[Runtime] Change ConcurrentReadableArray's API to provide iterable snapshots rather than using a callback-based read call.

rdar://problem/40230581

Author: mikeash

include/swift/Runtime/Concurrent.h

@@ -451,7 +451,37 @@ template <class ElemTy> struct ConcurrentReadableArray {
   Mutex WriterLock;
   std::vector<Storage *> FreeList;
 
+  void incrementReaders() {
+    ReaderCount.fetch_add(1, std::memory_order_acquire);
+  }
+  
+  void decrementReaders() {
+    ReaderCount.fetch_sub(1, std::memory_order_release);
+  }
+  
 public:
+  struct Snapshot {
+    ConcurrentReadableArray *Array;
+    const ElemTy *Start;
+    size_t Count;
+    
+    Snapshot(ConcurrentReadableArray *array, const ElemTy *start, size_t count)
+      : Array(array), Start(start), Count(count) {}
+    
+    Snapshot(const Snapshot &other)
+      : Array(other.Array), Start(other.Start), Count(other.Count) {
+      Array->incrementReaders();
+    }
+    
+    ~Snapshot() {
+      Array->decrementReaders();
+    }
+    
+    const ElemTy *begin() { return Start; }
+    const ElemTy *end() { return Start + Count; }
+    size_t count() { return Count; }
+  };
+  
   // This type cannot be safely copied, moved, or deleted.
   ConcurrentReadableArray(const ConcurrentReadableArray &) = delete;
   ConcurrentReadableArray(ConcurrentReadableArray &&) = delete;
@@ -486,28 +516,16 @@ template <class ElemTy> struct ConcurrentReadableArray {
         storage->deallocate();
   }
 
-  /// Read the contents of the array. The parameter `f` is called with
-  /// two parameters: a pointer to the elements in the array, and the
-  /// count. This represents a snapshot of the contents at the time
-  /// `read` was called. The pointer becomes invalid after `f` returns.
-  template <class F> auto read(F f) -> decltype(f(nullptr, 0)) {
-    ReaderCount.fetch_add(1, std::memory_order_acquire);
+  Snapshot snapshot() {
+    incrementReaders();
     auto *storage = Elements.load(SWIFT_MEMORY_ORDER_CONSUME);
+    if (storage == nullptr) {
+      return Snapshot(this, nullptr, 0);
+    }
+    
     auto count = storage->Count.load(std::memory_order_acquire);
     const auto *ptr = storage->data();
-    
-    decltype(f(nullptr, 0)) result = f(ptr, count);
-    
-    ReaderCount.fetch_sub(1, std::memory_order_release);
-    
-    return result;
-  }
-  
-  /// Get the current count. It's just a snapshot and may be obsolete immediately.
-  size_t count() {
-    return read([](const ElemTy *ptr, size_t count) -> size_t {
-      return count;
-    });
+    return Snapshot(this, ptr, count);
   }
 };
 

stdlib/public/runtime/ProtocolConformance.cpp

@@ -326,14 +326,11 @@ void ConformanceState::verify() const {
   // Iterate over all of the sections and verify all of the protocol
   // descriptors.
   auto &Self = const_cast<ConformanceState &>(*this);
-  Self.SectionsToScan.read([](const ConformanceSection *ptr, size_t count) -> char {
-    for (size_t i = 0; i < count; i++) {
-      for (const auto &Record : ptr[i]) {
-        Record.get()->verify();
-      }
+  for (const auto &Section : Self.SectionsToScan.snapshot()) {
+    for (const auto &Record : Section) {
+      Record.get()->verify();
     }
-    return 0;
-  });
+  }
 }
 #endif
 
@@ -445,7 +442,7 @@ searchInConformanceCache(const Metadata *type,
       }
 
       // Check if the negative cache entry is up-to-date.
-      if (Value->getFailureGeneration() == C.SectionsToScan.count()) {
+      if (Value->getFailureGeneration() == C.SectionsToScan.snapshot().count()) {
         // Negative cache entry is up-to-date. Return failure along with
         // the original query type's own cache entry, if we found one.
         // (That entry may be out of date but the caller still has use for it.)
@@ -546,100 +543,94 @@ swift_conformsToProtocolImpl(const Metadata * const type,
   auto failureEntry = FoundConformance.failureEntry;
 
