Implement Java 14 switch expressions

[niloc132]

Java switch expressions are implemented in JS as a function wrapped around a switch block, with return used for yield, and multi-valued case statements split into multiple entries. Most other optimizations that work on a switch statement at this time will not apply to switch expressions, these can be re-added at a later time, but other optimizations that have some specialization for switch statements have been tested or updated to handle switch expressions.

JEP https://openjdk.org/jeps/361
Fixes #9878

[niloc132]

See about rewriting to avoid the labeled break, maybe a stream/optional operation?

[niloc132]

Nightly build is passing https://github.com/niloc132/gwt/actions/runs/9978594294

[jnehlmeier]

I assume it is ready for review or do you want to address your second code comment first?

[niloc132]

I'd love some review yes, so we can get this shipped ASAP. I'm trying to finish up the closer-to-merging PRs, give people a chance to go over this, if interested, as its not as small as I had hoped.

[jnehlmeier]

Seems to look good except that empty method in CfgBuilder. It is a blind spot now for the compiler analysis, right?

[jnehlmeier]

Wondering about the consequences given that JSwitchStatement does quite a bit.

[niloc132]

Unlikely to matter in a meaningful way.

CfgBuilder is wrapped in DataflowOptimizer, which itself is wrapped in a compiler flag, and warns if it is run at all:

gwt/dev/core/src/com/google/gwt/dev/jjs/JavaToJavaScriptCompiler.java

Lines 1450 to 1455 in 7f42724

	if (options.shouldOptimizeDataflow()) {
	logger.log(TreeLogger.Type.WARN,
	"Unsafe dataflow optimization enabled, disable with -XdisableOptimizeDataflow.");
	// Just run it once, because it is very time consuming
	allOptimizerStats.add(DataflowOptimizer.exec(jprogram));
	}

The option is disabled by default:

gwt/dev/core/src/com/google/gwt/dev/jjs/JJSOptionsImpl.java

Line 43 in 7f42724

private boolean optimizeDataflow = false;

And while a compiler flag exists, its only purpose is to disable it:

gwt/dev/core/src/com/google/gwt/dev/util/arg/ArgHandlerDeprecatedOptimizeDataflow.java

Lines 27 to 52 in 7f42724

	public ArgHandlerDeprecatedOptimizeDataflow(OptionOptimizeDataflow option) {
	this.option = option;
	addTagValue("-XdisableOptimizeDataflow", false);
	}
	@Override
	public String getPurposeSnippet() {
	return "Analyze and optimize dataflow.";
	}
	@Override
	public String getLabel() {
	return "optimizeDataflow";
	}
	@Override
	public boolean isUndocumented() {
	return true;
	}
	@Override
	public boolean setFlag(boolean value) {
	option.setOptimizeDataflow(value);
	return true;
	}

I'm not clear how a normal user can turn it on without a fork or a custom build.

I still updated tests to reduce the risk of anything bad happening here for downstream users - but this would only negatively impact new switch-expr usage, and would not deoptimize existing switch statements (as the tests confirm).

[jnehlmeier]

Oh I see. I just found ea7228a

So maybe we should officially deprecate it for removal (or fix its bugs and keep it).