Add `x.py` option to skip rebuilding LLVM

[varkor]

The compiler LLVM is rebuilt whenever the built version of LLVM doesn't have the right hash. (Previously it was rebuilt based on a special marker file, which was easier to work around.)

rust/src/bootstrap/native.rs

Lines 88 to 108 in f042687

	if done_stamp.exists() {
	if let Some(llvm_commit) = llvm_info.sha() {
	let done_contents = t!(fs::read(&done_stamp));
	// If LLVM was already built previously and the submodule's commit didn't change
	// from the previous build, then no action is required.
	if done_contents == llvm_commit.as_bytes() {
	return build_llvm_config;
	}
	} else {
	builder.info(
	"Could not determine the LLVM submodule commit hash. \
	Assuming that an LLVM rebuild is not necessary.",
	);
	builder.info(&format!(
	"To force LLVM to rebuild, remove the file `{}`",
	done_stamp.display()
	));
	return build_llvm_config;
	}
	}

However, most of the time, having the latest LLVM version is not necessary, but makes rebuilding rustc after rebasing take significantly longer, which is a pain. It would be good to be able to disable this as an option in x.py for convenience.

This issue has been assigned to @matthew-healy via this comment.

[varkor]

To fix this, you'll want to add another option: you should be able to look at the logic for something like --keep-stage.

rust/src/bootstrap/flags.rs

Lines 130 to 131 in f042687

	opts.optmulti("", "keep-stage", "stage(s) to keep without recompiling \
	(pass multiple times to keep e.g., both stages 0 and 1)", "N");

[Centril]

It would be good if this could be changed in the config file so you don't have to pass in the option all the time.

[varkor]

@Centril: if it's a config file option, then there should be an x.py option to rebuild it if necessary. I.e. it shouldn't be necessarily to temporarily change the Config.toml to rebuild.

[Centril]

@varkor sure that seems good, although they can be implemented separately.

[spadaval]

The line you would need to change is near this one: (line 735 of bootstrap.py)

if self.get_toml('llvm-config') and self.get_toml('lld') != 'true':
        continue

It would probably be much easier to add a check against a key in the config, than to add a new flag.

[Walther]

@rustbot claim 🙂

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[Walther]

...sorry, my bad!

@rustbot claim

EDIT: oh dear, I'm making it worse 🤦‍♂