   // Prepare to scan conformance records.
-  size_t scannedCount;
-  auto returnNull = C.SectionsToScan
-    .read([&](const ConformanceSection *ptr, size_t count) -> bool {
-    scannedCount = count;
-    // Scan only sections that were not scanned yet.
-    // If we found an out-of-date negative cache entry,
-    // we need not to re-scan the sections that it covers.
-    auto startIndex = failureEntry ? failureEntry->getFailureGeneration() : 0;
-    auto endIndex = count;
+  auto snapshot = C.SectionsToScan.snapshot();
 
-    // If there are no unscanned sections outstanding
-    // then we can cache failure and give up now.
-    if (startIndex == endIndex) {
-      C.cacheFailure(type, protocol, count);
-      return true;
+  // Scan only sections that were not scanned yet.
+  // If we found an out-of-date negative cache entry,
+  // we need not to re-scan the sections that it covers.
+  auto startIndex = failureEntry ? failureEntry->getFailureGeneration() : 0;
+  auto endIndex = snapshot.count();
+
+  // If there are no unscanned sections outstanding
+  // then we can cache failure and give up now.
+  if (startIndex == endIndex) {
+    C.cacheFailure(type, protocol, snapshot.count());
+    return nullptr;
+  }
+
+  /// Local function to retrieve the witness table and record the result.
+  auto recordWitnessTable = [&](const ProtocolConformanceDescriptor &descriptor,
+                                const Metadata *type) {
+    switch (descriptor.getConformanceKind()) {
+    case ConformanceFlags::ConformanceKind::WitnessTable:
+      // If the record provides a nondependent witness table for all
+      // instances of a generic type, cache it for the generic pattern.
+      C.cacheSuccess(type, protocol, descriptor.getStaticWitnessTable());
+      return;
+
+    case ConformanceFlags::ConformanceKind::WitnessTableAccessor:
+      // If the record provides a dependent witness table accessor,
+      // cache the result for the instantiated type metadata.
+      C.cacheSuccess(type, protocol, descriptor.getWitnessTable(type));
+      return;
+
+    case ConformanceFlags::ConformanceKind::ConditionalWitnessTableAccessor: {
+      auto witnessTable = descriptor.getWitnessTable(type);
+      if (witnessTable)
+        C.cacheSuccess(type, protocol, witnessTable);
+      else
+        C.cacheFailure(type, protocol, snapshot.count());
+      return;
+    }
     }
 
-    /// Local function to retrieve the witness table and record the result.
-    auto recordWitnessTable = [&](const ProtocolConformanceDescriptor &descriptor,
-                                  const Metadata *type) {
-      switch (descriptor.getConformanceKind()) {
-      case ConformanceFlags::ConformanceKind::WitnessTable:
-        // If the record provides a nondependent witness table for all
-        // instances of a generic type, cache it for the generic pattern.
-        C.cacheSuccess(type, protocol, descriptor.getStaticWitnessTable());
-        return;
-
-      case ConformanceFlags::ConformanceKind::WitnessTableAccessor:
-        // If the record provides a dependent witness table accessor,
-        // cache the result for the instantiated type metadata.
-        C.cacheSuccess(type, protocol, descriptor.getWitnessTable(type));
-        return;
-
-      case ConformanceFlags::ConformanceKind::ConditionalWitnessTableAccessor: {
-        auto witnessTable = descriptor.getWitnessTable(type);
-        if (witnessTable)
-          C.cacheSuccess(type, protocol, witnessTable);
-        else
-          C.cacheFailure(type, protocol, count);
-        return;
-      }
-      }
+    // Always fail, because we cannot interpret a future conformance
+    // kind.
+    C.cacheFailure(type, protocol, snapshot.count());
+  };
 
