On Wed, 23 Mar 2022 12:25:08 GMT, Magnus Ihse Bursie <i...@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. No, unfortunately it won't. :( The problem started with the update to HarfBuzz 2.7.2, and there the opt level was HIGH. I also tested manually compiling with -O1 (which we don't really have a level for in the build system) and even that was too problematic for gcc. But another approach is possible. I talked off-list with Erik, and he had noticed (as had I) that a single file in harfbuzz was responsible for the lion's share of the build time regression. What he did, though, that I didn't come to think of, was to try to change optimization level for just that file. This fixes most of the build time regression, while being a much less intrusive fix. It also makes sense to do this fix for clang, since the 8 second regression seen there is basically also completely due to this file. So I'll upload an update too this fix that is more conservative, and only changes opt level for a single file. ------------- PR: https://git.openjdk.java.net/jdk/pull/7919
