Bug 278700 – Reduce ThreadSafeRefCounted compile time

• Component: Web Template Framework
• Status: RESOLVED FIXED

[Gerald Squelart]

Found with -ftime-trace and ClangBuildAnalyzer, the top-3 "templates that took longest to instantiate" are:
> 110395 ms (35802 times, avg 3 ms): WTF::Function<void ()>::Function<(lambda at /Volumes/WebKit/Internal/WebKit/WebKitBuild/Debug/usr/local/include/wtf/ThreadSafeRefCounted.h:143:27), void>
>  76389 ms (35802 times, avg 2 ms): WTF::makeUnique<WTF::Detail::CallableWrapper<(lambda at /Volumes/WebKit/Internal/WebKit/WebKitBuild/Debug/usr/local/include/wtf/ThreadSafeRefCounted.h:143:27), void>, (lambda at /Volumes/WebKit/Internal/WebKit/WebKitBuil...
>  66481 ms (35802 times, avg 1 ms): std::make_unique<WTF::Detail::CallableWrapper<(lambda at /Volumes/WebKit/Internal/WebKit/WebKitBuild/Debug/usr/local/include/wtf/ThreadSafeRefCounted.h:143:27), void>, (lambda at /Volumes/WebKit/Internal/WebKit/WebKitBui...

And at #25 of "Template sets that took longest to instantiate":
> 129369 ms (35838 times, avg 3 ms): WTF::ThreadSafeRefCounted<$>::deref

This is due to ThreadSafeRefCounted<T, DestructionThread>::deref doing a `switch` on the templated enum `DestructionThread`, and for some of them moving a lambda into a `WTF::Function`, which itself always calls WTF::makeUnique.

By changing this `switch` into a sequence of `if constexpr` blocks, it's possible to pretty much squash these big compile times out of the top-1000, only leaving a small template set taking less than 4% of the original time:
>  4666 ms (2065 times, avg 2 ms): WTF::ThreadSafeRefCounted<$>::deref

[Radar WebKit Bug Importer]

<rdar://problem/134756505>

[Gerald Squelart]

Pull request: https://github.com/WebKit/WebKit/pull/32749

[EWS]

Committed 282871@main (9ed2e706b2c7): <https://commits.webkit.org/282871@main>

Reviewed commits have been landed. Closing PR #32749 and removing active labels.