Commit 48b4790f authored by Heiko Carstens's avatar Heiko Carstens
Browse files

s390/preempt: Optimize __preempt_count_dec_and_test() 



Provide an inline assembly using alternatives to avoid the need of a
base register due to relocatable lowcore when adding or subtracting
small constants from preempt_count.

Main user is preempt_enable(), which subtracts one from preempt_count
and tests if the result is zero.

With this the generated code changes from

  1000b8:       a7 19 00 00             lghi    %r1,0
  1000bc:       eb ff 13 a8 00 6e       alsi    936(%r1),-1
  1000c2:       a7 54 00 05             jnhe    1000cc <__rcu_read_unlock+0x14>

to something like this:

  1000b8:       eb ff 03 a8 00 6e       alsi    936,-1
  1000be:       a7 54 00 05             jnhe    1000c8 <__rcu_read_unlock+0x10>

Kernel image size is reduced by 45kb (bloat-o-meter -t, defconfig, gcc15).

Reviewed-by: default avatarSven Schnelle <svens@linux.ibm.com>
Signed-off-by: default avatarHeiko Carstens <hca@linux.ibm.com>
parent 05405b8f

arch/s390/include/asm/preempt.h

+15 −0
Original line number Diff line number Diff line
@@ -113,7 +113,22 @@ static __always_inline void __preempt_count_sub(int val) 
 */

static __always_inline bool __preempt_count_dec_and_test(void)

{

#ifdef __HAVE_ASM_FLAG_OUTPUTS__

	unsigned long lc_preempt;

	int cc;



	lc_preempt = offsetof(struct lowcore, preempt_count);

	asm_inline(

		ALTERNATIVE("alsi	%[offzero](%%r0),%[val]\n",

			    "alsi	%[offalt](%%r0),%[val]\n",

			    ALT_FEATURE(MFEATURE_LOWCORE))

		: "=@cc" (cc), "+m" (((struct lowcore *)0)->preempt_count)

		: [offzero] "i" (lc_preempt), [val] "i" (-1),

		[offalt] "i" (lc_preempt + LOWCORE_ALT_ADDRESS));

	return (cc == 0) || (cc == 2);

#else

	return __atomic_add_const_and_test(-1, &get_lowcore()->preempt_count);

#endif

}



/*