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Re: Issue with LTO/-fwhole-program


On Mon, Jun 14, 2010 at 11:27 AM, David Brown <david@westcontrol.com> wrote:
> On 14/06/2010 11:22, Dave Korn wrote:
>>
>> On 14/06/2010 05:43, Ian Lance Taylor wrote:
>>>
>>> David Brown<david.brown@hesbynett.no> ?writes:
>>>
>>>> After doing a bit more reading and thinking, it seems to me that
>>>> -fwhole-program will be used in most cases where LTO is used. ?You use
>>>> -flto when compiling each source file, then link them with gcc with
>>>> -flto and -fwhole-program. ?Except in the case of libraries or other
>>>> files which need external symbols, you will want that combination to
>>>> generate optimal code. ?So if this combination alone, without common
>>>> symbols, is going to cause problems, then this would be a much bigger
>>>> issue than if it is only triggered by common symbols.
>>>
>>> That scenario is fine.
>>>
>>> You can look back to see the problematic case posted earlier. ?It was
>>> a case where one file was compiled with -flto, one file was compiled
>>> without -flto, both files defined a common symbol with the same name,
>>> the object files were linked together using -flto -fwhole-program, and
>>> the gold plugin was not used. ?All elements are essential to recreate
>>> the problem.
>>
>> ? Given how many standard libraries export common symbols, I wonder if it
>> won't actually happen quite often. ?"nm /usr/lib/*.a | grep ' C '" gives a
>> fair few hits for me.
>>
>
> It seems unlikely that common symbols in a library will be a problem, since
> it is very unlikely that the same common symbols will be defined in the
> library /and/ in user code, and it is very unlikely that different parts of
> the library would be compiled with different -flto settings.
>
> However, such libraries would give false (hopefully false!) positives in a
> warning message triggered by the use of common symbols. ?Is it possible for
> such a warning message to distinguish between common symbols from library
> files and common symbols from object files? ?Would that be too risky?

Commons between shared libraries and a program can't work.

Richard.


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