[Concurrency] Add environment variable for disabling async stack slab allocator.

Add SWIFT_DEBUG_ENABLE_TASK_SLAB_ALLOCATOR, which is on by default. When turned off, async stack allocations call through to malloc/free. This allows memory debugging tools to be used on async stack allocations.

include/swift/Runtime/EnvironmentVariables.h

@@ -14,6 +14,9 @@
 //
 //===----------------------------------------------------------------------===//
 
+#ifndef SWIFT_RUNTIME_ENVIRONMENTVARIABLES_H
+#define SWIFT_RUNTIME_ENVIRONMENTVARIABLES_H
+
 #include "swift/Threading/Once.h"
 #include "swift/shims/Visibility.h"
 
@@ -63,6 +66,12 @@ SWIFT_RUNTIME_STDLIB_SPI bool concurrencyValidateUncheckedContinuations();
 // Concurrency library can call.
 SWIFT_RUNTIME_STDLIB_SPI const char *concurrencyIsCurrentExecutorLegacyModeOverride();
 
+// Wrapper around SWIFT_DEBUG_ENABLE_TASK_SLAB_ALLOCATOR that the Concurrency
+// library can call.
+SWIFT_RUNTIME_STDLIB_SPI bool concurrencyEnableTaskSlabAllocator();
+
 } // end namespace environment
 } // end namespace runtime
 } // end namespace swift
+
+#endif // SWIFT_RUNTIME_ENVIRONMENTVARIABLES_H

stdlib/public/Concurrency/TaskAlloc.cpp

@@ -84,3 +84,8 @@ void swift::swift_job_deallocate(Job *job, void *ptr) {
 
   allocator(static_cast<AsyncTask *>(job)).dealloc(ptr);
 }
+
+#if !SWIFT_CONCURRENCY_EMBEDDED
+std::atomic<TaskAllocatorConfiguration::EnableState>
+    TaskAllocatorConfiguration::enableState = {EnableState::Uninitialized};
+#endif

stdlib/public/Concurrency/TaskPrivate.h

@@ -26,6 +26,7 @@
 #include "swift/Runtime/Atomic.h"
 #include "swift/Runtime/Concurrency.h"
 #include "swift/Runtime/DispatchShims.h"
+#include "swift/Runtime/EnvironmentVariables.h"
 #include "swift/Runtime/Error.h"
 #include "swift/Runtime/Exclusivity.h"
 #include "swift/Runtime/HeapObject.h"
@@ -744,14 +745,48 @@ public:
 static_assert(sizeof(ActiveTaskStatus) == ACTIVE_TASK_STATUS_SIZE,
   "ActiveTaskStatus is of incorrect size");
 
+struct TaskAllocatorConfiguration {
+#if SWIFT_CONCURRENCY_EMBEDDED
+
+  // Slab allocator is always enabled on embedded.
+  bool enableSlabAllocator() { return true; }
+
+#else
+
+  enum class EnableState : uint8_t {
+    Uninitialized,
+    Enabled,
+    Disabled,
+  };
+
+  static std::atomic<EnableState> enableState;
+
+  bool enableSlabAllocator() {
+    auto state = enableState.load(std::memory_order_relaxed);
+    if (SWIFT_UNLIKELY(state == EnableState::Uninitialized)) {
+      state = runtime::environment::concurrencyEnableTaskSlabAllocator()
+                  ? EnableState::Enabled
+                  : EnableState::Disabled;
+      enableState.store(state, std::memory_order_relaxed);
+    }
+
+    return SWIFT_UNLIKELY(state == EnableState::Enabled);
+  }
+
+#endif // SWIFT_CONCURRENCY_EMBEDDED
+};
+
 /// The size of an allocator slab. We want the full allocation to fit into a
 /// 1024-byte malloc quantum. We subtract off the slab header size, plus a
 /// little extra to stay within our limits even when there's overhead from
 /// malloc stack logging.
-static constexpr size_t SlabCapacity = 1024 - StackAllocator<0, nullptr>::slabHeaderSize() - 8;
+static constexpr size_t SlabCapacity =
+    1024 - 8 -
+    StackAllocator<0, nullptr, TaskAllocatorConfiguration>::slabHeaderSize();
 extern Metadata TaskAllocatorSlabMetadata;
 
-using TaskAllocator = StackAllocator<SlabCapacity, &TaskAllocatorSlabMetadata>;
+using TaskAllocator = StackAllocator<SlabCapacity, &TaskAllocatorSlabMetadata,
+                                     TaskAllocatorConfiguration>;
 
