GCC < 12.1 can miscompile printf_kunit's errptr() test when branch
profiling is enabled. BUILD_BUG_ON(IS_ERR(PTR)) is a constant false
expression, but CONFIG_TRACE_BRANCH_PROFILING and
CONFIG_PROFILE_ALL_BRANCHES make the IS_ERR() path side-effectful.
GCC's IPA splitter can then outline the cold assert arm into
errptr.part.* and leave that clone with an unconditional
__compiletime_assert_*() call, causing a false build failure.

This started showing up after test_hashed() became a macro and moved its
local buffer into errptr(), which changed GCC's inlining and splitting
decisions enough to expose the compiler bug.

Workaround the problem by disabling the branch profiling for
printf_kunit.o. It is a straightforward and acceptable solution.

The workaround can be removed once the minimum GCC includes commit
76fe49423047 ("Fix tree-optimization/101941: IPA splitting out
function with error attribute"), which first shipped in GCC 12.1.

Fixes: 9bfa52dac27a ("printf: convert test_hashed into macro")
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202604030636.NqjaJvYp-lkp@intel.com/
Cc: stable@vger.kernel.org
Signed-off-by: Petr Mladek <pmladek@suse.com>
---
Changes against v2:

  + Added info about gcc version and commit where the miscompilation
    was fixed. (Tamir)

Changes against v1:

  + Disable the branch profiling for the whole printf_kunit.o
    instead of using "noinline".

lib/tests/Makefile | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/lib/tests/Makefile b/lib/tests/Makefile
index 05f74edbc62b..7e9c2fa52e35 100644
--- a/lib/tests/Makefile
+++ b/lib/tests/Makefile
@@ -40,6 +40,8 @@ obj-$(CONFIG_MEMCPY_KUNIT_TEST) += memcpy_kunit.o
 obj-$(CONFIG_MIN_HEAP_KUNIT_TEST) += min_heap_kunit.o
 CFLAGS_overflow_kunit.o = $(call cc-disable-warning, tautological-constant-out-of-range-compare)
 obj-$(CONFIG_OVERFLOW_KUNIT_TEST) += overflow_kunit.o
+# GCC < 12.1 can miscompile errptr() test when branch profiling is enabled.
+CFLAGS_printf_kunit.o += -DDISABLE_BRANCH_PROFILING
 obj-$(CONFIG_PRINTF_KUNIT_TEST) += printf_kunit.o
 obj-$(CONFIG_RANDSTRUCT_KUNIT_TEST) += randstruct_kunit.o
 obj-$(CONFIG_SCANF_KUNIT_TEST) += scanf_kunit.o
-- 
2.53.0

On Tue, Apr 14, 2026 at 11:41 AM Petr Mladek <pmladek@suse.com> wrote:
>
> GCC < 12.1 can miscompile printf_kunit's errptr() test when branch
> profiling is enabled. BUILD_BUG_ON(IS_ERR(PTR)) is a constant false
> expression, but CONFIG_TRACE_BRANCH_PROFILING and
> CONFIG_PROFILE_ALL_BRANCHES make the IS_ERR() path side-effectful.
> GCC's IPA splitter can then outline the cold assert arm into
> errptr.part.* and leave that clone with an unconditional
> __compiletime_assert_*() call, causing a false build failure.
>
> This started showing up after test_hashed() became a macro and moved its
> local buffer into errptr(), which changed GCC's inlining and splitting
> decisions enough to expose the compiler bug.
>
> Workaround the problem by disabling the branch profiling for
> printf_kunit.o. It is a straightforward and acceptable solution.
>
> The workaround can be removed once the minimum GCC includes commit
> 76fe49423047 ("Fix tree-optimization/101941: IPA splitting out
> function with error attribute"), which first shipped in GCC 12.1.
>
> Fixes: 9bfa52dac27a ("printf: convert test_hashed into macro")
> Reported-by: kernel test robot <lkp@intel.com>
> Closes: https://lore.kernel.org/oe-kbuild-all/202604030636.NqjaJvYp-lkp@intel.com/
> Cc: stable@vger.kernel.org
> Signed-off-by: Petr Mladek <pmladek@suse.com>

Acked-by: Tamir Duberstein <tamird@kernel.org>

On Tue 2026-04-14 12:07:59, Tamir Duberstein wrote:
> On Tue, Apr 14, 2026 at 11:41 AM Petr Mladek <pmladek@suse.com> wrote:
> >
> > GCC < 12.1 can miscompile printf_kunit's errptr() test when branch
> > profiling is enabled. BUILD_BUG_ON(IS_ERR(PTR)) is a constant false
> > expression, but CONFIG_TRACE_BRANCH_PROFILING and
> > CONFIG_PROFILE_ALL_BRANCHES make the IS_ERR() path side-effectful.
> > GCC's IPA splitter can then outline the cold assert arm into
> > errptr.part.* and leave that clone with an unconditional
> > __compiletime_assert_*() call, causing a false build failure.
> >
> > This started showing up after test_hashed() became a macro and moved its
> > local buffer into errptr(), which changed GCC's inlining and splitting
> > decisions enough to expose the compiler bug.
> >
> > Workaround the problem by disabling the branch profiling for
> > printf_kunit.o. It is a straightforward and acceptable solution.
> >
> > The workaround can be removed once the minimum GCC includes commit
> > 76fe49423047 ("Fix tree-optimization/101941: IPA splitting out
> > function with error attribute"), which first shipped in GCC 12.1.
> >
> > Fixes: 9bfa52dac27a ("printf: convert test_hashed into macro")
> > Reported-by: kernel test robot <lkp@intel.com>
> > Closes: https://lore.kernel.org/oe-kbuild-all/202604030636.NqjaJvYp-lkp@intel.com/
> > Cc: stable@vger.kernel.org
> > Signed-off-by: Petr Mladek <pmladek@suse.com>
> 
> Acked-by: Tamir Duberstein <tamird@kernel.org>

Thanks a lot for checking.

JFYI, I have committed the patch into printk/linux.git,
branch for-7.1-printf-kunit-build.

Best Regards,
Petr