Stop allowing use of `SmallVectorBase::set_size()` outside of the
SmallVector implementation, which sets the size without calling
constructors or destructors.
Most callers should probably just use `resize()`. Or, if the new size is
guaranteed to be `<= size()`, then the new-ish `truncate()` works too
(and optimizes better).
Some callers want to avoid initializing memory before overwriting, but
need a pointer to the memory and so cannot use `push_back()`,
`emplace_back()`, or `append()`. Before this commit, this depended on
`reserve()` and `set_size()`:
```
V.reserve(V.size() + NumNew); // Reserve expected size.
NumNew = initialize(V.end(), ...); // Get number added.
V.set_size(V.size() + NumNew); // Set size to match.
```
Such code should be updated to use `resize_for_overwrite()` and
`truncate()`:
```
auto Size = V.size(); // Save initial size.
V.resize_for_overwrite(Size + NumNew); // Resize to expected size.
NumNew = initialize(V.begin() + Size, ...)); // Get number added.
V.truncate(Size + NumNew); // Truncate to match.
```
The new pattern is safe even for non-trivial types, since
`resize_for_overwrite()` calls constructors and `truncate()` calls
destructors. For trivial types, it should optimize the same way as the
old pattern.
Downstream code adapt to the disappearance of `set_size()` using this
new pattern should carefully audit uses of `V` between the resize and
the truncate:
- Change `V.size()` => `Size`.
- Change `V.capacity()` => `V.size()` (mostly).
- Change `V.end()` => `V.begin() + Size`.
- If `V` is an out-parameter, early returns need a `V.truncate()` or
`V.clear()`. A scope exit is recommended.
Differential Revision: https://reviews.llvm.org/D115380
LLVM_NODISCARD bool empty() const { return !Size; }
+protected:
/// Set the array size to \p N, which the current array must have enough
/// capacity for.
///
/// This does not construct or destroy any elements in the vector.
- ///
- /// Clients can use this in conjunction with capacity() to write past the end
- /// of the buffer when they know that more elements are available, and only
- /// update the size later. This avoids the cost of value initializing elements
- /// which will only be overwritten.
void set_size(size_t N) {
assert(N <= capacity());
Size = N;
}
private:
+ // Make set_size() private to avoid misuse in subclasses.
+ using SuperClass::set_size;
+
template <bool ForOverwrite> void resizeImpl(size_type N) {
if (N == this->size())
return;