RFR: 8283323: libharfbuzz optimization level results in extreme build times

[Aleksey Shipilev]

On Wed, 23 Mar 2022 12:25:08 GMT, Magnus Ihse Bursie <ihse at openjdk.org> wrote:

> [JDK-8247872](https://bugs.openjdk.java.net/browse/JDK-8247872) (upgrade HarfBuzz to 2.7.2) caused build time to go up with 24 seconds on my reference linux machine. This was one of the four culprits that caused a 25-30% build time regression over the last two years.
> 
> The problem here was that the new HarfBuzz code caught really bad behaviour from gcc when compiling with optimizations. The official HarfBuzz build does not use any -O flags at all for gcc, so presumably the HarfBuzz team is:
> 
> a) not thinking compiler optimization is important for the performance of this library, and
> b) unaware that their code causes such a headache for gcc.
> 
> (Other compilers fare much better: visual studio makes no difference at all, and for clang just a small regression was observed.)
> 
> The current optimization level was introduced by [JDK-8255790](https://bugs.openjdk.java.net/browse/JDK-8255790), which were really about moving libharfbuzz compilation back into libfontmanager. I could find no comments/discussion relating to the change of optimization level, so I assume it was incidental, and just seemed good at the time.
> 
> This patch changes the optimization level to `SIZE` (which is the closest thing we have to no optimization level) on gcc.

I have a question, though, because I suppose font rendering engine is performance sensitive. AFAICS, the `LIBFONTMANAGER_OPTIMIZATION` was changed from `HIGH` to `HIGHEST` here: https://github.com/openjdk/jdk/commit/05fe06a6#diff-c75e2d581b3730cf900075cedb66ac72eaba40fbbd92eaac5710161830b572b4L534-R491

Does it mean we should instead downgrade to `HIGH`, not `SIZE`? Would that recover the build times?

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PR: https://git.openjdk.java.net/jdk/pull/7919