Issue with LTO/-fwhole-program

[Richard Guenther]

On Sat, Jun 12, 2010 at 3:32 PM, David Brown <david.brown@hesbynett.no> wrote:
> Ian Lance Taylor wrote:
>>
>> Manuel López-Ibáñez <lopezibanez@gmail.com> writes:
>>
>>> This also means that linking your program with non-LTO+whole-program
>>> code may lead to miscompilations without any warning, which is really
>>> bad. I don't think it is a reasonable limitation and we will get bad
>>> press when programs start breaking for users. They won't care (and
>>> they won't even listen) about the reasons. The conclusion will be: LTO
>>> is broken in GCC, and just use another compiler.
>>
>> The limitation isn't all that bad.  If you want to use
>> -fwhole-program, I think the basic rule is this: compile all your code
>> with -flto, and don't define variables or functions which are
>> referenced by any code which is not yours.
>>
>> I think adding a warning for this case would be great, if we can
>> figure out how to do it.  But I also think that a clear statement in
>> the documentation will avoid most user problems.
>>
>> Ian
>>
>
> To me, this sounds like you are safe as long as you stick to the "one
> definition rule".  This is a requirement for C++ - the original example with
> "int v" definitions in two files would be illegal in C++, while it is legal
> in C.

Correct.  In failure mode with -fno-common you would get a linker
error instead of silent miscompiles.

> Assuming I'm correct here, then perhaps there could be a warning or error
> message that is triggered by breaking the ODR, and which could be enabled
> automatically by the -flto flag.  Perhaps existing checking mechanisms from
> C++ can be used here.

It's not easy without using the gold linker-plugin as we do not see the
other definition.  With using the gold linker-plugin we can detect the
situation and do the right thing (as proposed by the patches from Binfeng).

Richard.

> David
>
>