Link failing with attempt to add bitcode file after LTO

[tklajnscek]

Happens with 2.0.34 onward, latest I tried is 3.1.9, haven't tried earlier ones

When we compile our project with -flto we get:

[build] wasm-ld: error: D:\SDK\emsdk\upstream\emscripten\cache\sysroot\lib\wasm32-emscripten\lto\libc.a(fputc.o): attempt to add bitcode file after LTO.

I've looked at the relevant LLVM change log here https://reviews.llvm.org/rGb062fe181625bd1944ca9ca2a58246ffd7cd3536
And the previous issues #11545 and #15506

We don't use fputc, so it's most likely being generated by this transform https://github.com/llvm/llvm-project/blob/35cc2ec4ed4a7a0862e62ad009d561f61e2d12c3/llvm/lib/Transforms/Utils/SimplifyLibCalls.cpp#L2887

fwrite(S,1,1,F) -> fputc(S[0],F)

Adding a reference to it with Wl,-u,fputc fixes the issue, but I thought this has been fixed since #11545 is closed.

Does the non-LTO symbol list in system_libs.py need updating maybe to keep up with any transforms that may emit these symbols late? Or is there another fix for this on the horizon?

[tklajnscek]

Forgot to tag @sbc100 since you've worked on this as far as I can tell :)

[sbc100]

Yes the non-LTO symbol list in system_libs.py almost certainly needs updating. Sadly, I don't know of a good way to manage it automatically.

[tklajnscek]

Thanks for the lightning fast turnaround on this! ❤️

[tmm1]

I experienced a similar problem with emscripten v3.1.3:

wasm-ld: error: /project/bin/.emscripten_cache/sysroot/lib/wasm32-emscripten/lto/libnoexit.a(atexit_dummy.o): attempt to add bitcode file after LTO.

which was fixed by adding -Wl,-u,atexit -Wl,-u,__cxa_atexit

[sbc100]

Fix for __cxa_atexit is in #16923