[PATCH] libatomic: provide missing symbol __atomic_test_and_set [PR122078]

[Federico Terraneo]

As discussed in https://gcc.gnu.org/bugzilla/show_bug.cgi?id=122078
GCC defines the __atomic_test_and_set builtin in gcc/sync-builtins.def
On architectures without atomic operations this builtin gets implemented
as a call to a function with the same name.
Libatomic does not provide a symbol named __atomic_test_and_set
resulting in an "undefined reference to `__atomic_test_and_set'"
link-time error.

This bug affects different targets such as m68k-rtems and microblaze who
filed separate bug reports
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=118977
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=118280

It also affects armv6-m (cortex-m0), as we discovered testing our
target, arm-miosix-eabi which is for the Miosix RTOS for which we
maintain patches outside of the GCC repository.

This patch modifies libatomic to provide the __atomic_test_and_set



https://gcc.gnu.org/bugzilla/show_bug.cgi?id=118977#c9

PS: This is the first patch I propose to GCC, comments welcome.

Best regards,
Federico

From 4d715a78eed8a51dd363e0daafefb6a00170d03d Mon Sep 17 00:00:00 2001
From: Terraneo Federico <fede....@miosix.org>
Date: Thu, 6 Nov 2025 18:36:03 +0100
Subject: [PATCH] libatomic: provide missing symbol __atomic_test_and_set
 [PR122078]

---
 libatomic/tas_n.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/libatomic/tas_n.c b/libatomic/tas_n.c
index 036a3d23307..2ee7fca07ca 100644
--- a/libatomic/tas_n.c
+++ b/libatomic/tas_n.c
@@ -115,3 +115,10 @@ SIZE(libat_test_and_set) (UTYPE *mptr, int smodel UNUSED)
 
 EXPORT_ALIAS (SIZE(test_and_set));
 #undef LAT_TAS_N
+
+/* Provide __atomic_test_and_set, see
+   https://gcc.gnu.org/bugzilla/show_bug.cgi?id=122078 */
+#if N == 1 && !SIZE(HAVE_ATOMIC_TAS)
+extern bool __atomic_test_and_set (void *mptr, int smodel)
+  __attribute__((alias("libat_test_and_set_1")));
+#endif
-- 
2.25.1

symbol as an alias to __atomic_test_and_set_1, a compatible symbol already present in libatomic. We tested against our target (arm-miosix-eabi). The patch was also tested on rtems for both ARM and m68k:

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