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cast_user converts an arena offset into a user address by combining the low 32 bits of the pointer with the upper 32 bits of user_vm_start, while keeping a NULL pointer NULL. The current sequence always emits six instructions: it materializes user_vm_start >> 32 into a register, shifts it into place, and ORs in the offset. The upper half of user_vm_start is a constant, so it can be written directly onto the offset with MOVK. Move the 32-bit offset into dst (which also zeroes the upper 32 bits), then MOVK the non-zero halfwords of the upper address, branching over the MOVKs when the offset is zero so NULL is preserved. This emits at most four instructions, and only one when the upper half of user_vm_start is zero. The generated code is equivalent. Before: ; bpf_addr_space_cast(page1, 1, 0); 7c: mov w10, w8 80: mov w8, #1 84: lsl x8, x8, #32 88: cbz x10, 0xffff800087b80c20 8c: orr x10, x8, x10 90: mov x8, x10 After: ; bpf_addr_space_cast(page1, 1, 0); 7c: mov w8, w8 80: cbz w8, 0xffff800087b80c28 84: movk x8, #1, lsl #32 Signed-off-by: Puranjay Mohan <puranjay@kernel.org> Acked-by: Xu Kuohai <xukuohai@huawei.com> Link: https://lore.kernel.org/bpf/20260721105921.1070501-1-puranjay@kernel.org Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>