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Re: [GOOGLE] Disable -fdevirtualize by default


It was in upstream 4.9 branch, but got reverted in r215061 to fix
PR/61214 and PR/62224.

David

On Sun, Oct 19, 2014 at 1:21 AM, Jan Hubicka <hubicka@ucw.cz> wrote:
>> Hi Honza,
>>
>> As David says, we will do some more experiments with the change you
>> suggest and speculative devirtualization, but we needed to make this
>> change in part to get an internal release out. One of the issues was a
>> recent change to cp/decl2.c to make virtual function decls needed
>> under flag_devirtualize.
>
> I think that one is not in current 4.9 branch (only in mainline).
>
> Honza
>>
>> Thanks!
>> Teresa
>>
>> On Sat, Oct 18, 2014 at 4:22 PM, Jan Hubicka <hubicka@ucw.cz> wrote:
>> >>
>> >> The virtual functions will be emitted in some modules, right? If they
>> >> are hot, they will be included with the auxiliary modules. Note that
>> >> LIPO indirect call profile will point to the comdat copy that is
>> >> picked by the linker in the instrumentation build, so it guarantees to
>> >> exist.
>> >
>> > If you have COMDAT virtual inline function that is used by module FOO and
>> > LIPO's indirect inlining will work out that it goes to comdat copy of module BAR
>> > (that won the merging).  It will make C++ parser to also parse BAR to get
>> > the function, but callgarph builder will ignore it, too.
>> > (this is new logic in analyze_function that with -fno-devirtualize will just
>> > prevent all virtual functions not directly reachable to appear in symbol table)
>> >
>> > I am surprised you hit the size limits with 4.9 only - for quite some time
>> > we keep all virtual functions in callgarph until inlining. In fact 4.9 is first
>> > that works harder to drop them early (because I hit the problem with LTO
>> > where they artifically bloat the size of LTO object files)
>> >
>> > Honza
>> >>
>> >> David
>> >>
>> >>
>> >> >
>> >> > Honza
>> >> >>
>> >> >> David
>> >> >>
>> >> >>
>> >> >> On Sat, Oct 18, 2014 at 10:10 AM, Jan Hubicka <hubicka@ucw.cz> wrote:
>> >> >> >> Disabling devirtualization reduces code size, both for instrumentation (because
>> >> >> >> many more virtual functions are kept longer and therefore instrumented) and for
>> >> >> >> normal optimization.
>> >> >> >
>> >> >> > OK, with profile instrumentation (that you seem to try to minimize) i can see
>> >> >> > how you get noticeably more counters because virtual functions are kept longer.
>> >> >> > (note that 4.9 is a lot more agressive on removing unreacable virtual functions
>> >> >> > than earlier compilers).
>> >> >> >
>> >> >> > Instead of disabling -fdevirtualize completely (that will get you more indirect
>> >> >> > calls and thus more topn profiling) you may consider just hacking
>> >> >> > ipa.c:walk_polymorphic_call_targets to not make the possible targets as
>> >> >> > reachable. (see the conditional on before_inlining_p).
>> >> >> >
>> >> >> > Of course this will get you less devirtualization (but with LTO the difference
>> >> >> > should not be big - perhaps I could make switch for that for mainline) and less
>> >> >> > accurate profiles when you get speculative devirtualization via topn.
>> >> >> >
>> >> >> > I would be very interested to see how much difference this makes.
>> >> >> >
>> >> >> > Honza
>>
>>
>>
>> --
>> Teresa Johnson | Software Engineer | tejohnson@google.com | 408-460-2413


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