 /// Private storage in an AsyncTask object.
 struct AsyncTask::PrivateStorage {

stdlib/public/runtime/EnvironmentVariables.cpp

@@ -319,3 +319,7 @@ SWIFT_RUNTIME_STDLIB_SPI bool concurrencyValidateUncheckedContinuations() {
 SWIFT_RUNTIME_STDLIB_SPI const char *concurrencyIsCurrentExecutorLegacyModeOverride() {
   return runtime::environment::SWIFT_IS_CURRENT_EXECUTOR_LEGACY_MODE_OVERRIDE();
 }
+
+SWIFT_RUNTIME_STDLIB_SPI bool concurrencyEnableTaskSlabAllocator() {
+  return runtime::environment::SWIFT_DEBUG_ENABLE_TASK_SLAB_ALLOCATOR();
+}

stdlib/public/runtime/EnvironmentVariables.def

@@ -147,4 +147,9 @@ VARIABLE(SWIFT_DUMP_ACCESSIBLE_FUNCTIONS, bool, false,
          "record names, e.g. by the Distributed runtime to invoke distributed "
          "functions.")
 
+VARIABLE(SWIFT_DEBUG_ENABLE_TASK_SLAB_ALLOCATOR, bool, true,
+         "Use a slab allocator for the async stack. If not enabled, directly "
+         "call malloc/free. Direct use of malloc/free enables use of memory "
+         "debugging tools for async stack allocations.")
+
 #undef VARIABLE

stdlib/public/runtime/StackAllocator.h

@@ -57,7 +57,15 @@ namespace swift {
 ///
 /// SlabMetadataPtr specifies a fake metadata pointer to place at the beginning
 /// of slab allocations, so analysis tools can identify them.
-template <size_t SlabCapacity, Metadata *SlabMetadataPtr>
+///
+/// SlabAllocatorConfiguration allows customizing behavior. It currently allows
+/// one customization point: bool enableSlabAllocator():
+///   returns false - the stack allocator directly calls malloc/free
+///   returns true - the slab allocator is used
+/// This function MUST return the same value throughout the lifetime the stack
+/// allocator.
+template <size_t SlabCapacity, Metadata *SlabMetadataPtr,
+          typename SlabAllocatorConfiguration>
 class StackAllocator {
 private:
 
@@ -77,6 +85,8 @@ private:
   /// Used for unit testing.
   uint32_t numAllocatedSlabs:31;
 
+  /// The configuration object.
+  [[no_unique_address]] SlabAllocatorConfiguration configuration;
 
   /// The minimal alignment of allocated memory.
   static constexpr size_t alignment = MaximumAlignment;
@@ -227,9 +237,10 @@ private:
   };
 
   // Return a slab which is suitable to allocate \p size memory.
+  SWIFT_ALWAYS_INLINE
   Slab *getSlabForAllocation(size_t size) {
     Slab *slab = (lastAllocation ? lastAllocation->slab : firstSlab);
-    if (slab) {
+    if (SWIFT_LIKELY(slab)) {
       // Is there enough space in the current slab?
       if (slab->canAllocate(size))
         return slab;
@@ -249,6 +260,12 @@ private:
         size = std::max(size, alreadyAllocatedCapacity);
       }
     }
+
+    // This is only checked on the path that allocates a new slab, to minimize
+    // overhead when the slab allocator is enabled.
+    if (SWIFT_UNLIKELY(!configuration.enableSlabAllocator()))
+      return nullptr;
+
     size_t capacity = std::max(SlabCapacity,
                                Allocation::includingHeader(size));
     void *slabBuffer = malloc(Slab::includingHeader(capacity));
@@ -281,13 +298,15 @@ public:
   /// Construct a StackAllocator without a pre-allocated first slab.
   StackAllocator()
       : firstSlab(nullptr), firstSlabIsPreallocated(false),
-        numAllocatedSlabs(0) {}
+        numAllocatedSlabs(0), configuration() {}
 
