RFR: 8365053: Refresh hotspot precompiled.hpp with headers based on current frequency [v10]

[Francesco Andreuzzi]

On Fri, 8 Aug 2025 22:11:32 GMT, Francesco Andreuzzi <duke at openjdk.org> wrote:

>> In this PR I propose to refresh the included headers in hotspot `precompiled.hpp`. The current set of precompiled headers was refreshed in 2018, 7 years ago. I repeated the same operations and measurements after refreshing the set of precompiled headers according to the current usage frequency.
>> 
>> These are the results I observed. Depending on the platform, the improvement is between 10 and 20% in terms of total work (user+sys). The results are in seconds.
>> 
>> 
>> linux-x64 GCC
>> master      real 81.39 user 3352.15 sys 287.49
>> JDK-8365053 real 81.94 user 3030.24 sys 295.82
>> 
>> linux-x64 Clang
>> master      real 43.44 user 2082.93 sys 130.70
>> JDK-8365053 real 38.44 user 1723.80 sys 117.68
>> 
>> linux-aarch64 GCC
>> master      real 1188.08 user 2015.22 sys 175.53
>> JDK-8365053 real 1019.85 user 1667.45 sys 171.86
>> 
>> linux-aarch64 clang
>> master      real 981.77 user 1645.05 sys 118.60
>> JDK-8365053 real 791.96 user 1262.92 sys 101.50
>
> Francesco Andreuzzi has updated the pull request incrementally with two additional commits since the last revision:
> 
>  - magic number: 400
>  - inline

Some results:

| Features                         | Time1 (s) | Time2 (s) | Change (s) |
|---------------------------------|----------|----------|-------------|
| epsilongc                       | 490.77   | 489.50   | 1.27        |
| epsilongc cds                   | 546.27   | 537.91   | 8.36        |
| epsilongc compiler1             | 547.87   | 539.86   | 8.01        |
| epsilongc compiler2             | 785.18   | 735.13   | 50.05       |
| g1gc                            | 682.44   | 666.40   | 16.04       |
| epsilongc services              | 510.22   | 504.68   | 5.54        |
| epsilongc services jfr          | 675.49   | 636.87   | 38.62       |
| epsilongc jni-check              | 499.53   | 493.72   | 5.81        |
| epsilongc compiler1 jvmci       | 589.74   | 591.32   | -1.58       |
| epsilongc compiler2 jvmci       | 852.46   | 806.35   | 46.11       |
| epsilongc services jvmti        | 567.67   | 561.73   | 5.94        |
| epsilongc link-time-opt         | 535.96   | 526.00   | 9.97        |
| epsilongc management            | 508.63   | 504.53   | 4.10        |
| epsilongc opt-size              | 507.03   | 498.58   | 8.45        |
| parallelgc                      | 525.78   | 517.38   | 8.40        |
| serialgc                        | 512.59   | 511.88   | 0.71        |
| shenandoahgc                    | 739.44   | 722.72   | 16.72       |
| zgc                             | 681.20   | 672.28   | 8.92        |

- `Time1` is the time (sys+user) it takes to complete `make -h hotspot` with the precompiled headers in be25d3413b432b56e9789eae55920f1862008911 (>=400 includes in a `custom` build)
- `Time2`: same as `Time1`, with the precompiled headers having >=400 includes in a `custom` build with all features in the first column

So, the third column measures how much we improve by taking new precompiled headers due to a specific features. I'd say the following are worth keeping behind an include guard:
- `cds`
- `compiler1`
- `compiler2`
- `g1gc`
- `services`
- `jfr` ==> `services` (#26723)
- `jni-check`
- `link-time-opt`
- `opt-size`
- `parallelgc`
- `shenandoahgc`
- `zgc`

-------------

PR Comment: https://git.openjdk.org/jdk/pull/26681#issuecomment-3175208900