[libcxx-commits] [libcxx] [libc++] Add assumption for align of begin and end pointers of vector. (PR #108961)

[Louis Dionne via libcxx-commits]

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@@ -1027,6 +1027,10 @@ private:
   }
 
   _LIBCPP_CONSTEXPR_SINCE_CXX20 _LIBCPP_HIDE_FROM_ABI void __move_assign_alloc(vector&, false_type) _NOEXCEPT {}
+
+  _LIBCPP_CONSTEXPR_SINCE_CXX20 _LIBCPP_HIDE_FROM_ABI pointer __add_alignment_assumption(pointer __p) const _NOEXCEPT {
+    return static_cast<pointer>(__builtin_assume_aligned(__p, alignof(decltype(*__p))));
+  }
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ldionne wrote:

Reintroducing @philnik777 's comment:

> Does this work with fancy pointers? Also, shouldn't the compiler be able to infer this?

No, that wouldn't work AFAIU and I don't think this can work for fancy pointers (which could be implemented in very unusual ways). We may need to guard this with `if constexpr (is_pointer<pointer>::value)`.

https://github.com/llvm/llvm-project/pull/108961