Issue with LTO/-fwhole-program

[David Brown]

Dave Korn wrote:
> On 13/06/2010 20:55, Ian Lance Taylor wrote:
>> David Brown <david@westcontrol.com> writes:
>>
>>> If -flto were to activate the -fno-common flag, would that then catch 
>>> these potential problems with a linker error?
>> We could perhaps do that for C/C++ code, but Fortran relies on common 
>> symbols.
> 
> Well we shouldn't do it for plain C either, or at the very least should make
> it depend on the -std= option in effect, but since the code is entirely valid
> and legitimate C, I think we should acknowledge this is a weakness in our
> compiler. The original testcase is a perfectly straightforward bit of C89;
> there are two compatible tentative declarations of a variable of type int
> called "v". We don't want to have to argue that one is in fact a variable of
> type "int compiled with LTO" in order to back-justify some argument that they
> are not the same and this example violates some (vague and not
> standard-specified) C equivalent of the ODR.
> 
>     cheers,
>       DaveK
> 

I agree that banning common symbols is not a complete solution.  But 
perhaps it may be a partial solution until some more complete solution 
is found and implemented?

How about having a warning flag -Wcommon-lto which will produce a 
warning if LTO is used along with common data, and which is enabled by 
default by -flto?  For code that does not use common symbols, everything 
works as expected and the user gets LTO'ed code.  But if the code /does/ 
use common symbols, you get a warning that the code may be incorrect if 
a common symbol is declared in both LTO and non-LTO object files.  It is 
then up to the user to ignore the warning, or fix the common symbols.

This is not a complete fix - ideally, LTO should work fine even with 
common symbols.  And failing that, the warning (or error) message should 
only come if there really is a conflict, rather than just a potential 
conflict.  But I would think that a check like this is a relatively 
simple feature, and would add an extra level of safety to LTO.

Incidentally, the original poster used -fwhole-program.  Is this 
conflict only an issue when the -fwhole-program flag is used?  If that's 
the case, then the warning could be conditional on that flag too, 
meaning even rarer circumstances when false positives would be issued.

David