[Bug middle-end/36041] Speed up builtin_popcountll

["glisse at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36041

--- Comment #12 from Marc Glisse <glisse at gcc dot gnu.org> ---
Created attachment 30381
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=30381&action=edit
IFUNC proof of concept patch

Sadly, libgcc is compiled with gcc and not g++ so we can't use the recent
multiversioning support with the target attribute and we have to manually set
up ifunc. Note that the si/di difference is not a typo, just a wart in the way
libgcc is configured.

This is just a proof of concept, we'd want to replace also __popcountti2 at
least. And most importantly we need to restrict the inclusion of t-ifunc to
platforms where ifunc is supported (move it elsewhere in config.host, maybe
even include the content of t-ifunc in an existing t-*).

There are probably better ways to organize this, putting the generic
implementation in libgcc2.c protected by suitable macros (which ones?) so it
benefits also darwin/cygwin (no ifunc) and non-x86 platforms.

I didn't check the generic code, I just pasted it from one of the comments.

If you want this to happen, please work out a patch and post it to gcc-patches
(you can start from this one or not), don't wait for others to write one, I
won't have more time to spend on this. Don't be too afraid to test the wrong
macro, the reviewer will tell you if that is the case.