[PATCH] D48342: [libcxx] Optimize vectors construction of trivial types from an iterator range with const-ness mismatch.

[Erik Pilkington via Phabricator via cfe-commits]

erik.pilkington added inline comments.


================
Comment at: libcxx/include/memory:1479
+struct __has_construct_missing
+    : false_type
+{
----------------
vsapsai wrote:
> erik.pilkington wrote:
> > Shouldn't this be true_type?
> I see this is confusing and I'm still struggling how to express it. The issue is that in C++03 `__has_construct` should be something like unknown, so that neither `__has_construct` nor `! __has_construct` evaluate to true because we don't really know if allocator has construct. This case is covered by the added test, in C++03 the memcpy specialization was enabled when
> 
> ```
>     is_same<allocator_type, allocator<_Tp> >
>       || !false_type
> ```
> 
> So `is_same` check had no effect and we were using memcpy to convert between int and float.
> 
> I was considering using something like
> 
> ```lang=c++
>         typename enable_if
>         <
>             (is_same
>              <
>                 typename _VSTD::remove_const<typename allocator_type::value_type>::type,
>                 typename _VSTD::remove_const<_SourceTp>::type
>              >::value
> #ifndef _LIBCPP_CXX03_LANG
>                 || !__has_construct<allocator_type, _DestTp*, _SourceTp>::value
> #endif
>             ) &&
>              is_trivially_move_constructible<_DestTp>::value,
>             void
>         >::type
> ```
> 
> But that doesn't look readable to me, so I've introduced ad-hoc ternary logic with `__has_construct_missing`.
Oh I see, yikes! That's a pretty bad bug. I agree that this fix is best then, but can you add a comment explaining this to `__has_construct_missing` for future casual readers? Also, I think we should move the __has_construct_missing bugfix into a different (prerequisite) patch. Seems unrelated to the `const` related optimization below.


https://reviews.llvm.org/D48342