[Bug c++/14639] [3.4/3.5 Regression] [unit-at-a-time] Incorrect emission of unused compiler-generated destructor at -O2

["hubicka at ucw dot cz" <gcc-bugzilla at gcc dot gnu dot org>]

------- Additional Comments From hubicka at ucw dot cz  2004-03-26 10:01 -------
Subject: Re:  [3.4/3.5 Regression] [unit-at-a-time] Incorrect emission of unused compiler-generated destructor at -O2

> 
> ------- Additional Comments From mark at codesourcery dot com  2004-03-26 08:23 -------
> Subject: Re:  [3.4/3.5 Regression] [unit-at-a-time] Incorrect
>  emission of unused compiler-generated destructor at -O2
> 
> hubicka at ucw dot cz wrote:
> 
> > It will fix the problem because the thunk is only output at the time
> > function is assembled.  But it will make mark_referenced useless anyway
> > as when the function is assembled it is already mark_reference-d.
> > Removing the call as done by my patch should be fine then, but we must
> > ensure that referencing the vtable that references the thunk makes
> > cgraph code notice that the function referenced by the thunk is
> > referenced too, so the dependencies goes right.  I am not quite sure how
> > to construct testcase, but I will try to dig into it.  Obviously the
> > testsuite works so it seems to be fine in majority of case but my
> > experience is that there are common dead ends not covered by testsuite
> > in these areas, so I still feel unsafe about that patch, but all my
> > attempts to construct testcase seems to be failing.
> 
> I am not sure that the case you are worried about can happen.
> 
> For that to happen, there would have to be a vtable that contains a 
> reference to a thunk, but no references to the function to which the 
> thunk refers.  The only way a thunk to C::f can occur is in a vtable for 
>   C, or a base subobject of C.  But, in that case, C::f will also be 
> referenced directly from the vtable.
> 
> >>>How much incredible it is?  (ie having testcase that exhibits a lot of
> >>>unnecesary thunks/vtables would be usefull too). 
> >>
> >>Just take the original test case, but add thousands of classes.  That's 
> >>a very typical situation for some class frameworks.
> > 
> > 
> > We obvisouly emit unnecesary thunks, lets see what we can do about this.
> > I tried to compile mozilla with the patch fixing the testcase and
> > measured no difference in code size tought...
> 
> Many of the "unncessary" thunks are required by the ABI.  The ABI 
> guarantees that all thunks are put out with the function to which they 
> transfer control.
> 
> Based on our discussion, I think your patch is OK.  Please check it in, 
> but remove the call to mark_referenced copmletely, rather than 
> commenting it out.

OK, thanks!  I will try to investigate the remaining 3 calls if they can
cause similar showstopper too.

Honza
> 
> Thanks,
> 
> 
> 
> -- 
> 
> 
> http://gcc.gnu.org/bugzilla/show_bug.cgi?id=14639
> 
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=14639