Pass custom GCC optimization flags

[ReperakDev]

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Problem description: It doesn't seem like there's a way to pass custom compiler flags (e.g -O3) to the build script.
I've tried using the undocumented --make-args flag however that did not work. I'd like to see an easy and well documented way to enable experimental compiler flags.

[skjolber]

@M-Davies @sxa any hope for this? A few recent JDK benchmarks seem to indicate that tweaking the build can improve the performance considerably.

[karianna]

There are some generic optimisations that could be applied, especially for the Aarch64 based architectures. But we have to be careful not to optimize only for some architectures and disadvantage others.

@skjolber What's your use case here?

[skjolber]

@karianna my use-case is building for local developer machine execution, i.e. not distribution, on an AMD machine. I plan on copying compiler flags from one of the best performing distributions and running my benchmarks, which will at least include maven and gradle build performance.

The top performing distos typically offer docker images, so do not think there is much to gain from making customized images.

[sxa]

Which script are you interested in doing this for? For makejdk-any-platform.sh you'd likely be able to use:

--configure-args "--with-extra-cflags=-O3 --with-extra-cxxflags=-O3"

Or if you're running make-adopt-build-farm.sh then setting

CONFIGURE_ARGS="--with-extra-cflags=-O3 --with-extra-cxxflags=-O3"

first should do it.

@ReperakDev @skjolber Let me know whether that works for you. It would be interesting to hear the results of building with any such changes if you're willing and able to share.