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* [Bug libstdc++/78113] std::variant and std::visit's current implementations do not get optimized out (compared to "recursive visitation")
[not found] <bug-78113-4@http.gcc.gnu.org/bugzilla/>
@ 2021-05-12 9:52 ` redi at gcc dot gnu.org
2021-10-01 19:38 ` cvs-commit at gcc dot gnu.org
2021-10-01 19:54 ` redi at gcc dot gnu.org
2 siblings, 0 replies; 3+ messages in thread
From: redi at gcc dot gnu.org @ 2021-05-12 9:52 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78113
--- Comment #7 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Fixed in my fork (for visiting a single variant of up to 11 alternative types):
https://gitlab.com/jonathan-wakely/gcc/-/commit/f5de3d9055e47b73cd1c4d35c235ebaa3f93a5e6
^ permalink raw reply [flat|nested] 3+ messages in thread
* [Bug libstdc++/78113] std::variant and std::visit's current implementations do not get optimized out (compared to "recursive visitation")
[not found] <bug-78113-4@http.gcc.gnu.org/bugzilla/>
2021-05-12 9:52 ` [Bug libstdc++/78113] std::variant and std::visit's current implementations do not get optimized out (compared to "recursive visitation") redi at gcc dot gnu.org
@ 2021-10-01 19:38 ` cvs-commit at gcc dot gnu.org
2021-10-01 19:54 ` redi at gcc dot gnu.org
2 siblings, 0 replies; 3+ messages in thread
From: cvs-commit at gcc dot gnu.org @ 2021-10-01 19:38 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78113
--- Comment #8 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Jonathan Wakely <redi@gcc.gnu.org>:
https://gcc.gnu.org/g:cfb582f62791dfadc243d97d37f0b83ef77cf480
commit r12-4068-gcfb582f62791dfadc243d97d37f0b83ef77cf480
Author: Jonathan Wakely <jwakely@redhat.com>
Date: Tue May 4 23:31:48 2021 +0100
libstdc++: Optimize std::visit for the common case [PR 78113]
GCC does not do a good job of optimizing the table of function pointers
used for variant visitation. This avoids using the table for the common
case of visiting a single variant with a small number of alternative
types. Instead we use:
switch(v.index())
{
case 0: return visitor(get<0>(v));
case 1: return visitor(get<1>(v));
...
}
It's not quite that simple, because get<1>(v) is ill-formed if the
variant only has one alternative, and similarly for each get<N>. We
need to ensure each case only applies the visitor if the index is in
range for the actual type we're dealing with, and tell the compiler that
the case is unreachable otherwise. We also need to invoke the visitor
via the __gen_vtable_impl::__visit_invoke function, to handle the raw
visitation cases used to implement std::variant assignments and
comparisons.
Because that gets quite verbose and repetitive, a macro is used to stamp
out the cases.
We also need to handle the valueless_by_exception case, but only for raw
visitation, because std::visit already checks for it before calling
__do_visit.
Signed-off-by: Jonathan Wakely <jwakely@redhat.com>
libstdc++-v3/ChangeLog:
PR libstdc++/78113
* include/std/variant (__do_visit): Use a switch when we have a
single variant with a small number of alternatives.
^ permalink raw reply [flat|nested] 3+ messages in thread
* [Bug libstdc++/78113] std::variant and std::visit's current implementations do not get optimized out (compared to "recursive visitation")
[not found] <bug-78113-4@http.gcc.gnu.org/bugzilla/>
2021-05-12 9:52 ` [Bug libstdc++/78113] std::variant and std::visit's current implementations do not get optimized out (compared to "recursive visitation") redi at gcc dot gnu.org
2021-10-01 19:38 ` cvs-commit at gcc dot gnu.org
@ 2021-10-01 19:54 ` redi at gcc dot gnu.org
2 siblings, 0 replies; 3+ messages in thread
From: redi at gcc dot gnu.org @ 2021-10-01 19:54 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78113
Jonathan Wakely <redi at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Target Milestone|--- |12.0
Resolution|--- |FIXED
Status|NEW |RESOLVED
--- Comment #9 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Fixed for GCC 12.
^ permalink raw reply [flat|nested] 3+ messages in thread
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2021-05-12 9:52 ` [Bug libstdc++/78113] std::variant and std::visit's current implementations do not get optimized out (compared to "recursive visitation") redi at gcc dot gnu.org
2021-10-01 19:38 ` cvs-commit at gcc dot gnu.org
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