This is the mail archive of the gcc@gcc.gnu.org mailing list for the GCC project.


Index Nav: [Date Index] [Subject Index] [Author Index] [Thread Index]
Message Nav: [Date Prev] [Date Next] [Thread Prev] [Thread Next]
Other format: [Raw text]

Re: g++keeps unused objects with virtual functions


On Wed, Apr 8, 2015 at 11:59 AM, Stefan Ehrlich
<stefan.ehrlich@ehrlich.eu> wrote:
> But without the virtual keyword the global object never read is completely removed, so the statement "[...]but we don't have any pass removing stores to globals never read[...]" is only true with virtual functions.
> When I create the object on the stack, the optimization removes the object even with virtual functions completely (resp. partially when only a part of the object is used).
>
> The SomeData is removed completely, when created on stack (regardless I call the function virtFunction) - when the commented out object NotUsedObject is getting back into the code, the vtable, the virtual function, and the 100 ints are present in the executable.
>
> class CObject
> {
> public: virtual void virtFunction() {}
> public: int SomeData[100];
> };
>
> //CObject NotUsedObject;
>
> int main()
> {
>         CObject UsedObject;
>         UsedObject.virtFunction();
>         return 0;
> }
>
> So what can I do?

Nothing (well, improve GCC!).

Richard.

> Stefan
>
> -----UrsprÃngliche Nachricht-----
> Von: Jan Hubicka [mailto:hubicka@ucw.cz]
> Gesendet: Mittwoch, 08. April 2015 11:00
> An: Jan Hubicka
> Cc: Richard Biener; Stefan Ehrlich; GCC Development; zlynx@acm.org
> Betreff: Re: g++keeps unused objects with virtual functions
>
>> > which shows how the global objects initialization keeps things live.
>> > Early optimization turns it into
>> >
>> > (static initializers for t.C) ()
>> > {
>> >   <bb 2>:
>> >   NotUsedObject._vptr.CObject = &MEM[(void *)&_ZTV7CObject + 16B];
>> >   return;
>> >
>> > }
>> >
>> > but we don't have any pass removing stores to globals never read.
>> > IPA reference computes this in some way but doesn't get to export
>> > this in a reasonable way - its transform stage could DSE those
>> > though.
>>
>> We have write only variable detection but
>> > NotUsedObject/4 (CObject NotUsedObject) @0x7f70d41fe580
>> >   Type: variable definition analyzed
>> >   Visibility: public
>> >   References:
>> >   Referring: _Z41__static_initialization_and_destruction_0ii/9 (addr)
>> >   Availability: not-ready
>> >   Varpool flags:
>>
>> it stops on fact believing that address of NotUsedObject is taken. Why
>> it is not considered to be load?
>
> Too early dump I guess. At mainline I get empty constructor:
>         .type   _GLOBAL__sub_I_NotUsedObject, @function
> _GLOBAL__sub_I_NotUsedObject:
> .LFB6:
>         rep ret
>
> not ideal, but still an improvement ;)
> The problem is that write only var is removed only at late optimization and then it is too late to remove the unreachable function.
>
> Honza
>>
>> Honza


Index Nav: [Date Index] [Subject Index] [Author Index] [Thread Index]
Message Nav: [Date Prev] [Date Next] [Thread Prev] [Thread Next]