[llvm-dev] LLD doesn't handle globals with appending linkage

[Fangrui Song via llvm-dev]

On 2020-02-20, Gleb Popov via llvm-dev wrote:
>On Tue, Feb 18, 2020 at 12:28 PM Gleb Popov <6yearold at gmail.com> wrote:
>
>> Hello.
>>
>> I'm posting this question here, because there seem to be no LLD-specific
>> mailing list. Sorry in advance if this is wrong one.
>>
>> I compile two C source with following command:
>>
>> clang -flto -o %name.bc %name.c
>>
>> LLVM is augmented with my custom pass, which amongst other things create a
>> global with appending linkage:
>>
>> @myvar = appending constant [1 x [1 x i8]*] ...
>>
>> I also have another pass plugged into LTO pipeline, which turns this
>> global into internal one in the final module. I was hoping that LLD would
>> first link bitcodes like llvm-link, appending @myvar's from both modules
>> into a single one, then run my pass, and only then perform linking at
>> object level.
>>
>> However, LLD complains about duplicated symbol "myvar" and doesn't even
>> run any passes.
>>
>> I've tracked the problem down to BitcodeFile::parse() function from
>> https://github.com/llvm/llvm-project/blob/master/lld/COFF/InputFiles.cpp#L918
>> - it doesn't take linkage type into account.
>>
>> Can anything be done about this problem? Or was my approach broken from
>> the beginning?
>>
>> Thanks in advance.
>>
>
>
>The following patch has fixed the problem for me:
>
>diff --git a/llvm/lib/Object/ModuleSymbolTable.cpp
>b/llvm/lib/Object/ModuleSymbolTable.cpp
>index 33ce7d8109f..d1d74fa54bf 100644
>--- a/lib/Object/ModuleSymbolTable.cpp
>+++ b/lib/Object/ModuleSymbolTable.cpp
>@@ -201,7 +201,7 @@ uint32_t ModuleSymbolTable::getSymbolFlags(Symbol S)
>const {
>     Res |= BasicSymbolRef::SF_Executable;
>   if (isa<GlobalAlias>(GV))
>     Res |= BasicSymbolRef::SF_Indirect;
>-  if (GV->hasPrivateLinkage())
>+  if (GV->hasPrivateLinkage() || GV->hasAppendingLinkage())
>     Res |= BasicSymbolRef::SF_FormatSpecific;
>   if (!GV->hasLocalLinkage())
>     Res |= BasicSymbolRef::SF_Global;
>
>Is it a sane approach? Can it be upstreamed?

Can you paste the IR file for which you want AppendingLinkage to work?

 From an earlier command you pasted `clang -flto -o %name.bc %name.c`, it
seems that you have some way to generate `appending` from a .c file.