-      // Always fail, because we cannot interpret a future conformance
-      // kind.
-      C.cacheFailure(type, protocol, count);
-    };
-
-    // Really scan conformance records.
-    for (size_t i = startIndex; i < endIndex; i++) {
-      auto &section = ptr[i];
-      // Eagerly pull records for nondependent witnesses into our cache.
-      for (const auto &record : section) {
-        auto &descriptor = *record.get();
-
-        // If the record applies to a specific type, cache it.
-        if (auto metadata = descriptor.getCanonicalTypeMetadata()) {
-          auto P = descriptor.getProtocol();
-
-          // Look for an exact match.
-          if (protocol != P)
-            continue;
-
-          if (!isRelatedType(type, metadata, /*candidateIsMetadata=*/true))
-            continue;
-
-          // Record the witness table.
-          recordWitnessTable(descriptor, metadata);
-
-        // TODO: "Nondependent witness table" probably deserves its own flag.
-        // An accessor function might still be necessary even if the witness table
-        // can be shared.
-        } else if (descriptor.getTypeKind()
-                     == TypeMetadataRecordKind::DirectNominalTypeDescriptor ||
-                   descriptor.getTypeKind()
-                    == TypeMetadataRecordKind::IndirectNominalTypeDescriptor) {
-          auto R = descriptor.getTypeContextDescriptor();
-          auto P = descriptor.getProtocol();
-
-          // Look for an exact match.
-          if (protocol != P)
-            continue;
-
-          if (!isRelatedType(type, R, /*candidateIsMetadata=*/false))
-            continue;
-
-          recordWitnessTable(descriptor, type);
-        }
+  // Really scan conformance records.
+  for (size_t i = startIndex; i < endIndex; i++) {
+    auto &section = snapshot.Start[i];
+    // Eagerly pull records for nondependent witnesses into our cache.
+    for (const auto &record : section) {
+      auto &descriptor = *record.get();
+
+      // If the record applies to a specific type, cache it.
+      if (auto metadata = descriptor.getCanonicalTypeMetadata()) {
+        auto P = descriptor.getProtocol();
+
+        // Look for an exact match.
+        if (protocol != P)
+          continue;
+
+        if (!isRelatedType(type, metadata, /*candidateIsMetadata=*/true))
+          continue;
+
+        // Record the witness table.
+        recordWitnessTable(descriptor, metadata);
+
+      // TODO: "Nondependent witness table" probably deserves its own flag.
+      // An accessor function might still be necessary even if the witness table
+      // can be shared.
+      } else if (descriptor.getTypeKind()
+                   == TypeMetadataRecordKind::DirectNominalTypeDescriptor ||
+                 descriptor.getTypeKind()
+                  == TypeMetadataRecordKind::IndirectNominalTypeDescriptor) {
+        auto R = descriptor.getTypeContextDescriptor();
+        auto P = descriptor.getProtocol();
+
+        // Look for an exact match.
+        if (protocol != P)
+          continue;
+
+        if (!isRelatedType(type, R, /*candidateIsMetadata=*/false))
+          continue;
+
+        recordWitnessTable(descriptor, type);
       }
     }
-    return false;
-  });
-  
-  if (returnNull) return nullptr;
+  }
 
   // Conformance scan is complete.
   // Search the cache once more, and this time update the cache if necessary.
@@ -648,7 +639,7 @@ swift_conformsToProtocolImpl(const Metadata * const type,
   if (FoundConformance.isAuthoritative) {
     return FoundConformance.witnessTable;
   } else {
-    C.cacheFailure(type, protocol, scannedCount);
+    C.cacheFailure(type, protocol, snapshot.count());
     return nullptr;
   }
 }
@@ -657,19 +648,15 @@ const TypeContextDescriptor *
 swift::_searchConformancesByMangledTypeName(Demangle::NodePointer node) {
   auto &C = Conformances.get();
 
-  return C.SectionsToScan
-    .read([&](const ConformanceSection *ptr, size_t count) -> const TypeContextDescriptor * {
-    for (size_t i = 0; i < count; i++) {
-      auto &section = ptr[i];
-      for (const auto &record : section) {
-        if (auto ntd = record->getTypeContextDescriptor()) {
-          if (_contextDescriptorMatchesMangling(ntd, node))
-            return ntd;
-        }
+  for (auto &section : C.SectionsToScan.snapshot()) {
+    for (const auto &record : section) {
+      if (auto ntd = record->getTypeContextDescriptor()) {
+        if (_contextDescriptorMatchesMangling(ntd, node))
+          return ntd;
       }
     }
-    return nullptr;
-  });
+  }
+  return nullptr;
 }
 
 /// Resolve a reference to a generic parameter to type metadata.