[RFC] Building LLVM for WebAssembly

[MaskRay]

@whitequark

@MaskRay

While I appreciate the effort to increase LLVM portability and benefit WebAssembly communities, I’m concerned about the lack of support for some really basic functionalities:

pwd.h sys/wait.h alarm gethostname getpid fchown raise setjmp socket umask.

The lack of these APIs is a ground reality for the WASI platform: it very intentionally lacks discretionary access control, asynchronous signals, Unix process management, or Unix password database, and that’s not changing in foreseeable future (or in some cases, likely ever). Therefore this is something portable software must cope with.

For some of these APIs (notably setjmp and socket), a check cannot be removed, because you could conceivably want builds with or without those APIs enabled, targeting different embeddings and/or Wasm engines. For the rest of them, I can easily enough replace them with defined(__wasm__)—I do want feedback on the direction to take here.

If that is the ground reality for the platform, perhaps defined(__wasm__) might work better.

• to avoid cost of CMake compile-time tests
• make the platform requirement clear. HAVE_XXX is sometimes used for “we provide a similar API in a different name”

Is it cleaner to define LLVM_ON_UNIX for __wasm__ or not?