[cfe-dev] A stage2 build causes changes to libllvm impacting program using it (exemple: rustc)

[Sylvestre Ledru via cfe-dev]

Hello,

Lately, I have been working on moving Debian & Ubuntu packages to a
stage2 build.

This means that, instead of shipping llvm-toolchain packages built with
gcc, we are rebuilding
everything a second time using the newly built clang.

Now, when pushed to Debian, it caused some unexpected issues in
particular with rust reported here:
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=913271#35

rustc in Debian was building initially against libllvm built with gcc.
It was working fine.
The failure is now happening when the systems saw their libllvm upgraded
to the stage2 bootstrap one.

The beginning of the backtrace is the following (generating debug info):

Thread 1 "rustc" received signal SIGSEGV, Segmentation fault.
0x00007ffff1e273bc in llvm::StringMapImpl::LookupBucketFor(llvm::StringRef) () from /usr/lib/x86_64-linux-gnu/libLLVM-7.so.1
(gdb) bt
#0  0x00007ffff1e273bc in llvm::StringMapImpl::LookupBucketFor(llvm::StringRef) () from /usr/lib/x86_64-linux-gnu/libLLVM-7.so.1
#1  0x00007ffff1f654c9 in ?? () from /usr/lib/x86_64-linux-gnu/libLLVM-7.so.1
#2  0x00007ffff1f65491 in llvm::MDString::get(llvm::LLVMContext&, llvm::StringRef) () from /usr/lib/x86_64-linux-gnu/libLLVM-7.so.1
#3  0x00007ffff1ee69cd in ?? () from /usr/lib/x86_64-linux-gnu/libLLVM-7.so.1
#4  0x00007ffff1ee2d12 in llvm::DIBuilder::createFile(llvm::StringRef, llvm::StringRef, llvm::Optional<llvm::DIFile::ChecksumInfo<llvm::StringRef> >, llvm::Optional<llvm::StringRef>) () from /usr/lib/x86_64-linux-gnu/libLLVM-7.so.1
#5  0x00007ffff50a01f2 in LLVMRustDIBuilderCreateFile (Builder=0x5555555d4180, Filename=0x7ffffffeedd8 "src/main.rs", Directory=0x555555b3dd10 "$HOME/hello_world")
    at /usr/lib/llvm-7/include/llvm/ADT/StringRef.h:85
#6  0x00007ffff501d39e in rustc_codegen_llvm::debuginfo::metadata::compile_unit_metadata (tcx=..., codegen_unit_name=..., debug_context=0x7ffffffeeeb0) at librustc_codegen_llvm/debuginfo/metadata.rs:859

Disabling the debug generation for rustc fixes the issue (thanks to Ximin Luo for the investigation).

Why idea how to workaround this issue ?

Unfortunately, because rustc needs rustc to build itself, I cannot check if a simple recompilation fixes it or not.

Cheers,
S