Auto increase inlined capacity whenever it does not affect class' size.

PiperOrigin-RevId: 487292771
Change-Id: I2454e28fe91017fb2954a4ad194712fcafe893d2

absl/container/inlined_vector.h

@@ -661,10 +661,22 @@ class InlinedVector {
     ABSL_HARDENING_ASSERT(pos <= end());
 
     value_type dealias(std::forward<Args>(args)...);
+    // https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102329#c2
+    // It appears that GCC thinks that since `pos` is a const pointer and may
+    // point to uninitialized memory at this point, a warning should be
+    // issued. But `pos` is actually only used to compute an array index to
+    // write to.
+#if !defined(__clang__) && defined(__GNUC__)
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wmaybe-uninitialized"
+#endif
     return storage_.Insert(pos,
                            IteratorValueAdapter<A, MoveIterator<A>>(
                                MoveIterator<A>(std::addressof(dealias))),
                            1);
+#if !defined(__clang__) && defined(__GNUC__)
+#pragma GCC diagnostic pop
+#endif
   }
 
   // `InlinedVector::emplace_back(...)`

absl/container/inlined_vector_test.cc

@@ -1208,6 +1208,8 @@ TYPED_TEST_P(InstanceTest, CountConstructorsDestructorsOnMoveAssignment) {
 }
 
 TEST(CountElemAssign, SimpleTypeWithInlineBacking) {
+  const size_t inlined_capacity = absl::InlinedVector<int, 2>().capacity();
+
   for (size_t original_size = 0; original_size <= 5; ++original_size) {
     SCOPED_TRACE(original_size);
     // Original contents are [12345, 12345, ...]
@@ -1217,9 +1219,9 @@ TEST(CountElemAssign, SimpleTypeWithInlineBacking) {
                                   original_contents.end());
     v.assign(2, 123);
     EXPECT_THAT(v, AllOf(SizeIs(2u), ElementsAre(123, 123)));
-    if (original_size <= 2) {
+    if (original_size <= inlined_capacity) {
       // If the original had inline backing, it should stay inline.
-      EXPECT_EQ(2u, v.capacity());
+      EXPECT_EQ(v.capacity(), inlined_capacity);
     }
   }
 }
@@ -1360,6 +1362,8 @@ TEST(RangedConstructor, ElementsAreConstructed) {
 }
 
 TEST(RangedAssign, SimpleType) {
+  const size_t inlined_capacity = absl::InlinedVector<int, 3>().capacity();
+
   // Test for all combinations of original sizes (empty and non-empty inline,
   // and out of line) and target sizes.
   for (size_t original_size = 0; original_size <= 5; ++original_size) {
@@ -1367,13 +1371,13 @@ TEST(RangedAssign, SimpleType) {
     // Original contents are [12345, 12345, ...]
     std::vector<int> original_contents(original_size, 12345);
 
-    for (int target_size = 0; target_size <= 5; ++target_size) {
+    for (size_t target_size = 0; target_size <= 5; ++target_size) {
       SCOPED_TRACE(target_size);
 
       // New contents are [3, 4, ...]
       std::vector<int> new_contents;
-      for (int i = 0; i < target_size; ++i) {
-        new_contents.push_back(i + 3);
+      for (size_t i = 0; i < target_size; ++i) {
+        new_contents.push_back(static_cast<int>(i + 3));
       }
 
       absl::InlinedVector<int, 3> v(original_contents.begin(),
@@ -1382,9 +1386,10 @@ TEST(RangedAssign, SimpleType) {
 
       EXPECT_EQ(new_contents.size(), v.size());
       EXPECT_LE(new_contents.size(), v.capacity());
-      if (target_size <= 3 && original_size <= 3) {
+      if (target_size <= inlined_capacity &&
+          original_size <= inlined_capacity) {
         // Storage should stay inline when target size is small.
-        EXPECT_EQ(3u, v.capacity());
+        EXPECT_EQ(v.capacity(), inlined_capacity);
       }
       EXPECT_THAT(v, ElementsAreArray(new_contents));
     }
@@ -1470,9 +1475,12 @@ TEST(InitializerListConstructor, DisparateTypesInList) {
 }
 
