[PATCH] PR libstdc++/91788 improve codegen for std::variant<T...>::index()

[Marc Glisse]

On Mon, 23 Sep 2019, Jonathan Wakely wrote:

> If __index_type is a smaller type than size_t, then the result of
> size_t(__index_type(-1)) is not equal to size_t(-1), but to an incorrect
> value such as size_t(255) or size_t(65535). The old implementation of
> variant<T...>::index() uses (size_t(__index_type(_M_index + 1)) - 1)
> which is always correct, but generates suboptimal code for many common
> cases.
>
> When the __index_type is size_t or valueless variants are not possible
> we can just return the value directly.
>
> When the number of alternatives is sufficiently small the result of
> converting the _M_index value to the corresponding signed type will be
> either non-negative or -1. In those cases converting to the signed type
> and then to size_t will either produce the correct positive value or
> will sign extend -1 to (size_t)-1 as desired.
>
> For the remaining case we keep the existing arithmetic operations to
> ensure the correct result.
>
> 	PR libstdc++/91788 (partial)
> 	* include/std/variant (variant::index()): Improve codegen for cases
> 	where conversion to size_t already works correctly.
>
> Tested x86_64-linux, committed to trunk.

Thanks.

+       if constexpr (is_same_v<__index_type, size_t>)
+         return this->_M_index;

I don't think this special case is useful, gcc has no trouble optimizing 
the other 2 versions to nothing when the types are the same. Of course it 
won't hurt either.

-- 
Marc Glisse