Deprecate and remove the auto-linking feature (initially in STRICT; this would let us remove llvm-nm)

[kripken]

Talking to @dschuff it sounds like we can avoid the current llvm-nm stage in our linking. Currently we scan the symbols that way to tell which system libraries to link in. Instead, if we used .a files for system libraries and had a default linker command that included all system libraries, then wasm-ld would do everything for us. It would be simpler, and more efficient too probably. And we could remove the .symbols files.

Emscripten has non-default system libraries too, but those should be added manually anyhow with a -s command etc., that is, we don't use llvm-nm for them.

@sbc100 what do you think?

This might need to wait for after we remove fastcomp.

[sbc100]

I think there two different things being confused here.

1. We have a fake linking system for fastcomp today that uses llvm-nm at link time. This is fastcomp only and will go away when fastcomp goes away. We don't run llvm-nm at link time with the wasm backend.

2. We have a "auto-library" system tries to determine which libraries to link based on which symbols the program needs. This system is based on "symbols" files and we (the emscripten devs) use uses llvm-nm to keep those files up-to-date.

I'm all in favor of removing the auto-linking system. We already removed auto-linking of js libraries in STRICT mode. We should do that same for auto-linking of native libraries.

The idea with STRICT mode is that eventually thinking behind that flag will become the default.

If you agree we my assessment perhaps you want to re-name this issue "Remove auto-detection/auto-linking of system libraries"?

[kripken]

We have a "auto-library" system tries to determine which libraries to link based on which symbols the program needs. This system is based on "symbols" files and we (the emscripten devs) use uses llvm-nm to keep those files up-to-date.

True, but we also use llvm-nm on the program files to find out which system libraries we need (using those symbols files). Here is example logging when building hello_libcxx.cpp:

shared:DEBUG: executed /home/azakai/Dev/sdk/upstream/bin/llvm-nm /tmp/emscripten_temp/hello_libcxx_0.o
system_libs:DEBUG: including libc (libc.a)
system_libs:DEBUG: including libcompiler_rt (libcompiler_rt.a)
system_libs:DEBUG: including libc-wasm (libc-wasm.a)
system_libs:DEBUG: including libc++ (libc++-noexcept.a)
system_libs:DEBUG: including libc++abi (libc++abi.a)
system_libs:DEBUG: including libmalloc (libdlmalloc.a)
system_libs:DEBUG: including libpthreads (libpthreads_stub.a)
system_libs:DEBUG: including libcompiler_rt_wasm (libcompiler_rt_wasm.a)
system_libs:DEBUG: including libc_rt_wasm (libc_rt_wasm.a)

[sbc100]

I see. So the title of this bug should maybe be "Remove/deprecate the auto-library linking feature"?

I agree. We should probably remove it in STRICT mode initially.