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s390/preempt: Optimize __preemp_count_add()/__preempt_count_sub()

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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_disable(), which subtracts one from
preempt_count. With this the generated code changes from

  10012c:       a7 b9 00 00             lghi    %r11,0
  100130:       eb 01 b3 a8 00 6a       asi     936(%r11),1

to something like this:

 10012c:        eb 01 03 a8 00 6a       asi     936,1

Kernel image size is reduced by 13kb (bloat-o-meter -t, defconfig, gcc15).

Reviewed-by: Sven Schnelle <svens@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
This commit is contained in:
Heiko Carstens 2026-01-13 20:43:58 +01:00
parent 6cce3609a1
commit 23ba7d3163
1 changed files with 11 additions and 1 deletions
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12
arch/s390/include/asm/preempt.h
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@ -84,7 +84,17 @@ static __always_inline void __preempt_count_add(int val)
*/
if (!IS_ENABLED(CONFIG_PROFILE_ALL_BRANCHES)) {
if (__builtin_constant_p(val) && (val >= -128) && (val <= 127)) {
__atomic_add_const(val, &get_lowcore()->preempt_count);
unsigned long lc_preempt;
lc_preempt = offsetof(struct lowcore, preempt_count);
asm_inline(
ALTERNATIVE("asi %[offzero](%%r0),%[val]\n",
"asi %[offalt](%%r0),%[val]\n",
ALT_FEATURE(MFEATURE_LOWCORE))
: "+m" (((struct lowcore *)0)->preempt_count)
: [offzero] "i" (lc_preempt), [val] "i" (val),
[offalt] "i" (lc_preempt + LOWCORE_ALT_ADDRESS)
: "cc");
return;
}
}