Assertion failure with link-time optimization and link command that duplicates static library

[davidstone]

Given the following two translation units:

a.c:

b.c:

int main() {}

And compiling with

clang -flto=thin -c a.c
clang -flto=thin -c b.c
llvm-ar qc c.a a.o
clang -fuse-ld=mold -flto=thin b.o -o /dev/null c.a c.a

Causes a crash with

ld.mold: /home/david/llvm/llvm/tools/gold/gold-plugin.cpp:1069: std::vector<std::pair<SmallString<128>, bool>> runLTO(): Assertion `ObjFilename.second' failed.
clang: error: unable to execute command: Aborted (core dumped)
clang: error: linker command failed due to signal (use -v to see invocation)

I'm not sure if the bug is in llvm's gold-plugin code or mold calls that plugin incorrectly or if clang's compiler driver calls mold incorrectly.

[davidstone]

As a note, the same crash occurs if b.c is empty, but I added the int main() {} to make this a crash on valid instead of crash on invalid.

[benjamasu]

The corresponding issue in the mold repository: rui314/mold#1356.
The problem occurs not only with static libraries, but also with object files.

[rui314]

I think this is LLVMgold's bug. It is easy to deduplicate given filenames, but doing it is a bit odd from the linker's perspective, as a user explicitly pass the same file twice to the linker, and the most straightforward for the linker to do in that situation is just process them normally. In other words, the plugin should be prepared for the situation in which the same file is given more than once.