Re: [PATCH] eal: fix function versioning with LTO

[Stephen Hemminger <stephen@networkplumber.org>]

On Wed, 3 Jun 2026 12:01:48 +0200
David Marchand <david.marchand@redhat.com> wrote:

> Hello,
> 
> On Wed, 3 Jun 2026 at 00:57, Stephen Hemminger
> <stephen@networkplumber.org> wrote:
> >
> > When using function versioning and building with Link Time Optimization,
> > the compiler does not see the __asm__ annotation of symbols and
> > therefore thinks there are two versions of the same symbol.
> >
> > The fix is to use compiler symver attribute on the function which
> > was added in GCC 10. Keep the older method for backward compatibility
> > with older compilers.
> >
> > Bugzilla ID: 1949
> > Fixes: e30e194c4d06 ("eal: rework function versioning macros")  
> 
> We never used the symver stuff, so it seems unlikely the issue was
> introduced with this rework.
> 
> The fact that clang does not support this attribute is a concern.
> 
> 
> > Cc: stable@dpdk.org  
> 
> Why do we need to backport?
> 
> LTO is kind of experimental, so it seems good enough to reply "not
> expected to work in older LTS" if someone reported an issue.
> 
> And in practice, no LTS release call the versioning macros, since a
> LTS drops all compatibility.
> 
> > Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>  
> 
> I would like to reproduce, but I can't build main with LTO.
> What patches did you apply locally to avoid warnings on the hash library?
I get no warnings from current main with GCC 15

I was doing test build of net-next with this patch:
https://patchwork.dpdk.org/project/dpdk/patch/20260529000748.275863-1-stephen@networkplumber.org/

The LTO build is done by:
 $ meson setup build-lto -Db_lto=true

The result was:
cc  -o lib/librte_ethdev.so.26.2 lib/librte_ethdev.so.26.2.p/ethdev_ethdev_driver.c.o lib/librte_ethdev.so.26.2.p/ethdev_ethdev_private.c.o lib/librte_ethdev.so.26.2.p/ethdev_ethdev_profile.c.o lib/librte_ethdev.so.26.2.p/ethdev_ethdev_trace_points.c.o lib/librte_ethdev.so.26.2.p/ethdev_rte_class_eth.c.o lib/librte_ethdev.so.26.2.p/ethdev_rte_ethdev.c.o lib/librte_ethdev.so.26.2.p/ethdev_rte_ethdev_cman.c.o lib/librte_ethdev.so.26.2.p/ethdev_rte_ethdev_telemetry.c.o lib/librte_ethdev.so.26.2.p/ethdev_rte_flow.c.o lib/librte_ethdev.so.26.2.p/ethdev_rte_mtr.c.o lib/librte_ethdev.so.26.2.p/ethdev_rte_tm.c.o lib/librte_ethdev.so.26.2.p/ethdev_sff_telemetry.c.o lib/librte_ethdev.so.26.2.p/ethdev_sff_common.c.o lib/librte_ethdev.so.26.2.p/ethdev_sff_8079.c.o lib/librte_ethdev.so.26.2.p/ethdev_sff_8472.c.o lib/librte_ethdev.so.26.2.p/ethdev_sff_8636.c.o lib/librte_ethdev.so.26.2.p/ethdev_ethdev_linux_ethtool.c.o -flto=auto -Wl,--as-needed -Wl,--no-undefined -Wl,-O1 -shared -fPIC -Wl,-soname,librte_ethdev.so.26 -Wl,--no-as-needed -Wl,--undefined-version -pthread -Wl,--start-group -lm -ldl -lnuma -lfdt '-Wl,-rpath,$ORIGIN/' lib/librte_eal.so.26.2 lib/librte_kvargs.so.26.2 lib/librte_log.so.26.2 lib/librte_telemetry.so.26.2 lib/librte_argparse.so.26.2 lib/librte_net.so.26.2 lib/librte_mbuf.so.26.2 lib/librte_mempool.so.26.2 lib/librte_ring.so.26.2 lib/librte_meter.so.26.2 -Wl,--version-script=/home/shemminger/DPDK/lto/build-lto/lib/ethdev_exports.map /usr/lib/x86_64-linux-gnu/libbsd.so /usr/lib/x86_64-linux-gnu/libarchive.so -Wl,--end-group
/tmp/cc3RQyqL.s: Assembler messages:
/tmp

Fed the result into Claude, and it said "yeah, I see the same problem on
other projects, the answer is ...". With a little searching found an example
in Gentoo https://github.com/InBetweenNames/gentooLTO/pull/458

With this change symbol versioning with LTO works on both GCC >= 10 and Clang.
Clang LTO doesn't have the attribute, but it also doesn't have the same LTO issue.