   /// Construct a StackAllocator with a pre-allocated first slab.
   StackAllocator(void *firstSlabBuffer, size_t bufferCapacity) : StackAllocator() {
     // If the pre-allocated buffer can't hold a slab header, ignore it.
     if (bufferCapacity <= Slab::headerSize())
       return;
+    if (SWIFT_UNLIKELY(!configuration.enableSlabAllocator()))
+      return;
     char *start = (char *)llvm::alignAddr(firstSlabBuffer,
                                           llvm::Align(alignment));
     char *end = (char *)firstSlabBuffer + bufferCapacity;
@@ -317,6 +336,17 @@ public:
       size += sizeof(uintptr_t);
     size_t alignedSize = llvm::alignTo(size, llvm::Align(alignment));
     Slab *slab = getSlabForAllocation(alignedSize);
+
+    // If getSlabForAllocation returns null, that means that the slab allocator
+    // is disabled, and we should directly call malloc. We get this info
+    // indirectly rather than directly calling enableSlabAllocator() in order
+    // to minimize the overhead in the case where the slab allocator is enabled.
+    // When getSlabForAllocation gets inlined into this code, this ends up
+    // getting folded into its enableSlabAllocator() call, and the fast path
+    // where `slab` is non-null ends up with no extra conditionals at all.
+    if (SWIFT_UNLIKELY(!slab))
+      return malloc(size);
+
     Allocation *allocation = slab->allocate(alignedSize, lastAllocation);
     lastAllocation = allocation;
     assert(llvm::isAddrAligned(llvm::Align(alignment),
@@ -326,7 +356,10 @@ public:
 
   /// Deallocate memory \p ptr.
   void dealloc(void *ptr) {
-    if (!lastAllocation || lastAllocation->getAllocatedMemory() != ptr) {
+    if (SWIFT_UNLIKELY(!lastAllocation ||
+                       lastAllocation->getAllocatedMemory() != ptr)) {
+      if (!configuration.enableSlabAllocator())
+        return free(ptr);
       SWIFT_FATAL_ERROR(0, "freed pointer was not the last allocation");
     }
 

stdlib/toolchain/Compatibility56/include/Concurrency/TaskPrivate.h

@@ -25,7 +25,7 @@
 #include "swift/Runtime/Error.h"
 
 #define SWIFT_FATAL_ERROR swift_Concurrency_fatalError
-#include "public/runtime/StackAllocator.h"
+#include "Runtime/StackAllocator.h"
 
