test: Add a test for atomic load/store of large integers

Test atomically loading/storing 128 bit integers on 64 bit, and
64 bit integers on 32 bit.

This test case covers the root cause of
https://github.com/mstorsjo/llvm-mingw/issues/538.

test/Makefile

@@ -101,7 +101,7 @@ ifneq ($(HAVE_CFGUARD),)
     TESTS_CFGUARD = cfguard-test
     TESTS_ASAN_CFGUARD = $(TESTS_ASAN)
 endif
-TESTS_ATOMIC = atomic-helpers
+TESTS_ATOMIC = atomic-helpers atomic-load-store
 
 TARGETS_C = $(addsuffix $(EXEEXT), $(TESTS_C))
 TARGETS_C_DLL = $(addsuffix $(DLLEXT), $(TESTS_C_DLL))

test/atomic-load-store.c

@@ -0,0 +1,51 @@
+/*
+ * Copyright (c) 2026 Martin Storsjo
+ *
+ * This file is part of llvm-mingw.
+ *
+ * Permission to use, copy, modify, and/or distribute this software for any
+ * purpose with or without fee is hereby granted, provided that the above
+ * copyright notice and this permission notice appear in all copies.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+ * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+ * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+ * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+ * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
+ * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+ * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+ */
+
+#include <stdatomic.h>
+#include <stdint.h>
+#include <stdio.h>
+#include <string.h>
+
+#ifdef __SIZEOF_INT128__
+#define TYPE __uint128_t
+#else
+#define TYPE uint64_t
+#endif
+
+_Atomic(TYPE) atomic_var;
+TYPE regular_var;
+
+int main(int argc, char **argv) {
+    regular_var = 0x0123456789abcdef;
+#ifdef __SIZEOF_INT128__
+    regular_var <<= 64;
+    regular_var |= 0xfedcba9876543210;
+#endif
+    memcpy(&atomic_var, &regular_var, sizeof(regular_var));
+    regular_var = atomic_load(&atomic_var);
+    if (memcmp(&atomic_var, &regular_var, sizeof(regular_var))) {
+        printf("load failed\n");
+        return 1;
+    }
+    atomic_store(&atomic_var, regular_var);
+    if (memcmp(&atomic_var, &regular_var, sizeof(regular_var))) {
+        printf("store failed\n");
+        return 1;
+    }
+    return 0;
+}