[DPDK/core Bug 1949] function versioning is incompatible with LTO

[DPDK/core Bug 1949] function versioning is incompatible with LTO

[bugzilla]

http://bugs.dpdk.org/show_bug.cgi?id=1949

            Bug ID: 1949
           Summary: function versioning is incompatible with LTO
           Product: DPDK
           Version: 25.07
          Hardware: All
                OS: All
            Status: UNCONFIRMED
          Severity: minor
          Priority: Normal
         Component: core
          Assignee: dev@dpdk.org
          Reporter: stephen@networkplumber.org
  Target Milestone: ---

The default-version symver (@@) fails to assemble when build is using LTO.

Building a shared library with LTO fails when a function uses
RTE_DEFAULT_SYMBOL.
I introduces a new versioned symbol rte_eth_dev_get_name_by_port (ethdev).
This fails when doing LTO build with

invalid attempt to declare external version name as default
in symbol `rte_eth_dev_get_name_by_port@@DPDK_27'

Cause: the versioning macros emit a file-scope __asm__(".symver
…@@DPDK_<ver>"). Top-level asm is opaque to LTO, so GCC may place the directive
in a different ltrans partition than the _v<ver> definition. The implementation
symbol is then external in that partition, and gas rejects binding a default
version (@@) to an external symbol. Only the @@ (default) line fails; the
single-@ compat line is legal on an external symbol.

Fix: use the GCC symver function attribute (GCC ≥ 10) instead of top-level asm,
gated on __has_attribute(symver); retain the asm path as fallback. Clang lacks
the attribute and stays on the asm path (LTO + versioning remains unsupported
there).

Marking severity as minor since no current in-tree code uses versioned symbols.
But it is a trap waiting to happen.

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