 namespace swift {
 

stdlib/toolchain/Compatibility56/include/Runtime/StackAllocator.h

@@ -0,0 +1,354 @@
+//===--- StackAllocator.h - A stack allocator -----------------------------===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2014 - 2020 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+//
+//===----------------------------------------------------------------------===//
+//
+// A bump-pointer allocator that obeys a stack discipline.
+//
+//===----------------------------------------------------------------------===//
+
+// Define __STDC_WANT_LIB_EXT1__ to get memset_s on platforms that have it.
+// Other files may have included string.h without it already, so we also set
+// this with a -D flag when building, but this allows tests to build without
+// additional trouble.
+#define __STDC_WANT_LIB_EXT1__ 1
+#include <string.h>
+
+#include "swift/ABI/MetadataValues.h"
+#include "swift/Runtime/Debug.h"
+#include "llvm/Support/Alignment.h"
+#include <cstddef>
+#include <new>
+
+// Notes: swift::fatalError is not shared between libswiftCore and libswift_Concurrency
+// and libswift_Concurrency uses swift_Concurrency_fatalError instead.
+#ifndef SWIFT_FATAL_ERROR
+#define SWIFT_FATAL_ERROR swift::fatalError
+#endif
+
+namespace swift {
+
+/// A bump-pointer allocator that obeys a stack discipline.
+///
+/// StackAllocator performs fast allocation and deallocation of memory by
+/// implementing a bump-pointer allocation strategy.
+///
+/// This isn't strictly a bump-pointer allocator as it uses backing slabs of
+/// memory rather than relying on a boundless contiguous heap. However, it has
+/// bump-pointer semantics in that it is a monotonically growing pool of memory
+/// where every allocation is found by merely allocating the next N bytes in
+/// the slab, or the next N bytes in the next slab.
+///
+/// In contrast to a pure bump-pointer allocator, it's possible to free memory.
+/// Allocations and deallocations must follow a strict stack discipline. In
+/// general, slabs which become unused are _not_ freed, but reused for
+/// subsequent allocations.
+///
+/// It's possible to place the first slab into pre-allocated memory.
+///
+/// The SlabCapacity specifies the capacity for newly allocated slabs.
+///
+/// SlabMetadataPtr specifies a fake metadata pointer to place at the beginning
+/// of slab allocations, so analysis tools can identify them.
+template <size_t SlabCapacity, Metadata *SlabMetadataPtr>
+class StackAllocator {
+private:
+
+  struct Allocation;
+  struct Slab;
+
+  /// The last active allocation.
+  ///
+  /// A deallocate() must free this allocation.
+  Allocation *lastAllocation = nullptr;
+
+  /// The first slab.
+  Slab *firstSlab;
+
+  /// True if the first slab is pre-allocated.
+  uint32_t firstSlabIsPreallocated:1;
+  /// Used for unit testing.
+  uint32_t numAllocatedSlabs:31;
+
+
+  /// The minimal alignment of allocated memory.
+  static constexpr size_t alignment = MaximumAlignment;
+
+  /// If set to true, memory allocations are checked for buffer overflows and
+  /// use-after-free, similar to guard-malloc.
+  static constexpr bool guardAllocations =
+#ifdef NDEBUG
+    false;
+#else
+    true;
+#endif
+
+  static constexpr uintptr_t magicUninitialized =   (uintptr_t)0xcdcdcdcdcdcdcdcdull;
+  static constexpr uintptr_t magicEndOfAllocation = (uintptr_t)0xdeadbeafdeadbeafull;
+
+  /// A memory slab holding multiple allocations.
+  ///
+  /// This struct is actually just the slab header. The slab buffer is tail
+  /// allocated after Slab.
+  struct Slab {
+    /// A fake metadata pointer that analysis tools can use to identify slab
+    /// allocations.
+    const void *metadata;
+
+    /// A single linked list of all allocated slabs.
+    Slab *next = nullptr;
+
+    // Capacity and offset do not include these header fields.
+    uint32_t capacity;
+    uint32_t currentOffset = 0;
+
+    // Here starts the tail allocated memory buffer of the slab.
+
+    Slab(size_t newCapacity)
+        : metadata(SlabMetadataPtr), capacity(newCapacity) {
+      assert((size_t)capacity == newCapacity && "capacity overflow");
+    }
+
+    /// The size of the slab header.
+    static constexpr size_t headerSize() {
+      return (sizeof(Slab) + alignment - 1) & ~(alignment - 1);
+    }
+
+    /// Return \p size with the added overhead of the slab header.
+    static size_t includingHeader(size_t size) {
+      return headerSize() + size;
+    }
+
+    /// Clear the fake metadata pointer. Call before freeing so that leftover
+    /// heap garbage doesn't have slab metadata pointers in it.
+    void clearMetadata() {
+      // Use memset_s on Apple platforms. Fall back to a plain
+      // assignment on other platforms. This is not necessary for
+      // correctness, just as an aid to analysis tools, so it's OK if
+      // the fallback gets optimized out.
+#if defined(__APPLE__)
+      memset_s(&metadata, sizeof(metadata), 0, sizeof(metadata));
+#else
+      metadata = 0;
+#endif
+    }
+
+    /// Return the payload buffer address at \p atOffset.
+    ///
+    /// Note: it's valid to call this function on a not-yet-constructed slab.
+    char *getAddr(size_t atOffset) {
+      return (char *)this + headerSize() + atOffset;
+    }
+
+    /// Return true if this slab can fit an allocation of \p size.
+    ///
+    /// \p size does not include the allocation header, but must include the
+    /// overhead for guardAllocations (if enabled).
+    inline bool canAllocate(size_t size) const {
+      return currentOffset + Allocation::includingHeader(size) <= capacity;
+    }
+
+    /// Return true, if no memory is allocated in this slab.
+    bool isEmpty() const { return currentOffset == 0; }
+
+    /// Allocate \p alignedSize of bytes in this slab.
+    ///
+    /// \p alignedSize does not include the allocation header, but must include
+    /// the overhead for guardAllocations (if enabled).
+    ///
+    /// Precondition: \p alignedSize must be aligned up to
+    ///               StackAllocator::alignment.
+    /// Precondition: there must be enough space in this slab to fit the
+    ///               allocation.
+    Allocation *allocate(size_t alignedSize, Allocation *lastAllocation) {
+      assert(llvm::isAligned(llvm::Align(alignment), alignedSize));
