[PATCH v2] overflow: disable failing tests for older clang versions

[Nick Desaulniers <ndesaulniers@google.com>]

Building the overflow kunit tests with clang-11 fails with:

$ ./tools/testing/kunit/kunit.py run --arch=arm --make_options LLVM=1 \
overflow
...
ld.lld: error: undefined symbol: __mulodi4
...

Clang 11 and earlier generate unwanted libcalls for signed output,
unsigned input.

Disable these tests for now, but should these become used in the kernel
we might consider that as justification for dropping clang-11 support.
Keep the clang-11 build alive a little bit longer.

Avoid -Wunused-function warnings via __maybe_unused. To test W=1:

$ make LLVM=1 -j128 defconfig
$ ./scripts/config -e KUNIT -e KUNIT_ALL
$ make LLVM=1 -j128 olddefconfig lib/overflow_kunit.o W=1

Link: https://github.com/ClangBuiltLinux/linux/issues/1711
Link: https://github.com/llvm/llvm-project/commit/3203143f1356a4e4e3ada231156fc6da6e1a9f9d
Reported-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
---
Changes v1 -> v2:
* Put back previous clang-13 related BITS_PER_LONG == 64 guards; all
  BITS_PER_LONG guards would need to be updated with the condition I was
  trying to change. Otherwise kernel test robot reported instances of
  "use of undeclared identifier" errors.
* Simplify my condition; the issue has nothing to do with lp64 targets.
* Remove link from comment.
* Add instructions for W=1 testing.
* Tested with clang-11 and clang-16 (ToT).

 lib/overflow_kunit.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/lib/overflow_kunit.c b/lib/overflow_kunit.c
index f385ca652b74..2914c9d36b0f 100644
--- a/lib/overflow_kunit.c
+++ b/lib/overflow_kunit.c
@@ -254,6 +254,7 @@ static void do_test_ ## n(struct kunit *test, const struct test_ ## n *p) \
 	check_one_op(t, fmt, mul, "*", p->b, p->a, p->prod, p->p_of);	\
 }									\
 									\
+__maybe_unused								\
 static void n ## _overflow_test(struct kunit *test) {			\
 	unsigned i;							\
 									\
@@ -720,8 +721,14 @@ static struct kunit_case overflow_test_cases[] = {
 	KUNIT_CASE(u64_u64__u64_overflow_test),
 	KUNIT_CASE(s64_s64__s64_overflow_test),
 #endif
-	KUNIT_CASE(u32_u32__u8_overflow_test),
+/*
+ * Clang 11 and earlier generate unwanted libcalls for signed output, unsigned
+ * input.
+ */
+#if !(defined(CONFIG_CC_IS_CLANG) && __clang_major__ <= 11)
 	KUNIT_CASE(u32_u32__int_overflow_test),
+#endif
+	KUNIT_CASE(u32_u32__u8_overflow_test),
 	KUNIT_CASE(u8_u8__int_overflow_test),
 	KUNIT_CASE(int_int__u8_overflow_test),
 	KUNIT_CASE(shift_sane_test),

base-commit: 833477fce7a14d43ae4c07f8ddc32fa5119471a2
-- 
2.38.0.rc2.412.g84df46c1b4-goog