 TEST(InitializerListConstructor, ComplexTypeWithInlineBacking) {
-  EXPECT_THAT((absl::InlinedVector<CopyableMovableInstance, 1>{
+  const size_t inlined_capacity =
+      absl::InlinedVector<CopyableMovableInstance, 1>().capacity();
+  EXPECT_THAT(
+      (absl::InlinedVector<CopyableMovableInstance, 1>{
           CopyableMovableInstance(0)}),
-              AllOf(SizeIs(1u), CapacityIs(1u), ElementsAre(ValueIs(0))));
+      AllOf(SizeIs(1u), CapacityIs(inlined_capacity), ElementsAre(ValueIs(0))));
 }
 
 TEST(InitializerListConstructor, ComplexTypeWithReallocationRequired) {
@@ -2022,4 +2030,31 @@ TEST(NonSwappableSwapTest, SwapThis) {
   EXPECT_THAT(v, Pointwise(HasValue(), {1, 2, 3}));
 }
 
+template <size_t N>
+using CharVec = absl::InlinedVector<char, N>;
+
+// Warning: This struct "simulates" the type `InlinedVector::Storage::Allocated`
+// to make reasonable expectations for inlined storage capacity optimization. If
+// implementation changes `Allocated`, then `MySpan` and tests that use it need
+// to be updated accordingly.
+template <typename T>
+struct MySpan {
+  T* data;
+  size_t size;
+};
+
+TEST(StorageTest, InlinedCapacityAutoIncrease) {
+  // The requested capacity is auto increased to `sizeof(MySpan<char>)`.
+  EXPECT_GT(CharVec<1>().capacity(), 1);
+  EXPECT_EQ(CharVec<1>().capacity(), sizeof(MySpan<char>));
+  EXPECT_EQ(CharVec<1>().capacity(), CharVec<2>().capacity());
+  EXPECT_EQ(sizeof(CharVec<1>), sizeof(CharVec<2>));
+
+  // The requested capacity is auto increased to
+  // `sizeof(MySpan<int>) / sizeof(int)`.
+  EXPECT_GT((absl::InlinedVector<int, 1>().capacity()), 1);
+  EXPECT_EQ((absl::InlinedVector<int, 1>().capacity()),
+            sizeof(MySpan<int>) / sizeof(int));
+}
+
 }  // anonymous namespace

absl/container/internal/inlined_vector.h

@@ -379,7 +379,9 @@ class Storage {
     return data_.allocated.allocated_capacity;
   }
 
-  SizeType<A> GetInlinedCapacity() const { return static_cast<SizeType<A>>(N); }
+  SizeType<A> GetInlinedCapacity() const {
+    return static_cast<SizeType<A>>(kOptimalInlinedSize);
+  }
 
   StorageView<A> MakeStorageView() {
     return GetIsAllocated() ? StorageView<A>{GetAllocatedData(), GetSize(),
@@ -483,8 +485,15 @@ class Storage {
     SizeType<A> allocated_capacity;
   };
 
+  // `kOptimalInlinedSize` is an automatically adjusted inlined capacity of the
+  // `InlinedVector`. Sometimes, it is possible to increase the capacity (from
+  // the user requested `N`) without increasing the size of the `InlinedVector`.
+  static constexpr size_t kOptimalInlinedSize =
+      (std::max)(N, sizeof(Allocated) / sizeof(ValueType<A>));
+
   struct Inlined {
-    alignas(ValueType<A>) char inlined_data[sizeof(ValueType<A>[N])];
+    alignas(ValueType<A>) char inlined_data[sizeof(
+        ValueType<A>[kOptimalInlinedSize])];
   };
 
   union Data {