Commit 63678eec authored May 06, 2022 by Heiko Carstens

s390/preempt: disable __preempt_count_add() optimization for PROFILE_ALL_BRANCHES



gcc 12 does not (always) optimize away code that should only be generated
if parameters are constant and within in a certain range. This depends on
various obscure kernel config options, however in particular
PROFILE_ALL_BRANCHES can trigger this compile error:

In function ‘__atomic_add_const’,
    inlined from ‘__preempt_count_add.part.0’ at ./arch/s390/include/asm/preempt.h:50:3:
./arch/s390/include/asm/atomic_ops.h:80:9: error: impossible constraint in ‘asm’
   80 |         asm volatile(                                                   \
      |         ^~~

Workaround this by simply disabling the optimization for
PROFILE_ALL_BRANCHES, since the kernel will be so slow, that this
optimization won't matter at all.

Reported-by: Thomas Richter <tmricht@linux.ibm.com>
Reviewed-by: Sven Schnelle <svens@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>

parent 5ace65eb

arch/s390/include/asm/preempt.h

+11 −4

Original line number	Diff line number	Diff line
		@@ -46,9 +46,16 @@ static inline bool test_preempt_need_resched(void)

		static inline void __preempt_count_add(int val)
		{
		if (__builtin_constant_p(val) && (val >= -128) && (val <= 127))
		/*
		* With some obscure config options and CONFIG_PROFILE_ALL_BRANCHES
		* enabled, gcc 12 fails to handle __builtin_constant_p().
		*/
		if (!IS_ENABLED(CONFIG_PROFILE_ALL_BRANCHES)) {
		if (__builtin_constant_p(val) && (val >= -128) && (val <= 127)) {
		__atomic_add_const(val, &S390_lowcore.preempt_count);
		else
		return;
		}
		}
		__atomic_add(val, &S390_lowcore.preempt_count);
		}