[Bug middle-end/36041] Speed up builtin_popcountll

["crrodriguez at opensuse dot org" <gcc-bugzilla@gcc.gnu.org>]

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

--- Comment #19 from Cristian Rodríguez <crrodriguez at opensuse dot org> ---
(In reply to Jakub Jelinek from comment #18)
> I think it is a bad idea to introduce the IFUNC into libgcc_s, because then
> while you speed up the few users of this builtin, you slow down all users of
> libgcc_s (pretty much all C++ programs and lots of C programs), because they
> will need to resolve the ifunc.  For a very heavily used builtin perhaps,
> but for a rarely used one it just isn't a good idea.  User's can just use
> multi-versioning themselves and use __builtin_popcount* in the
> multi-versioned function.

Hold on..Apparently I used ambiguous language in my comment.. adding ifuncs to
libgcc* was not my real suggestion, but to EMIT such IFUNC s in the resulting
final user code when the target environment allows it. One generic, one
hardware/arch specific.
>From gcc-bugs-return-425262-listarch-gcc-bugs=gcc.gnu.org@gcc.gnu.org Thu Jun 27 06:43:39 2013
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From: "glisse at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug middle-end/36041] Speed up builtin_popcountll
Date: Thu, 27 Jun 2013 06:43:00 -0000
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36041

--- Comment #20 from Marc Glisse <glisse at gcc dot gnu.org> ---
(In reply to Jakub Jelinek from comment #18)
> I think it is a bad idea to introduce the IFUNC into libgcc_s, because then
> while you speed up the few users of this builtin, you slow down all users of
> libgcc_s (pretty much all C++ programs and lots of C programs), because they
> will need to resolve the ifunc.

I assume it is only those that use the builtin at least once, no? At least
LD_DEBUG seems to say so. I have no idea how heavy the ifunc resolution is, so
ok. We are back to only considering the non-table version... (By the way,
shouldn't these builtins act like C99 inline functions, so we can sometimes
inline them at -O3 (it could also enable vectorization)? Or maybe they already
do and it's just that I didn't test hard enough)


(In reply to Cristian Rodríguez from comment #19)
> Hold on..Apparently I used ambiguous language in my comment.. adding ifuncs
> to libgcc* was not my real suggestion, but to EMIT such IFUNC s in the
> resulting final user code when the target environment allows it. One
> generic, one hardware/arch specific.

Not sure if that's much better. Ideally we'd clone the hot loop that uses it
and propagate the versioning to that, not just the instruction, but I don't
think we have any code for that. Although if gcc saw the full code:
if(__builtin_cpu_supports("popcnt"))_mm_popcnt_u64(x);else{call lib}, it might
already manage to clone the loop.
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From: "jbeulich at novell dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c/57725] conflicting language extensions
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http://gcc.gnu.org/bugzilla/show_bug.cgi?idW725

--- Comment #5 from jbeulich at novell dot com ---
How that? How is code supposed to find out then?

Perhaps briefly explaining where this is coming from originally might help: The
Xen hypervisor (as much as Linux) has a number of linker script constructs like

  .xsm_initcall.init : {
       __xsm_initcall_start = .;
       *(.xsm_initcall.init)
       __xsm_initcall_end = .;
  } :text

If there's no matching input section at all, the two boundary symbols will end
up equal. How would C code be supposed to find out if the comparison result is
unspecified?

And remember, this is not a problem with default visibility (presumably because
the code needs to be prepared for link time overrides of the symbols), but Xen
likes to get built with non-default visibility in order to avoid expensive GOT
indirections when accessing data despite the necessary use of -fPIC.