[llvm-dev] [ThinLTO] static library failure with object files with the same name

Teresa Johnson via llvm-dev llvm-dev at lists.llvm.org
Tue Sep 5 14:09:08 PDT 2017

Hi Johan,

Right, per the bug this is fixed in lld (and was already handled in
gold-plugin), but I guess not in ld64. Note that lld and gold-plugin use
the new LTO API, while ld64 (and probably other linkers) are still using
the legacy libLTO (which is what ThinLTOCodeGenerator.cpp is part of).
Fixing it in the location you propose could work for all legacy libLTO
users. But I don't think that adding just the size will (always) be enough
to disambiguate (couldn't the 2 same named members have the same size?) -
although lld is doing the same thing so this may be as good as what is done
there. For gold-plugin we add the byte offset into the archive where the
member starts, which will be unique.
+davide for thoughts since he fixed it on the lld side.


On Tue, Sep 5, 2017 at 1:09 PM, Johan Engelen via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Hi all,
>   I have a static library with object files with the same name (not the
> same full path, but the archive made with llvm-ar does not store the full
> path). The library contains object files that have been compiled with
> `-flto=thin` (compiled with LDC, not clang, but that shouldn't matter).
> When linking to that static library, I get the error:
> Assertion failed: (ModuleMap.find(ModuleBuffer.getBufferIdentifier()) ==
> ModuleMap.end() && "Expect unique Buffer Identifier"), function
> generateModuleMap, file ../lib/LTO/ThinLTOCodeGenerator.cpp, line 138.
> The error occurs because the buffer identifier uses the filename of the
> objects inside the archive, and those are identical for the two files with
> different source path.
> This problem appears to be fixed for LLD here:
> https://reviews.llvm.org/D25495
> https://bugs.llvm.org/show_bug.cgi?id=30665
> But it persists for linking with the system linker on OSX (while manually
> passing libLTO.dylib to the linker).
> If I modify lib/LTO/ThinLTOCodeGenerator.cpp to do a poor-man's
> uniquefying of the buffer identifier:
> ```
> void ThinLTOCodeGenerator::addModule(StringRef Identifier, StringRef
> Data) {
> -  ThinLTOBuffer Buffer(Data, Identifier.str()));
> +  ThinLTOBuffer Buffer(Data, Identifier.str() + utostr(Data.size()));
> ```
> then the problem is solved.
> What would be a proper way to fix this issue?
> Can it be fixed in lib/LTO, or should I not create an (afaict valid)
> archive containing duplicate object file names?
> (Note: this issue makes it impossible to use an LTO version of the
> standard library with LDC)
> Thanks,
>   Johan
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Teresa Johnson |  Software Engineer |  tejohnson at google.com |  408-460-2413
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