[PATCH] futex: Optimise the size check get_futex_key()

[Sebastian Andrzej Siewior]

The futex address must be naturally aligned and this is checked via
"address % size" where `address' is the supplied address and `size' is
the expected size of futex. It is guaranteed that `size' is power of two
but the compiler does not see it and creates here a `div' operation
(x86, arm, gcc-15).

We can take advantage of the pow2 property and rewrite it as
"address & (size-1)".

As per testing, the command
|perf bench futex hash -f 1 -b 16384 -t 1 -r 30

improved from
| [thread  0] futex: 0x5619f931f740 [ 7001583 ops/sec ]
to
| [thread  0] futex: 0x55da173e5740 [ 7376137 ops/sec ]

or by 5.3%

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
---

Could someone verify this, please? The 5% look a bit high. This is on
top of the series (but not worsen by the series).

 kernel/futex/core.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/futex/core.c b/kernel/futex/core.c
index 179b26e9c9341..2b00ab510e7d2 100644
--- a/kernel/futex/core.c
+++ b/kernel/futex/core.c
@@ -520,7 +520,7 @@ int get_futex_key(u32 __user *uaddr, unsigned int flags, union futex_key *key,
 	 * The futex address must be "naturally" aligned.
 	 */
 	key->both.offset = address % PAGE_SIZE;
-	if (unlikely((address % size) != 0))
+	if (unlikely((address & (size-1)) != 0))
 		return -EINVAL;
 	address -= key->both.offset;
 
-- 
2.53.0