SERVER-44095 Initialize TypeBits buffer to a smaller size

This lowers the memory overhead of index key generation by reducing the
initial buffer size in TypeBits from 512 to 8 bytes.

3 files changed, 28 insertions, 27 deletions

diff --git a/src/mongo/bson/bsonobjbuilder.cpp b/src/mongo/bson/bsonobjbuilder.cpp
index 1eab3f0b93c..39538ecf6c5 100644
--- a/src/mongo/bson/bsonobjbuilder.cpp
+++ b/src/mongo/bson/bsonobjbuilder.cpp
@@ -235,25 +235,9 @@ BSONObjBuilder::~BSONObjBuilder() {
     }
 }
 
-template <typename Alloc>
-void BasicBufBuilder<Alloc>::grow_reallocate(int minSize) {
-    if (minSize > BufferMaxSize) {
-        std::stringstream ss;
-        ss << "BufBuilder attempted to grow() to " << minSize << " bytes, past the 64MB limit.";
-        msgasserted(13548, ss.str().c_str());
-    }
-
-    int a = 64;
-    while (a < minSize)
-        a = a * 2;
-
-    _buf.realloc(a);
-    size = a;
-}
-
 template class BasicBufBuilder<SharedBufferAllocator>;
-template class BasicBufBuilder<StackAllocator>;
+template class BasicBufBuilder<StackAllocator<StackSizeDefault>>;
 template class StringBuilderImpl<SharedBufferAllocator>;
-template class StringBuilderImpl<StackAllocator>;
+template class StringBuilderImpl<StackAllocator<StackSizeDefault>>;
 
 }  // namespace mongo
diff --git a/src/mongo/bson/util/builder.h b/src/mongo/bson/util/builder.h
index a6c821c5b0f..64b578faafd 100644
--- a/src/mongo/bson/util/builder.h
+++ b/src/mongo/bson/util/builder.h
@@ -111,6 +111,8 @@ private:
     SharedBuffer _buf;
 };
 
+enum { StackSizeDefault = 512 };
+template <size_t SZ>
 class StackAllocator {
     StackAllocator(const StackAllocator&) = delete;
     StackAllocator& operator=(const StackAllocator&) = delete;
@@ -121,7 +123,6 @@ public:
         free();
     }
 
-    enum { SZ = 512 };
     void malloc(size_t sz) {
         if (sz > SZ)
             _ptr = mongoMalloc(sz);
@@ -340,7 +341,21 @@ private:
         DataView(grow(sizeof(t))).write(tagLittleEndian(t));
     }
     /* "slow" portion of 'grow()'  */
-    void grow_reallocate(int minSize);
+    void grow_reallocate(int minSize) {
+        if (minSize > BufferMaxSize) {
+            std::stringstream ss;
+            ss << "BufBuilder attempted to grow() to " << minSize << " bytes, past the 64MB limit.";
+            msgasserted(13548, ss.str().c_str());
+        }
+
+        int a = 64;
+        while (a < minSize)
+            a = a * 2;
+
+        _buf.realloc(a);
+        size = a;
+    }
+
 
     BufferAllocator _buf;
     int l;
@@ -360,11 +375,13 @@ MONGO_STATIC_ASSERT(std::is_move_constructible_v<BufBuilder>);
       nothing bad would happen.  In fact in some circumstances this might make sense, say,
       embedded in some other object.
 */
-class StackBufBuilder : public BasicBufBuilder<StackAllocator> {
+template <size_t SZ>
+class StackBufBuilderBase : public BasicBufBuilder<StackAllocator<SZ>> {
 public:
-    StackBufBuilder() : BasicBufBuilder<StackAllocator>(StackAllocator::SZ) {}
+    StackBufBuilderBase() : BasicBufBuilder<StackAllocator<SZ>>(SZ) {}
     void release() = delete;  // not allowed. not implemented.
 };
+using StackBufBuilder = StackBufBuilderBase<StackSizeDefault>;
 MONGO_STATIC_ASSERT(!std::is_move_constructible<StackBufBuilder>::value);
 
 /** std::stringstream deals with locale so this is a lot faster than std::stringstream for UTF8 */
@@ -524,11 +541,11 @@ private:
 };
 
 using StringBuilder = StringBuilderImpl<SharedBufferAllocator>;
-using StackStringBuilder = StringBuilderImpl<StackAllocator>;
+using StackStringBuilder = StringBuilderImpl<StackAllocator<StackSizeDefault>>;
 
 extern template class BasicBufBuilder<SharedBufferAllocator>;
-extern template class BasicBufBuilder<StackAllocator>;
+extern template class BasicBufBuilder<StackAllocator<StackSizeDefault>>;
 extern template class StringBuilderImpl<SharedBufferAllocator>;
-extern template class StringBuilderImpl<StackAllocator>;
+extern template class StringBuilderImpl<StackAllocator<StackSizeDefault>>;
 
 }  // namespace mongo
diff --git a/src/mongo/bson/util/builder_test.cpp b/src/mongo/bson/util/builder_test.cpp
index a1c1f127eb0..a4ed4c87115 100644
--- a/src/mongo/bson/util/builder_test.cpp
+++ b/src/mongo/bson/util/builder_test.cpp
@@ -79,8 +79,8 @@ TEST(Builder, BooleanOstreamOperator) {
 }
 
 TEST(Builder, StackAllocatorShouldNotLeak) {
-    StackAllocator stackAlloc;
-    stackAlloc.malloc(StackAllocator::SZ + 1);  // Force heap allocation.
+    StackAllocator<StackSizeDefault> stackAlloc;
+    stackAlloc.malloc(StackSizeDefault + 1);  // Force heap allocation.
     // Let the builder go out of scope. If this leaks, it will trip the ASAN leak detector.
 }