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

[Jonathan Wakely]

On 16/05/19 12:43 +0100, Jonathan Wakely wrote:
>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.

Here's what I've tested and am about to commit.


-------------- next part --------------
A non-text attachment was scrubbed...
Name: patch.txt
Type: text/x-patch
Size: 2395 bytes
Desc: not available
URL: <http://gcc.gnu.org/pipermail/gcc-patches/attachments/20190516/3a0beb28/attachment.bin>