName);
      if (Name.empty())
        Name = Die.getName(DINameKind::ShortName);
      Prefix = Name;
      // Skip over abstract origins.
      if (Die.find(dwarf::DW_AT_inline))
        return;
      // We've seen an (inlined) instance of this function.
      auto &FnStats = FnStatMap[Name];
      FnStats.NumFnInlined++;
      FnStats.IsFunction = true;
    }

    // PC Ranges.
    auto RangesOrError = Die.getAddressRanges();
    if (!RangesOrError) {
      llvm::consumeError(RangesOrError.takeError());
      return;
    }

    auto Ranges = RangesOrError.get();
    uint64_t BytesInThisScope = 0;
    for (auto Range : Ranges)
      BytesInThisScope += Range.HighPC - Range.LowPC;
    ScopeLowPC = getLowPC(Die);

    if (BytesInThisScope)
      BytesInScope = BytesInThisScope;
  } else {
    // Not a scope, visit the Die itself. It could be a variable.
    collectStatsForDie(Die, Prefix, ScopeLowPC, BytesInScope, FnStatMap,
                       GlobalStats);
  }

  // Traverse children.
  DWARFDie Child = Die.getFirstChild();
  while (Child) {
    collectStatsRecursive(Child, Prefix, ScopeLowPC, BytesInScope, FnStatMap,
                          GlobalStats);
    Child = Child.getSibling();
  }
}

/// Print machine-readable output.
/// The machine-readable format is single-line JSON output.
/// \{
static void printDatum(raw_ostream &OS, const char *Key, StringRef Value) {
  OS << ",\"" << Key << "\":\"" << Value << '"';
  LLVM_DEBUG(llvm::dbgs() << Key << ": " << Value << '\n');
}
static void printDatum(raw_ostream &OS, const char *Key, uint64_t Value) {
  OS << ",\"" << Key << "\":" << Value;
  LLVM_DEBUG(llvm::dbgs() << Key << ": " << Value << '\n');
}
/// \}

/// Collect debug info quality metrics for an entire DIContext.
///
/// Do the impossible and reduce the quality of the debug info down to a few
/// numbers. The idea is to condense the data into numbers that can be tracked
/// over time to identify trends in newer compiler versions and gauge the effect
/// of particular optimizations. The raw numbers themselves are not particularly
/// useful, only the delta between compiling the same program with different
/// compilers is.
bool collectStatsForObjectFile(ObjectFile &Obj, DWARFContext &DICtx,
                               Twine Filename, raw_ostream &OS) {
  StringRef FormatName = Obj.getFileFormatName();
  GlobalStats GlobalStats;
  StringMap<PerFunctionStats> Statistics;
  for (const auto &CU : static_cast<DWARFContext *>(&DICtx)->compile_units())
    if (DWARFDie CUDie = CU->getUnitDIE(false))
      collectStatsRecursive(CUDie, "/", 0, 0, Statistics, GlobalStats);

  /// The version number should be increased every time the algorithm is changed
  /// (including bug fixes). New metrics may be added without increasing the
  /// version.
  unsigned Version = 1;
  unsigned VarTotal = 0;
  unsigned VarUnique = 0;
  unsigned VarWithLoc = 0;
  unsigned NumFunctions = 0;
  unsigned NumInlinedFunctions = 0;
  for (auto &Entry : Statistics) {
    PerFunctionStats &Stats = Entry.getValue();
    unsigned TotalVars = Stats.VarsInFunction.size() * Stats.NumFnInlined;
    unsigned Constants = Stats.ConstantMembers;
    VarWithLoc += Stats.TotalVarWithLoc + Constants;
    VarTotal += TotalVars + Constants;
    VarUnique += Stats.VarsInFunction.size();
    LLVM_DEBUG(for (auto V
                    : Stats.VarsInFunction) llvm::dbgs()
               << Entry.getKey() << ": " << V << "\n");
    NumFunctions += Stats.IsFunction;
    NumInlinedFunctions += Stats.IsFunction * Stats.NumFnInlined;
  }

  // Print summary.
  OS.SetBufferSize(1024);
  OS << "{\"version\":\"" << Version << '"';
  LLVM_DEBUG(llvm::dbgs() << "Variable location quality metrics\n";
             llvm::dbgs() << "---------------------------------\n");
  printDatum(OS, "file", Filename.str());
  printDatum(OS, "format", FormatName);
  printDatum(OS, "source functions", NumFunctions);
  printDatum(OS, "inlined functions", NumInlinedFunctions);
  printDatum(OS, "unique source variables", VarUnique);
  printDatum(OS, "source variables", VarTotal);
  printDatum(OS, "variables with location", VarWithLoc);
  printDatum(OS, "scope bytes total",
             GlobalStats.ScopeBytesFromFirstDefinition);
  printDatum(OS, "scope bytes covered", GlobalStats.ScopeBytesCovered);
  OS << "}\n";
  LLVM_DEBUG(
      llvm::dbgs() << "Total Availability: "
                   << (int)std::round((VarWithLoc * 100.0) / VarTotal) << "%\n";
      llvm::dbgs() << "PC Ranges covered: "
                   << (int)std::round((GlobalStats.ScopeBytesCovered * 100.0) /
                                      GlobalStats.ScopeBytesFromFirstDefinition)
                   << "%\n");
  return true;
}