[llvm-dwarf] Make --verify for .debug_names multithreaded.

This PR makes verification of .debug_names acceleration table multithreaded.
In local testing it improves verification of clang .debug_names
from four minutes to under a minute.
This PR relies on a current mechanism of extracting DIEs into a vector.
Future improvements can include creating API to extract DIE at a time,
or grouping Entires into buckets by CUs and extracting before parallel
step.

Single Thread
4:12.37 real,   246.88 user,    3.54 sys,       0 amem,
10232004 mmem
0:49.40 real,   612.84 user,    515.73 sys,     0 amem, 11226292 mmem

Author: ayermolo

llvm/include/llvm/DebugInfo/DWARF/DWARFAcceleratorTable.h

@@ -770,11 +770,12 @@ class DWARFDebugNames : public DWARFAcceleratorTable {
     }
 
   public:
+    using size_type = size_t;
     using iterator_category = std::input_iterator_tag;
     using value_type = NameTableEntry;
     using difference_type = uint32_t;
     using pointer = NameTableEntry *;
-    using reference = NameTableEntry; // We return entries by value.
+    using reference = NameTableEntry;
 
     /// Creates an iterator whose initial position is name CurrentName in
     /// CurrentIndex.
@@ -793,6 +794,14 @@ class DWARFDebugNames : public DWARFAcceleratorTable {
       next();
       return I;
     }
+    reference operator[](size_type idx) {
+      return CurrentIndex->getNameTableEntry(idx + 1);
+    }
+
+    difference_type operator-(const NameIterator &other) const {
+      assert(CurrentIndex == other.CurrentIndex);
+      return this->CurrentName - other.CurrentName;
+    }
 
     friend bool operator==(const NameIterator &A, const NameIterator &B) {
       return A.CurrentIndex == B.CurrentIndex && A.CurrentName == B.CurrentName;

llvm/include/llvm/DebugInfo/DWARF/DWARFVerifier.h

@@ -16,6 +16,7 @@
 #include "llvm/DebugInfo/DWARF/DWARFUnitIndex.h"
 #include <cstdint>
 #include <map>
+#include <mutex>
 #include <set>
 
 namespace llvm {
@@ -32,7 +33,9 @@ struct DWARFSection;
 
 class OutputCategoryAggregator {
 private:
+  std::mutex WriteMutex;
   std::map<std::string, unsigned> Aggregation;
+  unsigned NumErrors = 0;
   bool IncludeDetail;
 
 public:
@@ -42,6 +45,8 @@ class OutputCategoryAggregator {
   size_t GetNumCategories() const { return Aggregation.size(); }
   void Report(StringRef s, std::function<void()> detailCallback);
   void EnumerateResults(std::function<void(StringRef, unsigned)> handleCounts);
+  /// Return the number of errors that have been reported.
+  unsigned GetNumErrors() const { return NumErrors; }
 };
 
 /// A class that verifies DWARF debug information given a DWARF Context.
@@ -104,6 +109,7 @@ class DWARFVerifier {
   bool IsObjectFile;
   bool IsMachOObject;
   using ReferenceMap = std::map<uint64_t, std::set<uint64_t>>;
+  std::mutex AccessMutex;
 
   raw_ostream &error() const;
   raw_ostream &warn() const;
@@ -264,21 +270,22 @@ class DWARFVerifier {
   /// \param SectionName the name of the table we're verifying
   ///
   /// \returns The number of errors occurred during verification
-  unsigned verifyAppleAccelTable(const DWARFSection *AccelSection,
-                                 DataExtractor *StrData,
-                                 const char *SectionName);
-
-  unsigned verifyDebugNamesCULists(const DWARFDebugNames &AccelTable);
-  unsigned verifyNameIndexBuckets(const DWARFDebugNames::NameIndex &NI,
-                                  const DataExtractor &StrData);
-  unsigned verifyNameIndexAbbrevs(const DWARFDebugNames::NameIndex &NI);
-  unsigned verifyNameIndexAttribute(const DWARFDebugNames::NameIndex &NI,
-                                    const DWARFDebugNames::Abbrev &Abbr,
-                                    DWARFDebugNames::AttributeEncoding AttrEnc);
-  unsigned verifyNameIndexEntries(const DWARFDebugNames::NameIndex &NI,
-                                  const DWARFDebugNames::NameTableEntry &NTE);
-  unsigned verifyNameIndexCompleteness(const DWARFDie &Die,
-                                       const DWARFDebugNames::NameIndex &NI);
+  void verifyAppleAccelTable(const DWARFSection *AccelSection,
+                             DataExtractor *StrData, const char *SectionName);
+
+  void verifyDebugNamesCULists(const DWARFDebugNames &AccelTable);
+  void verifyNameIndexBuckets(const DWARFDebugNames::NameIndex &NI,
+                              const DataExtractor &StrData);
+  void verifyNameIndexAbbrevs(const DWARFDebugNames::NameIndex &NI);
+  void verifyNameIndexAttribute(const DWARFDebugNames::NameIndex &NI,
+                                const DWARFDebugNames::Abbrev &Abbr,
+                                DWARFDebugNames::AttributeEncoding AttrEnc);
+  void verifyNameIndexEntries(
+      const DWARFDebugNames::NameIndex &NI,
+      const DWARFDebugNames::NameTableEntry &NTE,
+      const DenseMap<uint64_t, DWARFUnit *> &CUOffsetsToDUMap);
+  void verifyNameIndexCompleteness(const DWARFDie &Die,
+                                   const DWARFDebugNames::NameIndex &NI);
 
   /// Verify that the DWARF v5 accelerator table is valid.
   ///
@@ -297,8 +304,8 @@ class DWARFVerifier {
   /// \param StrData string section
   ///
   /// \returns The number of errors occurred during verification
-  unsigned verifyDebugNames(const DWARFSection &AccelSection,
-                            const DataExtractor &StrData);
+  void verifyDebugNames(const DWARFSection &AccelSection,
+                        const DataExtractor &StrData);
 
 public:
   DWARFVerifier(raw_ostream &S, DWARFContext &D,