[ADT] Add resize_for_overwrite method to SmallVector.

Analagous to the std::make_(unqiue|shared)_for_overwrite added in c++20.
If T is POD, and the container gets larger, any new values added wont be initialized.
This is useful when using SmallVector as a buffer where its planned to overwrite any potential new values added.
If T is not POD, `new (Storage) T` functions identically to `new (Storage) T()` so this will function identically to `resize(size_type)`.

Reviewed By: dexonsmith

Differential Revision: https://reviews.llvm.org/D93532

1 files changed, 25 insertions, 0 deletions

diff --git a/llvm/unittests/ADT/SmallVectorTest.cpp b/llvm/unittests/ADT/SmallVectorTest.cpp
index 957412f..e02c3e7 100644
--- a/llvm/unittests/ADT/SmallVectorTest.cpp
+++ b/llvm/unittests/ADT/SmallVectorTest.cpp
@@ -341,6 +341,31 @@ TYPED_TEST(SmallVectorTest, ResizeFillTest) {
   this->assertValuesInOrder(this->theVector, 3u, 77, 77, 77);
 }
 
+TEST(SmallVectorTest, ResizeForOverwrite) {
+  {
+    // Heap allocated storage.
+    SmallVector<unsigned, 0> V;
+    V.push_back(5);
+    V.pop_back();
+    V.resize_for_overwrite(V.size() + 1);
+    EXPECT_EQ(5, V.back());
+    V.pop_back();
+    V.resize(V.size() + 1);
+    EXPECT_EQ(0, V.back());
+  }
+  {
+    // Inline storage.
+    SmallVector<unsigned, 2> V;
+    V.push_back(5);
+    V.pop_back();
+    V.resize_for_overwrite(V.size() + 1);
+    EXPECT_EQ(5, V.back());
+    V.pop_back();
+    V.resize(V.size() + 1);
+    EXPECT_EQ(0, V.back());
+  }
+}
+
 // Overflow past fixed size.
 TYPED_TEST(SmallVectorTest, OverflowTest) {
   SCOPED_TRACE("OverflowTest");