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

[Peter Zijlstra]

On Wed, Jul 01, 2026 at 06:17:36PM +0200, Sebastian Andrzej Siewior wrote:
> 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).

That's pretty stupid, since size = futex_size() := 1 << (blah); so it
has every opportunity to actually see this. Also, see below, clang does!

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

Bah, I tried to reproduce and couldn't. Then I noticed I did a clang
build and that is in fact clever enough to do this optimization itself.

/me tries again with a GCC build.

pre:  [thread  0] futex: 0x561f14430680 [ 9021408 ops/sec ]
post: [thread  0] futex: 0x55feadbbb680 [ 8977527 ops/sec ]

(and this seems to be well inside the error threshold of this test).

So I see the GCC build do the DIV, and no longer with his patch applied,
but for some reason I cannot get the runtime performance to actually
improve much of anything on my system.