+      assert(canAllocate(alignedSize));
+      void *buffer = getAddr(currentOffset);
+      auto *allocation = ::new (buffer) Allocation(lastAllocation, this);
+      currentOffset += Allocation::includingHeader(alignedSize);
+      if (guardAllocations) {
+        uintptr_t *endOfCurrentAllocation = (uintptr_t *)getAddr(currentOffset);
+        endOfCurrentAllocation[-1] = magicEndOfAllocation;
+      }
+      return allocation;
+    }
+
+    /// Deallocate \p allocation.
+    ///
+    /// Precondition: \p allocation must be an allocation in this slab.
+    void deallocate(Allocation *allocation) {
+      assert(allocation->slab == this);
+      if (guardAllocations) {
+        auto *endOfAllocation = (uintptr_t *)getAddr(currentOffset);
+        if (endOfAllocation[-1] != magicEndOfAllocation)
+          SWIFT_FATAL_ERROR(0, "Buffer overflow in StackAllocator");
+        for (auto *p = (uintptr_t *)allocation; p < endOfAllocation; ++p)
+          *p = magicUninitialized;
+      }
+      currentOffset = (char *)allocation - getAddr(0);
+    }
+  };
+
+  /// A single memory allocation.
+  ///
+  /// This struct is actually just the allocation header. The allocated
+  /// memory buffer is located after Allocation.
+  struct Allocation {
+    /// A single linked list of previous allocations.
+    Allocation *previous;
+    /// The containing slab.
+    Slab *slab;
+
+    // Here starts the tail allocated memory.
+
+    Allocation(Allocation *previous, Slab *slab) :
+      previous(previous), slab(slab) {}
+
+    void *getAllocatedMemory() {
+      return (char *)this + headerSize();
+    }
+
+    /// The size of the allocation header.
+    static size_t headerSize() {
+      return llvm::alignTo(sizeof(Allocation), llvm::Align(alignment));
+    }
+
+    /// Return \p size with the added overhead of the allocation header.
+    static size_t includingHeader(size_t size) {
+      return headerSize() + size;
+    }
+  };
+
+  // Return a slab which is suitable to allocate \p size memory.
+  Slab *getSlabForAllocation(size_t size) {
+    Slab *slab = (lastAllocation ? lastAllocation->slab : firstSlab);
+    if (slab) {
+      // Is there enough space in the current slab?
+      if (slab->canAllocate(size))
+        return slab;
+
+      // Is there a successor slab, which we allocated before (and became free
+      // in the meantime)?
+      if (Slab *nextSlab = slab->next) {
+        assert(nextSlab->isEmpty());
+        if (nextSlab->canAllocate(size))
+          return nextSlab;
+
+        // No space in the next slab. Although it's empty, the size exceeds its
+        // capacity.
+        // As we have to allocate a new slab anyway, free all successor slabs
+        // and allocate a new one with the accumulated capacity.
+        size_t alreadyAllocatedCapacity = freeAllSlabs(slab->next);
+        size = std::max(size, alreadyAllocatedCapacity);
+      }
+    }
+    size_t capacity = std::max(SlabCapacity,
+                               Allocation::includingHeader(size));
+    void *slabBuffer = malloc(Slab::includingHeader(capacity));
+    Slab *newSlab = ::new (slabBuffer) Slab(capacity);
+    if (slab)
+      slab->next = newSlab;
+    else
+      firstSlab = newSlab;
+    numAllocatedSlabs++;
+    return newSlab;
+  }
+
+  /// Deallocate all slabs after \p first and set \p first to null.
+  size_t freeAllSlabs(Slab *&first) {
+    size_t freedCapacity = 0;
+    Slab *slab = first;
+    first = nullptr;
+    while (slab) {
+      Slab *next = slab->next;
+      freedCapacity += slab->capacity;
+      slab->clearMetadata();
+      free(slab);
+      numAllocatedSlabs--;
+      slab = next;
+    }
+    return freedCapacity;
+  }
+
+public:
+  /// Construct a StackAllocator without a pre-allocated first slab.
+  StackAllocator()
+      : firstSlab(nullptr), firstSlabIsPreallocated(false),
+        numAllocatedSlabs(0) {}
+
+  /// Construct a StackAllocator with a pre-allocated first slab.
+  StackAllocator(void *firstSlabBuffer, size_t bufferCapacity) : StackAllocator() {
+    // If the pre-allocated buffer can't hold a slab header, ignore it.
+    if (bufferCapacity <= Slab::headerSize())
+      return;
+    char *start = (char *)llvm::alignAddr(firstSlabBuffer,
+                                          llvm::Align(alignment));
+    char *end = (char *)firstSlabBuffer + bufferCapacity;
+    assert(start + Slab::headerSize() <= end &&
+           "buffer for first slab too small");
+    firstSlab = ::new (start) Slab(end - start - Slab::headerSize());
+    firstSlabIsPreallocated = true;
+    numAllocatedSlabs = 0;
+  }
+
+  ~StackAllocator() {
+    if (lastAllocation)
+      SWIFT_FATAL_ERROR(0, "not all allocations are deallocated");
+    if (firstSlabIsPreallocated)
+      firstSlab->clearMetadata();
+    (void)freeAllSlabs(firstSlabIsPreallocated ? firstSlab->next : firstSlab);
+    assert(getNumAllocatedSlabs() == 0);
+  }
+
+  static constexpr size_t slabHeaderSize() {
+    return Slab::headerSize();
+  }
+
+  /// Allocate a memory buffer of \p size.
+  void *alloc(size_t size) {
+    if (guardAllocations)
+      size += sizeof(uintptr_t);
+    size_t alignedSize = llvm::alignTo(size, llvm::Align(alignment));
+    Slab *slab = getSlabForAllocation(alignedSize);
+    Allocation *allocation = slab->allocate(alignedSize, lastAllocation);
+    lastAllocation = allocation;
+    assert(llvm::isAddrAligned(llvm::Align(alignment),
+                               allocation->getAllocatedMemory()));
+    return allocation->getAllocatedMemory();
+  }
+
+  /// Deallocate memory \p ptr.
+  void dealloc(void *ptr) {
+    if (!lastAllocation || lastAllocation->getAllocatedMemory() != ptr) {
+      SWIFT_FATAL_ERROR(0, "freed pointer was not the last allocation");
+    }
+
+    Allocation *prev = lastAllocation->previous;
+    lastAllocation->slab->deallocate(lastAllocation);
+    lastAllocation = prev;
+  }
+
+  void deallocThrough(void *ptr) {
+    void *current = nullptr;
+    do {
+      if (!lastAllocation) {
+        SWIFT_FATAL_ERROR(0, "freed pointer not among allocations");
+      }
+      current = lastAllocation->getAllocatedMemory();
+      dealloc(current);
+    } while (current != ptr);
+  }
+
+  /// For unit testing.
+  int getNumAllocatedSlabs() { return numAllocatedSlabs; }
+};
+
+} // namespace swift
+

