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futex: Optimise the size check get_futex_key()

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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@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://patch.msgid.link/20260701161736.xYYizA0e@linutronix.de
This commit is contained in:
Sebastian Andrzej Siewior 2026-07-01 18:17:36 +02:00 committed by Peter Zijlstra
parent b78b0b6582
commit 5e601ab361
1 changed files with 1 additions and 1 deletions
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2
kernel/futex/core.c
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@ -516,7 +516,7 @@ 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;