On Tue, 24 Sep 2019, Jonathan Wakely wrote:
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.
I thought the code only allowed unsigned char and unsigned short, so it
would require a platform where size_t is the same as one of those...
So doing the (sizeof...(_Types) <= __index_type(-1)/2
case first might make more sense.
Er, from a codegen point of view, I would rather start with the simplest
version (the zero-extension, as in the current code).
--
Marc Glisse
