Bug 311831 – Build time improvement: Use extern template instantiation for high-traffic mpark::variant instantiations

• Status: RESOLVED
• Resolution: FIXED
• Component: New Bugs

[Brent Fulgham]

Reduce redundant template instantiation across translation units by adding explicit template instantiation for commonly used mpark::variant types in widely-included WebCore headers.

This reduces compile time by avoiding the repeated template expansions (which have to be culled at link time). Currently every translation unit does a template expansion of the variant machinery, which has to be removed at a later build step. Both operations add time to the build.

On my system, this current patch reduced build time by 5.4%.

[Radar WebKit Bug Importer]

<rdar://problem/174419851>

[Brent Fulgham]

Pull request: https://github.com/WebKit/WebKit/pull/62367

[Brent Fulgham]

Results from my M4 MBP:

```
====================================================================================================
AGGREGATE CATEGORY COMPARISON (matched TUs only)
====================================================================================================
  Frontend                                12256.4s ->    12232.8s  (-23.6s, -0.2%)
  Backend                                  4271.6s ->     4273.3s  (+1.7s, +0.0%)
  ParseClass                               4611.2s ->     4537.8s  (-73.4s, -1.6%)
  InstantiateFunction                     12829.7s ->    12745.9s  (-83.7s, -0.7%)
  InstantiateClass                         5116.3s ->     5150.7s  (+34.4s, +0.7%)
  PerformPendingInstantiations             2382.6s ->     2315.5s  (-67.1s, -2.8%)
  CodeGen Function                          516.6s ->      537.4s  (+20.8s, +4.0%)
  OptFunction                              1413.2s ->     1426.0s  (+12.8s, +0.9%)
  OptModule                                2554.0s ->     2532.5s  (-21.5s, -0.8%)

====================================================================================================
FINAL SUMMARY  —  build IMPROVED by 18.0s (0.8%)
====================================================================================================
  Total compile time:                     16674.0s ->    16655.9s  (-18.0s, -0.8%)

  Frontend                                -23.6s  (-0.2%)  <<<<<<<<<<<<<<<<<<<<<<<<<<<<<<
  Backend                                  +1.7s  (+0.0%)  >>
  ParseClass                              -73.4s  (-1.6%)  <<<<<<<<<<<<<<<<<<<<<<<<<<<<<<
  InstantiateFunction                     -83.7s  (-0.7%)  <<<<<<<<<<<<<<<<<<<<<<<<<<<<<<
  InstantiateClass                        +34.4s  (+0.7%)  >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
  PerformPendingInstantiations            -67.1s  (-2.8%)  <<<<<<<<<<<<<<<<<<<<<<<<<<<<<<
  CodeGen Function                        +20.8s  (+4.0%)  >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
  OptFunction                             +12.8s  (+0.9%)  >>>>>>>>>>>>>>>>>>>>>
  OptModule                               -21.5s  (-0.8%)  <<<<<<<<<<<<<<<<<<<<<<<<<<<<<<

  Template instantiations (net):       -49.3s
    time saved:                        -1266.0s  (templates faster or removed)
    time added:                        +1216.7s  (templates slower or new)

```

[Brent Fulgham]

Note: The 5.4% was for a different patch. This small change only nets 0.8%.

[EWS]

Committed 311479@main (c69a763f29d8): <https://commits.webkit.org/311479@main>

Reviewed commits have been landed. Closing PR #62367 and removing active labels.