Native library optimization level research

This page will discuss all our native JDK libraries (except Hotspot) with regard to optimization levels, both what is currently used, and material for discussing a change.

Current C/C++ JDK native libraries, default (bold) and other optimization levels and the size of the libs on various platforms ; info is from JDK25

Optimizing levels marked with * have special settings e.g. regarding compiler version/debug level

TODOs: 

• check LIBMANAGEMENT special optimization settings for Linux with debug symbols →  removed this special handling  https://bugs.openjdk.org/browse/JDK-8351542
• check LIBVERIFY special optimization settings for Linux with debug symbols  →  removed this special handling https://bugs.openjdk.org/browse/JDK-8352015

• the lib sizes on AIX are  HUGE ;  this is because we have the debug info stored inside the lib and not separated like on other platforms;  it should be possible to separate the debug info on AIX to stabfiles but currently there are issues with tool support

• macOS / clang : try -Oz size optimization (see https://clang.llvm.org/docs/CommandGuide/clang.html  Code Generation options )  for SIZE =>  this sometimes leads to slightly smaller libs, but no huge decreases in lib size
• add some measurements with lto (link time optimization) ; this could lead to good results regarding lib-size when gcc is used and HIGHEST is the default optimization level of a lib
• check and compare the build times for some example lib(s) e.g. libjdwp for different Opt-level  ;   see  https://github.com/openjdk/jdk/pull/23563#issuecomment-2786694168  for  LIB_JDWP build times on Linux x86_64 / gcc-13.2.0 ; looks like LOW and HIGHEST have rather similar build times on this platform, SIZE is somewhat faster
• check LTO build option for (some?) libs; this can potentially decrease  lib size a lot (at least with gcc) and also produce faster code

VS2022 17.6.5 (SAP)