Rust does not perform cross language LTO for C functions forwarding to C++

[Time0o]

I believe that cross language LTO does not work correctly when calling C++ code from Rust via intermediate C functions.

Consider this example, here a C++ function foo::do_something (defined in cc/foo_cc.h) is wrapped in a C function foo_do_something (declared in c/foo_c.h) which is then called from Rust via its FFI (in rust/foo_main.rs).

For reference, when linking the static library libfoo_c.a that exposes foo_do_something to a binary created from another C file (c/foo_main.c), a call to foo_do_something is correctly reduced to a call to foo::do_something in the resulting machine code. However, this is not the case for the binary created by rustc, here the main function first jumps to foo_do_something which then does nothing but to jump to foo::do_something.

I've tested this with rustc 1.53.0 and clang 12.0.0

[michaelwoerister]

Thanks for the bug report, @Time0o! It would be helpful if you could provide the LLVM IR files for all files involved. The Rust compiler can generate those via --emit llvm-ir.

[HKalbasi]

Might be related to llvm/llvm-project#141038