From 2908d9bb9afda3325ed778ce0ba3833d91ba438e Mon Sep 17 00:00:00 2001
From: Dan Calavrezo <195309321+dcalavrezo-qorix@users.noreply.github.com>
Date: Fri, 17 Jul 2026 09:02:38 +0300
Subject: [PATCH] aarch64-linux: link libgcc for outline atomic helpers

rust_binary targets built with the ferrocene_aarch64_unknown_linux_gnu
toolchain fail to link when they pull in C/C++ code that performs atomic
operations (e.g. std::atomic, std::shared_ptr refcounts, mutexes via C++
interop):

    undefined reference to `__aarch64_ldadd4_acq_rel'
    undefined reference to `__aarch64_cas2_acq_rel'
    undefined reference to `__aarch64_swp4_acq_rel'

Root cause: GCC 10+ defaults to "outline atomics" on aarch64. Rather than
emitting LSE (ARMv8.1) or LL/SC atomics inline, the compiler emits calls to
helper functions in libgcc that select an LSE or a backward-compatible path
at runtime. Those helpers are defined only in the static libgcc.a; the shared
libgcc_s.so does not export them. rustc links with -nodefaultlibs and adds
-lgcc_s but not -lgcc, so the helpers are never offered to the linker.

Fix: append -Clink-arg=-lgcc to the aarch64-unknown-linux-gnu toolchain's
extra_rustc_flags so libgcc.a is on the link line. It is placed last because
the linker resolves a static archive only against symbols already referenced
to its left, so libgcc.a must follow the C/C++ objects and libraries
(-lstdc++ etc.) that reference the helpers.

This resolves the outline-atomics half of the aarch64 GCC link failure
without requiring downstream repositories to inject the flag via .bazelrc.

MODULE.bazel.lock is regenerated (bazel mod deps --lockfile_mode=update) to
reflect the added flag.

Refs: eclipse-score/toolchains_rust#27, eclipse-score/communication#328
---
 MODULE.bazel      | 1 +
 MODULE.bazel.lock | 5 +++--
 2 files changed, 4 insertions(+), 2 deletions(-)

diff --git a/MODULE.bazel b/MODULE.bazel
index 06f6b27..56fbf2a 100644
--- a/MODULE.bazel
+++ b/MODULE.bazel
@@ -67,6 +67,7 @@ ferrocene.toolchain(
         "-Clink-arg=-lstdc++",
         "-Clink-arg=-lm",
         "-Clink-arg=-lc",
+        "-Clink-arg=-lgcc",
     ],
     miri_sysroot_sha256 = "74f90eabcb34809e44300535016f25eb0cf4a500763c0d18e7f587583b5b9908",
     miri_sysroot_strip_prefix = "aarch64-unknown-linux-gnu",
diff --git a/MODULE.bazel.lock b/MODULE.bazel.lock
index 30d0d9e..27450e9 100644
--- a/MODULE.bazel.lock
+++ b/MODULE.bazel.lock
@@ -348,7 +348,7 @@
     "//extensions:ferrocene_toolchain_ext.bzl%ferrocene_toolchain_ext": {
       "general": {
         "bzlTransitiveDigest": "JTR6scmBZuukTYspnbvaNFU27CfBHiQyu6OywcI1QPE=",
-        "usagesDigest": "3kOi9ZQ7CwvyfvR2TUOe9FHbsxa796pSC1jmnoIxy/g=",
+        "usagesDigest": "rWEZaM0xWw7yeL7B75jmGTsU4xi83MP5olBN4FBicSo=",
         "recordedFileInputs": {},
         "recordedDirentsInputs": {},
         "envVariables": {},
@@ -409,7 +409,8 @@
                 "-Clink-arg=-Wl,--no-as-needed",
                 "-Clink-arg=-lstdc++",
                 "-Clink-arg=-lm",
-                "-Clink-arg=-lc"
+                "-Clink-arg=-lc",
+                "-Clink-arg=-lgcc"
               ],
               "extra_exec_rustc_flags": [],
               "env": {},