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

["mark at codesourcery dot com" <gcc-bugzilla at gcc dot gnu dot org>]

------- 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.

Thanks,



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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=14639