[llvm-dev] LLD doesn't handle globals with appending linkage (original) (raw)

Fangrui Song via llvm-dev llvm-dev at lists.llvm.org
Thu Feb 20 10:42:52 PST 2020

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 @@ uint32t ModuleSymbolTable::getSymbolFlags(Symbol S) const { Res |= BasicSymbolRef::SFExecutable; if (isa(GV)) Res |= BasicSymbolRef::SFIndirect; - if (GV->hasPrivateLinkage()) + if (GV->hasPrivateLinkage() || GV->hasAppendingLinkage()) Res |= BasicSymbolRef::SFFormatSpecific; if (!GV->hasLocalLinkage()) Res |= BasicSymbolRef::SFGlobal; 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.

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