Have linking look for .imports alongside each .a.

[sbc100]

Currently we rely on the clang driver adding -allow-undefined-file wasm.syms along with -lc. Without this (if -lc is used on its own) symbols such as syscalls will cause the link to fail.

I propose that we have the linker looks for an optional .imports files alongside each library can can list symbols which that library expects the wasm embedder to provide. We can then rename wasm.syms to libc.imports and remove the -allow-undefined-file from the driver.

[sbc100]

LLD change: https://reviews.llvm.org/D41922

[sbc100]

Clang driver change: https://reviews.llvm.org/D41923

[sbc100]

waterfall change: WebAssembly/waterfall#309

[sbc100]

musl change: jfbastien/musl#34

[dschuff]

This seems fine for the short term, seems like a better/more general mechanism than -allow-undefined-file. Longer term I think it will make more sense to have another kind of symbol, to indicate that a symbol should not participate in the usual linking but should be made into an import in the final binary. This kind of symbol would also be used by dynamically-linked executables (and dynamic libraries) for the same purpose (e.g. syscalls or other calls that come from the host environment). The dynamic loader could then have the same behavior without having to have extra files to go along with the dylibs.
We'll want to think carefully about how these symbols interact with "regular" symbols.

[NWilson]

Funny, I have a branch I haven't yet turned into a PR where I was thinking along the same lines! I was going to take the easier route of renaming the (extremely generic) "wasm.syms" to "libc.syms". But I hadn't considered removing the -allow-undefined-file argument.