Add -ftime-trace instrumentation for the inliner pass

[suyashkumar102]

After PR #22825 I kept looking at traces and noticed the inliner pass just
doesn't show up. When compiling with -inline -ftime-trace, the trace shows Semantic analysis followed by Code generation— nothing in between.

The frontend inliner can contribute to compile-time slowdowns in some codebases, and having visibility into its activity may help when investigating such cases. For example, #20315 highlights a situation where the inliner was suspected to play a role in increased compile times, and there was nothing in the trace to look at when diagnosing it.

This adds two new event types:

• Inlining — top-level span covering the entire inliner pass
• Inline: <function> — per-function span for each function scanned

The Inlining span covers both the pragma-inline scan and the full -inline scan.

1. ftimetrace_inline.d compiled with -inline:

2. ftimetrace_pragma.d compiled without -inline :

3. Recursive template case (reduction from DMD frontend inliner causes major slowdown #20315) —
   spans showing the inliner working through the divide-and-conquer tree:

Updated ftimetrace.d and added ftimetrace_inline.d and ftimetrace_pragma.d
to cover all cases.

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If your PR contains non-trivial changes, please reference a Bugzilla issue or create a manual changelog.

Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub run digger -- build "master + dmd#22911"

[nordlow]

Thanks for doing this. Btw, while you have your brain cache partially filled with details on inlining, would it be possible for you to investigate why -cov coverage for a function is lost when it's qualified with pragma(inline, true);?

[suyashkumar102]

@nordlow Tried it out.
int square(int x) shows 0000000. Zero hits even though it's called

Seems this is Issue #18337. Good timing since I'm already deep in the inliner code.
I'll take a proper look at where the coverage data needs to be preserved during the inline expansion and see if there's a clean fix!

[nordlow]

Seems this is Issue #18337. Good timing since I'm already deep in the inliner code. I'll take a proper look at where the coverage data needs to be preserved during the inline expansion and see if there's a clean fix!

Awesome! See also #22830.

[suyashkumar102]

Awesome! See also #22830.

Got a fix working — opened PR #22922. Followed @dkorpel's suggestion from #22830.