RFR: 8293020: jmod should not be treated as "small" tool for large modules [v2]

[Aleksey Shipilev]

> This is similar to [JDK-8245168](https://bugs.openjdk.org/browse/JDK-8245168), but the blanket change to allow all modules be compiled with default options is a net loss. Instead, we can hand-pick the major offenders (large modules) where running jmod with normal tool options improves performance.
> 
> I instrumented the jmod to tell me the times it needs to create individual modules, and hand-picked three top modules that take multiple seconds to run.
> 
> Motivational `make clean-images images` times:
> 
> 
> # x86_64 Server, release
> 
> # Baseline
> real	0m12.040s
> user	1m4.872s
> sys	0m10.805s
> 
> # Patched
> real	0m10.785s  ; <--- 1.2s faster
> user	1m7.031s
> sys	0m10.985s
> 
> # x86_64 Server, fastdebug
> 
> # Baseline
> real	0m19.263s
> user	2m42.317s
> sys	0m18.537s
> 
> # Patched
> real	0m17.911s ;  <--- 1.1s faster
> user	2m52.810s
> sys	0m19.092s
> 
> 
> # x86_64 Server, slowdebug
> 
> # Baseline
> real	0m44.799s
> user	10m7.106s
> sys	0m17.578s
> 
> # Patched
> real	0m46.975s   ; <--- 2.5 sec slower
> user	11m1.155s
> sys	0m17.060s
> 
> 
> I think we can accept the `slowdebug` regression in favor of improvements on `release` and `fastdebug` that most people seem to be building every day.

Aleksey Shipilev has updated the pull request incrementally with one additional commit since the last revision:

  Indent

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Changes:
  - all: https://git.openjdk.org/jdk/pull/10062/files
  - new: https://git.openjdk.org/jdk/pull/10062/files/3be7680d..50ae27e7

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=10062&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=10062&range=00-01

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/10062.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10062/head:pull/10062

PR: https://git.openjdk.org/jdk/pull/10062