RFR: 8374521: Support fine-grained native debug levels

[Aleksey Shipilev]

On Tue, 6 Jan 2026 08:28:35 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

> [JDK-8374507](https://bugs.openjdk.org/browse/JDK-8374507) forced me to look into debug info sizes again. And I remembered that GCC actually have debug-level support, which can select what kind of diagnostic data is in the debug info. See: https://gcc.gnu.org/onlinedocs/gcc-4.5.0/gcc/Debugging-Options.html
> 
> Allowing users to configure the debug level allows two major deviations from the default level (2):
>  level=1: significantly reduce the debug info size, but still allow backtrace reconstruction
>  level=3: add even more debugging info, like macro expansion, into the debug data
> 
> This RFE upstreams the variant the patch that I have been using in builds.shipilev.net binaries: the fastdebug variants there are done with GCC level=1, greatly optimizing the debug binary sizes, while still maintaining the major benefit of debug binaries -- rich crash diagnostics.
> 
> I threw in this option into current GHA builds to conserve more disk space on runners and/or transfers. 
> 
> I am open for bikeshedding on the option name :)

Motivational improvements in debug info sizes:


$ ls -sh ./build/linux-x86_64-server-fastdebug/images/jdk/lib/server/libjvm.debuginfo
# level=1: 144M
# level=2: 860M
# level=3: 892M


Since debuginfo-s are duplicated several times in our build dir, we are scratching against the 14G guaranteed free space on GHA runners, and reducing to level=1 buys us a lot of headroom:


$ du -sh build/linux-x86_64-server-fastdebug/
# level=1:  4.0G
# level=2: 14.0G
# level=3: 16.0G

Similar thing on MacOS AArch64 (Clang):


% du -sh ./build/macosx-aarch64-server-fastdebug/images/jdk/lib/server/libjvm.dylib.dSYM/ 
# level=1:  92M
# level=2: 202M
# level=3: 202M

% du -sh ./build/macosx-aarch64-server-fastdebug/     
# level=1: 3.6G
# level=2: 5.6G
# level=3: 5.6G

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PR Comment: https://git.openjdk.org/jdk/pull/29056#issuecomment-3713722791
PR Comment: https://git.openjdk.org/jdk/pull/29056#issuecomment-3714487776