[PATCH] Changes to std::variant to reduce code size

[Jonathan Wakely]

On 16/05/19 12:29 +0100, Jonathan Wakely wrote:
>These two changes both result in smaller code for std::variant.
>
>The first one means smaller tables of function pointers, because we
>don't generate an instantiation for the valueless state. Instead we do
>a runtime branch, marked [[unlikely]] to make _M_reset() a no-op if
>it's already valueless. In a microbenchmark I couldn't measure any
>performance difference due to the extra branch, so the code size
>reduction seems worthwhile.
>
>The second one removes a branch from the index() member by relying on
>unsigned arithmetic. That also results in smaller code and I can't see
>any downside.
>
>	* include/std/variant (_Variant_storage<false, _Types...>::_M_reset):
>	Replace raw visitation with a runtime check for the valueless state
>	and a non-raw visitor.
>	(_Variant_storage<false, _Types...>::_M_reset_impl): Remove.
>	(variant::index()): Remove branch.

We might also want:

--- a/libstdc++-v3/include/std/variant
+++ b/libstdc++-v3/include/std/variant
@@ -1503,7 +1503,7 @@ namespace __variant
                  }
                else
                  {
-                   if (this->index() != variant_npos)
+                   if (!this->valueless_by_exception()) [[__likely__]]
                      {
                        auto __tmp(std::move(__rhs_mem));
                        __rhs = std::move(*this);
@@ -1520,7 +1520,7 @@ namespace __variant
              }
            else
              {
-               if (this->index() != variant_npos)
+               if (!this->valueless_by_exception()) [[__likely__]]
                  {
                    __rhs = std::move(*this);
                    this->_M_reset();


This results in smaller code too, because for some specializations
valueless_by_exception() always returns false, so the branch can be
removed.

(This suggests that it's generally better to ask the yes/no question
"are you valid?" rather than "what is your index, and does it equal
this magic number?")

For specializations where a valueless state is possible we still
expect it to be very unlikely in practice, so the attribute should
help there.