On Thu, Oct 31, 2024 at 7:20 AM Jakub Jelinek <ja...@redhat.com> wrote: > > On Thu, Oct 31, 2024 at 10:57:02AM +0000, Peter Robinson wrote: > > On Thu, 31 Oct 2024 at 07:32, Jakub Jelinek <ja...@redhat.com> wrote: > > > > > > On Wed, Oct 30, 2024 at 10:46:01PM -0400, Neal Gompa wrote: > > > > I know the idea of moving to -O3 has been briefly mentioned before in > > > > other contexts when we've discussed uplifting the flags, but it looks > > > > like Ubuntu is moving to -O3 for Ubuntu 25.04[1]. Is there a reason > > > > why we shouldn't consider doing the same for Fedora Linux 42? > > > > > > Yes, this is a very bad idea. > > > > > > -O3 significantly increases code size (and the speed up gains aren't > > > really > > > guaranteed), which is highly undesirable when the vast majority of code in > > > the distro isn't performance critical, the I-cache footprint is then more > > > important. Especially when -O2 in GCC performs some vectorization already > > > for years and it is going to be used more at -O2 in GCC 15 (but the > > > heuristics for -O2 vectorization is avoid significant code increases). > > > > > > -O3 should be used just for performance critical code, which is found to > > > be > > > hot in profiling and proved to help performance of the code. > > > Better yet, performance criticial code should use PGO (profile guided > > > optimizations) so that only the hot parts of the code are automatically > > > optimized for speed and cold parts for size. With reasonable workload > > > used > > > during package build for the profile feedback generation. > > > > > > There are some packages in the distro which use PGO (e.g. gcc itself), but > > > I think e.g. SUSE packages use PGO far more often than in Fedora. > > > > How does SUSE choose to use PGO? Is that a manual process where the > > maintainer chooses to turn it on for a particular library or binary? > > I think so. > It needs some package specific know-how. > Basically, one needs to build the binaries and/or libraries with > -fprofile-generate flag in addition to normal compiler flags, then > run the binaries with the libraries on some typical workload and > finally rebuild again with -fprofile-use flag instead of -fprofile-generate. > It can be combined with LTO and generally LTO+PGO results in better speedups > over just one of those separately. > Testsuites often aren't very good workloads because they usually spend more > time on unlikely corner cases than on the common cases, but can be used when > there isn't anything better. > E.g. GCC when building itself has as the workload building itself, so is > trained mostly on C++ code and far less on C code, for shells I'd imagine one > should run a few configure scripts with the newly built shell, etc. > I guess one could look at SUSE debuginfo packages and see what packages have > been built with -fprofile-use and learn from their spec files (if the > corresponding packages in Fedora already don't use that). > Where PGO isn't ideal is multi-versioned code where the binaries/libraries > decide based on the build box hw capabilities on if one or another code > should be used; in that case depending on what the build box hw capabilities > were, some code can look unused even if it could be hot on other hw. >
SUSE has the benefit of their build system taking care of automatic rebuilds and simple build loops for them. This allows them to have a much more hands-off approach than we do, and gives their packagers significantly more breathing room to take advantage of things like this. I don't think we can reasonably do so at scale as long as we don't have features like this. -- 真実はいつも一つ！/ Always, there's only one truth! -- _______________________________________________ devel mailing list -- devel@lists.fedoraproject.org To unsubscribe send an email to devel-le...@lists.fedoraproject.org Fedora Code of Conduct: https://docs.fedoraproject.org/en-US/project/code-of-conduct/ List Guidelines: https://fedoraproject.org/wiki/Mailing_list_guidelines List Archives: https://lists.fedoraproject.org/archives/list/devel@lists.fedoraproject.org Do not reply to spam, report it: https://pagure.io/fedora-infrastructure/new_issue
