[libcxx-commits] [libcxx] [lldb] [libcxx] adds size-based `__split_buffer` representation to unstable ABI (PR #139632)

[Christopher Di Bella via libcxx-commits]

================
@@ -78,23 +80,232 @@ public:
                       __split_buffer,
                       void>;
 
-  pointer __first_;
-  pointer __begin_;
-  pointer __end_;
-  _LIBCPP_COMPRESSED_PAIR(pointer, __cap_, allocator_type, __alloc_);
+  struct __data {
+    pointer __first_ = nullptr;
+    pointer __begin_ = nullptr;
+#ifdef _LIBCPP_ABI_SIZE_BASED_CONTAINERS
+    size_type __size_ = 0;
+    size_type __cap_ = 0;
+    allocator_type __alloc_;
+#else
+    pointer __end_ = nullptr;
+    _LIBCPP_COMPRESSED_PAIR(pointer, __cap_ = nullptr, allocator_type, __alloc_);
+#endif
+
+    _LIBCPP_CONSTEXPR_SINCE_CXX20 _LIBCPP_HIDE_FROM_ABI __data() = default;
+
+    _LIBCPP_CONSTEXPR_SINCE_CXX20 _LIBCPP_HIDE_FROM_ABI explicit __data(const allocator_type& __alloc)
+    : __alloc_(__alloc)
+    {}
+
+    _LIBCPP_CONSTEXPR_SINCE_CXX20 _LIBCPP_HIDE_FROM_ABI pointer begin() _NOEXCEPT {
+      return __begin_;
+    }
+    _LIBCPP_CONSTEXPR_SINCE_CXX20 _LIBCPP_HIDE_FROM_ABI const_pointer begin() const _NOEXCEPT {
+      return __begin_;
+    }
+
+    _LIBCPP_CONSTEXPR_SINCE_CXX20 _LIBCPP_HIDE_FROM_ABI pointer end() _NOEXCEPT {
+#ifdef _LIBCPP_ABI_SIZE_BASED_CONTAINERS
+      return __begin_ + __size_;
+#else
+      return __end_;
+#endif
+    }
+
+    _LIBCPP_CONSTEXPR_SINCE_CXX20 _LIBCPP_HIDE_FROM_ABI pointer end() const _NOEXCEPT {
+#ifdef _LIBCPP_ABI_SIZE_BASED_CONTAINERS
+      return __begin_ + __size_;
+#else
+      return __end_;
+#endif
+    }
----------------
cjdb wrote:

Here's what calling `end()` when [optimisations are disabled](https://godbolt.org/z/96Ph6sT1r) looks like with both implementations, across a handful of popular targets (or targets with increasing popularity). It looks like a fairly noticeable increase in code size, which risks pessimising debug performance. It's impossible to know if this is indeed a performance regression without benchmarks, and our load test infrastructure isn't set up to evaluate debug performance, which is why I wanted to keep the code consistent with what's currently there.

There isn't a need to debate the trade-off any further though. IIUC your goal here was to reduce the `#ifdef` granularity, which the current implementation does in a different way.

https://github.com/llvm/llvm-project/pull/139632