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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 |
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