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

[Jonathan Wakely]

On 24/09/19 09:57 +0100, Jonathan Wakely wrote:
>On 23/09/19 19:39 +0200, Marc Glisse wrote:
>>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.
>
>My rationale was that it's much cheaper to instantiate is_same_v than
>the __never_valueless<T...>() check (and will be even cheaper after
>the concepts-cxx2a branch merges, as I plan to make is_same_v use the
>__is_same_as built-in to avoid instantiating the std::is_same class
>template).
>
>That's probably not a big saving, as the __never_valueless function
>template will almost certainly be used by some other member function
>anyway.

On the other hand ... a variant with size_t as the index type is
probably vanishingly rare, because it would need tens of thousands of
alternatives. So doing the (sizeof...(_Types) <= __index_type(-1)/2
case first might make more sense.