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

[Gleb Popov via llvm-dev]

On Thu, Feb 20, 2020 at 10:43 PM Fangrui Song via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> 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?
>

There is nothing special about it. It just has a GV with appending linkage
like I wrote earlier:

@myvar = appending constant [1 x [1 x i8]*] ...


>  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.
>

I have my own custom pass plugged into Clang's pass pipeline, which
generates that @myvar. Of course, vanilla Clang doesn't generate GVs with
appending linkage.

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