MdePkg/Library/BaseMemoryLibOptDxe/Arm/CompareMem.asm

;
; Copyright (c) 2013 - 2016, Linaro Limited
; All rights reserved.
; SPDX-License-Identifier: BSD-2-Clause-Patent
;

; Parameters and result.
#define src1      r0
#define src2      r1
#define limit     r2
#define result    r0

; Internal variables.
#define data1     r3
#define data2     r4
#define limit_wd  r5
#define diff      r6
#define tmp1      r7
#define tmp2      r12
#define pos       r8
#define mask      r14

    EXPORT  InternalMemCompareMem
    THUMB
    AREA    CompareMem, CODE, READONLY

InternalMemCompareMem
    push    {r4-r8, lr}
    eor     tmp1, src1, src2
    tst     tmp1, #3
    bne     Lmisaligned4
    ands    tmp1, src1, #3
    bne     Lmutual_align
    add     limit_wd, limit, #3
    nop.w
    lsr     limit_wd, limit_wd, #2

    ; Start of performance-critical section  -- one 32B cache line.
Lloop_aligned
    ldr     data1, [src1], #4
    ldr     data2, [src2], #4
Lstart_realigned
    subs    limit_wd, limit_wd, #1
    eor     diff, data1, data2        ; Non-zero if differences found.
    cbnz    diff, L0
    bne     Lloop_aligned
    ; End of performance-critical section  -- one 32B cache line.

    ; Not reached the limit, must have found a diff.
L0
    cbnz    limit_wd, Lnot_limit

    // Limit % 4 == 0 => all bytes significant.
    ands    limit, limit, #3
    beq     Lnot_limit

    lsl     limit, limit, #3              // Bits -> bytes.
    mov     mask, #~0
    lsl     mask, mask, limit
    bic     data1, data1, mask
    bic     data2, data2, mask

    orr     diff, diff, mask

Lnot_limit
    rev     diff, diff
    rev     data1, data1
    rev     data2, data2

    ; The MS-non-zero bit of DIFF marks either the first bit
    ; that is different, or the end of the significant data.
    ; Shifting left now will bring the critical information into the
    ; top bits.
    clz     pos, diff
    lsl     data1, data1, pos
    lsl     data2, data2, pos

    ; But we need to zero-extend (char is unsigned) the value and then
    ; perform a signed 32-bit subtraction.
    lsr     data1, data1, #28
    sub     result, data1, data2, lsr #28
    pop     {r4-r8, pc}

Lmutual_align
    ; Sources are mutually aligned, but are not currently at an
    ; alignment boundary.  Round down the addresses and then mask off
    ; the bytes that precede the start point.
    bic     src1, src1, #3
    bic     src2, src2, #3
    add     limit, limit, tmp1          ; Adjust the limit for the extra.
    lsl     tmp1, tmp1, #2              ; Bytes beyond alignment -> bits.
    ldr     data1, [src1], #4
    neg     tmp1, tmp1                  ; Bits to alignment -32.
    ldr     data2, [src2], #4
    mov     tmp2, #~0

    ; Little-endian.  Early bytes are at LSB.
    lsr     tmp2, tmp2, tmp1            ; Shift (tmp1 & 31).
    add     limit_wd, limit, #3
    orr     data1, data1, tmp2
    orr     data2, data2, tmp2
    lsr     limit_wd, limit_wd, #2
    b       Lstart_realigned

Lmisaligned4
    sub     limit, limit, #1
L1
    // Perhaps we can do better than this.
    ldrb    data1, [src1], #1
    ldrb    data2, [src2], #1
    subs    limit, limit, #1
    it      cs
    cmpcs   data1, data2
    beq     L1
    sub     result, data1, data2
    pop     {r4-r8, pc}

    END