llvm dev - Dec 2015 - weak definitions in LTO

Hi,

I have a question regarding the behavior of weak symbol resolution in LTO:

Suppose there are weak definitions in both the source code and some 
native lib.
In non-LTO path, we will use the version from source code.
In LTO path, LLVM may discard the definition as it has "linkeonce" 
linkage type. So the native version will be selected by linker.
Now, non-LTO and LTO build may have different behavior.

The question is do we need to make non-LTO and LTO have the same 
behavior? If so, the LTO part needs to create pseudo uses (or some other 
mechanism) to keep the symbol from deletion.

Thanks,
Weiming

On 8 December 2015 at 18:04, Zhao, Weiming via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
> Hi,
>
> I have a question regarding the behavior of weak symbol resolution in LTO:
>
> Suppose there are weak definitions in both the source code and some native
> lib.
> In non-LTO path, we will use the version from source code.
> In LTO path, LLVM may discard the definition as it has
"linkeonce" linkage
> type. So the native version will be selected by linker.
> Now, non-LTO and LTO build may have different behavior.
>
> The question is do we need to make non-LTO and LTO have the same behavior?
> If so, the LTO part needs to create pseudo uses (or some other mechanism)
to
> keep the symbol from deletion.

On the gold plugin this was pr19901.

It was fixed by having the plugin convert linkonce to weak when the
resolution is PREVAILING_DEF (i.e. there is an use from a native
file).

Cheers,
Rafael

Hi Rafael,

Nice.
That should fix the problem.

Thanks,
Weiming



On 12/11/2015 3:11 PM, Rafael Espíndola wrote:
> On 8 December 2015 at 18:04, Zhao, Weiming via llvm-dev
> <llvm-dev at lists.llvm.org> wrote:
>> Hi,
>>
>> I have a question regarding the behavior of weak symbol resolution in
LTO:
>>
>> Suppose there are weak definitions in both the source code and some
native
>> lib.
>> In non-LTO path, we will use the version from source code.
>> In LTO path, LLVM may discard the definition as it has
"linkeonce" linkage
>> type. So the native version will be selected by linker.
>> Now, non-LTO and LTO build may have different behavior.
>>
>> The question is do we need to make non-LTO and LTO have the same
behavior?
>> If so, the LTO part needs to create pseudo uses (or some other
mechanism) to
>> keep the symbol from deletion.
>
> On the gold plugin this was pr19901.
>
> It was fixed by having the plugin convert linkonce to weak when the
> resolution is PREVAILING_DEF (i.e. there is an use from a native
> file).
>
> Cheers,
> Rafael