https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78113
Nir Friedman <quicknir at gmail dot com> changed:
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CC| |quicknir at gmail dot com
--- Comment #3 from Nir Friedman <quicknir at gmail dot com> ---
I noticed this issue independently recently, and in fact just mentioned it in
my cppcon presentation.
I think the optimal implementation is to do switch case on the index as a
special case when there is only one variant of up to N types (suggested value
N=10) passed to visit. Whatever the boost implementation is (someone mentioned
recursive if?), I do not think it is optimal. On the other hand, switch case
optimizes well: https://gcc.godbolt.org/z/ysKr5s. Note that clang with libc++
yields basically the same results. Also note the optimal assembly for Antony's
example.
I'm willing to write the implementation (basically, use a switch case for a
single visitor of up to 10 types and fall back to previous implementation
otherwise) if there's interest in this approach, or otherwise I'd be curious if
there are any objections to it. I also plan to approach clang and MSVC library
maintainers.
One final thing to note: even if gcc/clang get to the point that they can
inline the function pointers and get a jump table, that still won't really be
as fast as switch case. Switch case for instance won't typically turn into a
jump table for 2-3 options, presumably that's because the compiler thinks it's
actually faster to branch. It's unlikely that you'd see the same
transformations from an array of function pointers. Bottom line is that switch
case seems to give the compiler the most information, has the most man-years of
optimization behind it, so to me it makes a lot of sense to optimize variant by
using it for the common case.
