[libcxx-commits] [libcxx] [libc++] Refactor how we do amortized growth (PR #171117)

[Louis Dionne via libcxx-commits]

================
@@ -2285,6 +2285,25 @@ private:
     return __long(__buffer, __capacity);
   }
 
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 __long
+  __allocate_long_buffer_for_amortized_growth(size_type __required_size) {
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ldionne wrote:

Instead, we could take the required size as an additional parameter of `__allocate_long_buffer`. That would address cases like in `reserve` where we need the same functionality without growing for amortized growth.

```
__long __buffer  = __allocate_long_buffer(__alloc_, /* used size */ __requested_capacity,
                                                    /* min capacity */ __requested_capacity);
```

This also has the benefit of documenting that we're growing to exactly what we need, no more.

The major benefit I see here is to keep allocation and the determination of the capacity we need separate, which I think is really neat in the current API.

https://github.com/llvm/llvm-project/pull/171117