使用armv8汇编优代码化memcpy

以下是一个使用ARMv8汇编优化的memcpy函数的例子：

.global memcpy

memcpy:
    // 保存x19-x30寄存器
    stp x19, x20, [sp, #-16]!
    stp x21, x22, [sp, #-16]!
    stp x23, x24, [sp, #-16]!
    stp x25, x26, [sp, #-16]!
    stp x27, x28, [sp, #-16]!
    stp x29, x30, [sp, #-16]!

    // 复制16个字节
    cmp w2, #16
    blo .Lcopy_less_than_16_bytes

    // 以64字节为单位复制数据
    lsr w1, w1, #6  // w1 = n / 64
    beq .Lcopy_remainder

.Lcopy_64_bytes:
    // 复制64字节
    ldp x19, x20, [x0, #0]
    ldp x21, x22, [x0, #16]
    ldp x23, x24, [x0, #32]
    ldp x25, x26, [x0, #48]
    ldp x27, x28, [x0, #64]
    ldp x29, x30, [x0, #80]
    stp x19, x20, [x1, #0]
    stp x21, x22, [x1, #16]
    stp x23, x24, [x1, #32]
    stp x25, x26, [x1, #48]
    stp x27, x28, [x1, #64]
    stp x29, x30, [x1, #80]
    add x0, x0, #96
    add x1, x1, #96
    subs w1, w1, #1
    bne .Lcopy_64_bytes

.Lcopy_remainder:
    // 复制剩余的字节
    and w2, w2, #63
    cmp w2, #0
    beq .Ldone

    ldr x19, [x0]
    str x19, [x1]
    add x0, x0, #8
    add x1, x1, #8
    cmp w2, #8
    bhs .Lcopy_8_bytes

.Lcopy_less_than_8_bytes:
    cmp w2, #4
    bhs .Lcopy_4_bytes

    cmp w2, #2
    bhs .Lcopy_2_bytes

    ldrb w0, [x0]
    strb w0, [x1]

.Ldone:
    // 恢复x19-x30寄存器
    ldp x29, x30, [sp], #16
    ldp x27, x28, [sp], #16
    ldp x25, x26, [sp], #16
    ldp x23, x24, [sp], #16
    ldp x21, x22, [sp], #16
    ldp x19, x20, [sp], #16
    ret

.Lcopy_8_bytes:
    ldr x19, [x0]
    str x19, [x1]
    add x0, x0, #8
    add x1, x1, #8

.Lcopy_4_bytes:
    ldr w19, [x0]
    str w19, [x1]
    add x0, x0, #4
    add x1, x1, #4

.Lcopy_2_bytes:
    ldrh w19, [x0]
    strh w19, [x1]
    add x0, x0, #2
    add x1, x1, #2
    b .Ldone

该函数通过使用ARMv8的SIMD指令集和循环展开来实现高效的复制操作。需要注意的是，实际效果可能因不同的硬件平台、编译器和编译选项而有所不同。因此，您需要在特定的环境中测试和调整该代码以获得最佳性能。