test/abi/macOS/arm64/stdlib.swift

@@ -1148,3 +1148,6 @@ Added: __swift_debug_metadataAllocatorPageSize
 Added: __swift_retainRelease_slowpath_mask_v1
 Added: _swift_release_preservemost
 Added: _swift_retain_preservemost
+
+// New debug environment variable for the concurrency runtime.
+Added: _concurrencyEnableTaskSlabAllocator

test/abi/macOS/x86_64/stdlib.swift

@@ -1143,3 +1143,6 @@ Removed: _$sSS10_wordIndex6beforeSS0B0VAD_tF
 // Internal info exposed for swift-inspect.
 Added: __swift_debug_allocationPoolSize
 Added: __swift_debug_metadataAllocatorPageSize
+
+// New debug environment variable for the concurrency runtime.
+Added: _concurrencyEnableTaskSlabAllocator

unittests/runtime/StackAllocator.cpp

@@ -25,11 +25,15 @@ static constexpr size_t exceedsSlab = slabCapacity + 16;
 
 static Metadata SlabMetadata;
 
+struct SlabAllocatorConfiguration {
+  bool enableSlabAllocator() { return true; }
+};
+
 TEST(StackAllocatorTest, withPreallocatedSlab) {
 
   char firstSlab[firstSlabBufferCapacity];
-  StackAllocator<slabCapacity, &SlabMetadata> allocator(
-      firstSlab, firstSlabBufferCapacity);
+  StackAllocator<slabCapacity, &SlabMetadata, SlabAllocatorConfiguration>
+      allocator(firstSlab, firstSlabBufferCapacity);
 
   char *mem1 = (char *)allocator.alloc(fitsIntoFirstSlab);
   EXPECT_EQ(allocator.getNumAllocatedSlabs(), 0);
@@ -74,7 +78,8 @@ TEST(StackAllocatorTest, withoutPreallocatedSlab) {
 
   constexpr size_t slabCapacity = 256;
 
-  StackAllocator<slabCapacity, &SlabMetadata> allocator;
+  StackAllocator<slabCapacity, &SlabMetadata, SlabAllocatorConfiguration>
+      allocator;
 
   size_t fitsIntoSlab = slabCapacity - 16;
   size_t twoFitIntoSlab = slabCapacity / 2 - 32;