ADT: Add SmallVectorImpl::truncate() to replace uses of set_size()

Add `SmallVectorImpl::truncate()`, a variant of `resize()` that cannot
increase the size.

- Compared to `resize()`, this has no code path for growing the
  allocation and can be better optimized.
- Compared to `set_size()`, this formally calls destructors, and does
  not skip any constructors.
- Compared to `pop_back_n()`, this takes the new desired size, which in
  many contexts is more intuitive than the number of elements to remove.

The immediate motivation is to pair this with `resize_for_overwrite()`
to remove uses of `set_size()`, which can then be made private.

Differential Revision: https://reviews.llvm.org/D115383

1 files changed, 26 insertions, 0 deletions

diff --git a/llvm/unittests/ADT/SmallVectorTest.cpp b/llvm/unittests/ADT/SmallVectorTest.cpp
index 06b98ef..3fbea52 100644
--- a/llvm/unittests/ADT/SmallVectorTest.cpp
+++ b/llvm/unittests/ADT/SmallVectorTest.cpp
@@ -309,6 +309,32 @@ TYPED_TEST(SmallVectorTest, ResizeShrinkTest) {
   EXPECT_EQ(5, Constructable::getNumDestructorCalls());
 }
 
+// Truncate test.
+TYPED_TEST(SmallVectorTest, TruncateTest) {
+  SCOPED_TRACE("TruncateTest");
+
+  this->theVector.reserve(3);
+  this->makeSequence(this->theVector, 1, 3);
+  this->theVector.truncate(1);
+
+  this->assertValuesInOrder(this->theVector, 1u, 1);
+  EXPECT_EQ(6, Constructable::getNumConstructorCalls());
+  EXPECT_EQ(5, Constructable::getNumDestructorCalls());
+
+#if !defined(NDEBUG) && GTEST_HAS_DEATH_TEST
+  EXPECT_DEATH(this->theVector.truncate(2), "Cannot increase size");
+#endif
+  this->theVector.truncate(1);
+  this->assertValuesInOrder(this->theVector, 1u, 1);
+  EXPECT_EQ(6, Constructable::getNumConstructorCalls());
+  EXPECT_EQ(5, Constructable::getNumDestructorCalls());
+
+  this->theVector.truncate(0);
+  this->assertEmpty(this->theVector);
+  EXPECT_EQ(6, Constructable::getNumConstructorCalls());
+  EXPECT_EQ(6, Constructable::getNumDestructorCalls());
+}
+
 // Resize bigger test.
 TYPED_TEST(SmallVectorTest, ResizeGrowTest) {
   SCOPED_TRACE("ResizeGrowTest");