RFR: 8370438: Offer link time optimization support on library level

[Matthias Baesken]

On Fri, 24 Oct 2025 13:24:43 GMT, Erik Joelsson <erikj at openjdk.org> wrote:

>> We currently have support for LTO (link time optimization) for Hotspot/libjvm, that can be enabled as a JVM feature.
>> But for other JDK native libs, we do not have support for this feature.
>> LTO and sometimes lead to faster and also in some cases smaller binaries, so support for this might be interesting also for other libs and not only libjvm.
>
> make/autoconf/flags-ldflags.m4 line 72:
> 
>> 70:         -fPIC"
>> 71: 
>> 72:     LDFLAGS_LTO="-flto=auto -fuse-linker-plugin -fno-strict-aliasing"
> 
> I notice that the compiler args for GCC and Clang are different, but the linker args are the same. Just want to make sure that's intentional.

Yes they are different, this is 'borrowed' from Hotspot flags, see
https://github.com/openjdk/jdk/blob/9625993611bb6acf84d428bea4a65d33b9d66e5f/make/hotspot/lib/JvmFeatures.gmk#L178

where we supported LTO for some time.

> make/common/NativeCompilation.gmk line 101:
> 
>> 99: #   SYSROOT_LDFLAGS the linker flags for using the specific sysroot
>> 100: #   OPTIMIZATION sets optimization level to NONE, LOW, HIGH, HIGHEST, HIGHEST_JVM, SIZE
>> 101: #   LINK_TIME_OPTIMIZATION if set to YES, it enables additionally link time optimization
> 
> For boolean options, we use the vales `true`/`false`.

Thanks for the advice, I'll change it!

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PR Review Comment: https://git.openjdk.org/jdk/pull/27976#discussion_r2468661804
PR Review Comment: https://git.openjdk.org/jdk/pull/27976#discussion_r2468644852