Issue #2148 - Shrink Vector from (usually) four pointers in size to three

when no inline storage is used.

See Bug 1338374

1 files changed, 43 insertions, 0 deletions

diff --git a/mfbt/tests/TestVector.cpp b/mfbt/tests/TestVector.cpp
index 24baf8eae7..9aed9e617c 100644
--- a/mfbt/tests/TestVector.cpp
+++ b/mfbt/tests/TestVector.cpp
@@ -396,6 +396,49 @@ mozilla::detail::VectorTesting::testInsert()
   MOZ_RELEASE_ASSERT(S::destructCount == 1);
 }
 
+// Declare but leave (permanently) incomplete.
+struct Incomplete;
+
+// We could even *construct* a Vector<Incomplete, 0> if we wanted.  But we can't
+// destruct it, so it's not worth the trouble.
+static_assert(sizeof(Vector<Incomplete, 0>) > 0,
+              "Vector of an incomplete type will compile");
+
+// Vector with no inline storage should occupy the absolute minimum space in
+// non-debug builds.  (Debug adds a laundry list of other constraints, none
+// directly relevant to shipping builds, that aren't worth precisely modeling.)
+#ifndef DEBUG
+
+template<typename T>
+struct NoInlineStorageLayout
+{
+  T* mBegin;
+  size_t mLength;
+  struct CRAndStorage {
+    size_t mCapacity;
+  } mTail;
+};
+
+// Only one of these should be necessary, but test a few of them for good
+// measure.
+static_assert(sizeof(Vector<int, 0>) == sizeof(NoInlineStorageLayout<int>),
+              "Vector of int without inline storage shouldn't occupy dead "
+              "space for that absence of storage");
+
+static_assert(sizeof(Vector<bool, 0>) == sizeof(NoInlineStorageLayout<bool>),
+              "Vector of bool without inline storage shouldn't occupy dead "
+              "space for that absence of storage");
+
+static_assert(sizeof(Vector<S, 0>) == sizeof(NoInlineStorageLayout<S>),
+              "Vector of S without inline storage shouldn't occupy dead "
+              "space for that absence of storage");
+
+static_assert(sizeof(Vector<Incomplete, 0>) == sizeof(NoInlineStorageLayout<Incomplete>),
+              "Vector of an incomplete class without inline storage shouldn't "
+              "occupy dead space for that absence of storage");
+
+#endif // DEBUG
+
 int
 main()
 {