RFR: 8349638: Build libjdwp with SIZE optimization

[Matthias Baesken]

On Tue, 1 Apr 2025 11:59:07 GMT, Magnus Ihse Bursie <ihse at openjdk.org> wrote:

> It would be interesting to also see how compilation times varies with optimization level. At least some kind of hint if HIGHEST is like 2x slower than LOW, or if SIZE is slower than LOW at all, etc. The relative speed difference is interesting, but so is it in absolute terms. If a library takes 0.5 seconds on LOW but 1.1 seconds on HIGH on a particular system, it is unlikely to matter much to overall build time anywhere. But if it goes from 15s to 30s on a fast machine, it might be a problem if such performance regressions stack up, especially on slower machines (which includes the ones running GHA).

This is what I got from my Linux x86_64 system using gcc 13.2.0 devkit (opt build). Note that the build operates on a relatively slow filer, this will slow the build time somewhat  but that is true for all  opt-levels.


rm -rf  ./support/modules_libs/jdk.jdwp.agent/libjdwp.so  ./jdk/lib/libjdwp.so   ./support/native/jdk.jdwp.agent/libjdwp
time make jdk.jdwp.agent-libs-only  JOBS=1



gave me these times 

**default (LOW)**
real    0m15.661s
user    0m8.763s
sys     0m2.012s

**HIGHEST**
real    0m15.201s
user    0m9.005s
sys	    0m2.003s

**SIZE**
real    0m14.263s
user    0m7.905s
sys     0m1.891s

So it looks like SIZE is a little faster than the other, and LOW  and  HIGHEST are rather similar.
LOW is `-O2` on Linuxx86_64 and HIGHEST is `-O3` , those are maybe rather similar  (LOW is a bit misleading because `-O2` is not really that 'low' the gcc docu says about it  : 'Optimize even more. GCC performs nearly all supported optimizations that do not involve a space-speed tradeoff.').

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PR Comment: https://git.openjdk.org/jdk/pull/23563#issuecomment-2786694168