arm64/runtime-const: Introduce runtime_const_mask_32()

Futex hash computation requires a mask operation with read-only after
init data that will be converted to a runtime constant in the subsequent
commit.

Introduce runtime_const_mask_32 to further optimize the mask operation
in the futex hash computation hot path. Since all the current use-cases
are of the form GENMASK(n, 0), with n > 0, a single:

  ubfx  w0, w0, #0, #widthm1     // w0 = w0 [widthm1:0]

instruction is used for amd64 to improve instruction dinsity and
performance.

"Arm A-profile A64 Instruction Set Architecture" manual, Sec.
"A64 -- Base Instructions" [1] for UBFX instruction highlights the
immediate "width" is encoded as width minus 1 in imms (Bits [15:10])
which is patched by __runtime_fixup_mask() once the mask is known.

If a future use case arises that needs to tackle arbitrary mask,
consider using:

  movz  w1, #lo16, lsl #0
  movk  w1, #hi16, lsl #16

to patch the 32-bit mask in the asm block and return "__ret & (val)"
from runtime_const_mask_32() which allows compiler to further optimize
the logical and operation. __runtime_fixup_ptr() already patches a
"movz, + movk lsl #16" sequence which can be reused when the need
arises.

A possible implementation for this alternate scheme can be found at [2].

Suggested-by: Samuel Holland <samuel.holland@sifive.com>
Suggested-by: Charlie Jenkins <thecharlesjenkins@gmail.com>
Assisted-by: Claude:claude-sonnet-4-6
Signed-off-by: K Prateek Nayak <kprateek.nayak@amd.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Charlie Jenkins <thecharlesjenkins@gmail.com>
Tested-by: Charlie Jenkins <thecharlesjenkins@gmail.com>
Link: https://developer.arm.com/documentation/ddi0602/2026-03/Base-Instructions/ [1]
Link: https://lore.kernel.org/lkml/20260430094730.31624-4-kprateek.nayak@amd.com/ [2]
Link: https://patch.msgid.link/20260728052540.4728-4-kprateek.nayak@amd.com

0 files changed, 0 insertions, 0 deletions