Add section for cranelift

[janhohenheim]

Adopts #801 by @MalekiRe and changes some text based on feedback by @bjorn3

Blocked by:

• cranelift fails to build Bevy 0.13.2 on macOS rust-lang/rustc_codegen_cranelift#1504
• Bevy crashes at startup on macOS rust-lang/rustc_codegen_cranelift#1507
• Distribute rustc_codegen_cranelift for arm64 macOS rust-lang/rust#127177

Edit: leaving it as a primary Linux recommendation for now.

[BD103]

This looks very good right now, but one nit I have is to only compile your binary with Cranelift. In the October 2023 progress report, it mentions how you can enable Cranelift for specific packages.

In this case, I think it would be best to recommend to only compile your binary with Cranelift, so that Bevy and the rest still receive better optimizations. This is the same reason why we recommend opt-level = 3 for dependencies.

[janhohenheim]

@BD103 that makes sense, thanks for the suggestion!

[janhohenheim]

Blocked by rust-lang/rustc_codegen_cranelift#1504

[janhohenheim]

While we are on the subject, @bjorn3, is there a way to compile my dependencies with LLVM and use cranelift only for my binary on Windows? The way I've set it up locally, I need to run cargo clif manually and I don't know how I would tell it to compile in the specified way. There's no way to do a similar setup to Linux and macOS by just changing one's config.toml on Windows, right?

Also, I heard Bevy stuff on Wasm didn't build or didn't run with cranelift. I don't have the time to verify this. Is this still the case? If so, I can point it out in the readme as well.

[bjorn3]

is there a way to compile my dependencies with LLVM and use cranelift only for my binary on Windows?

Not really. The codegen-backends option in .cargo/config.toml only works for codegen backend shipped with rustc. Rustc itself supports passing the path to a codegen backend dylib (which cargo-clif internally uses), but for cargo I was asked to restrict this unfortunately.

Also, I heard Bevy stuff on Wasm didn't build or didn't run with cranelift. I don't have the time to verify this. Is this still the case? If so, I can point it out in the readme as well.

This is still the case.

[bjorn3]

rust-lang/rust#127177 would also be really nice to land first. This will ship cg_clif for arm64 macOS.

[janhohenheim]

@bjorn3 I've added both links to the PR description for visibility :)

[janhohenheim]

@bjorn3 since we discovered a runtime crash on macOS in the meantime, would it be fine if we changed the section to recommend cranelift specifically for Linux for now? I'd like to have this section merged in time for the Bevy jam.
If that's okay with you, I'll set up a tracking issue for mentioning macOS as well when it's ready.

[bjorn3]

Yeah, that is fine. Maybe also explicitly mention why it shouldn't be used on Windows and macOS to avoid people trying to do it anyway.