On Tue, 28 Nov 2023 01:37:01 GMT, Xiaohong Gong <xg...@openjdk.org> wrote:
>>> In fact, I am not even sure why it seems to the PR author to be a good idea >>> to let the default be dependent on the build machine at all. My personal >>> opinion is that it would be better to select either "SVE enabled" or "SVE >>> disabled" as the default, and then let the user override this on the >>> configure command line, if they target a platform with different SVE >>> availability. >> >> SVE support should be enabled regardless of the build machine. The same >> binary must run on both SVE and non-SVE machines, using SVE if it is >> advantageous. I suppose some ancient C++ compilers without SVE support still >> exist, but I see no very good reason to support them building JDK 22+. >> >> Making a configure option to disable SVE support for vector math is a >> mistake, but IMO mostly harmless because no-one will ever turn it off. > >> That's fine, but we must make sure that SVE is not used by the compiler in >> any other places. If you've already built on non-SVE and tested the result >> on both SVE and non-SVE, I'm happy. > > Agree. > > Since it contains both NEON and SVE functions in libvmath.so, we have to > disable SVE feature when building those NEON functions. We want to separate > NEON/SVE functions in two files, build them with different cflags (i.e. only > build SVE sources with `-march=armv8-a+sve`), and then link to the single > `libvmath.so`. Do we have such similar examples or functions in current jdk > make system? I'm still struggling on finding out an effective way for it. > >> SVE support should be enabled regardless of the build machine. The same >> binary must run on both SVE and non-SVE machines, using SVE if it is >> advantageous. I suppose some ancient C++ compilers without SVE support still >> exist, but I see no very good reason to support them building JDK 22+. >> >> Making a configure option to disable SVE support for vector math is a >> mistake, but IMO mostly harmless because no-one will ever turn it off. > > Yes, SVE feature is also always enabled in jdk hotspot on SVE machines. If we > add the option to give people disable SVE, it's weird that we disabling the > SVE just in libvmath.so, and enabling it in hotspot. Besides, we choose the > NEON stubs for smaller than 128-bit vector operations no matter whether the > runtime machine supports SVE or not. So performance may not be an issue. > Hence, I don't think people have reason disabling SVE features. It makes no sense to configure any of this at build time. Postpone all of the decisions to runtime. Don't touch the make system.Instead, try to open the library at runtime with `os::dll_open()`, after (or inside) `VM_Version::initialize()`. If you're not running on an SVE system, none of the SVE routines will be called, so it doesn't matter, right? ------------- PR Review Comment: https://git.openjdk.org/jdk/pull/16234#discussion_r1407511713
