MdePkg/Library/BaseMemoryLibOptDxe/Arm/CompareMem.S

   1 //
   2 // Copyright (c) 2013 - 2016, Linaro Limited
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  27 //
  28
  29 // Parameters and result.
  30 #define src1      r0
  31 #define src2      r1
  32 #define limit     r2
  33 #define result    r0
  34
  35 // Internal variables.
  36 #define data1     r3
  37 #define data2     r4
  38 #define limit_wd  r5
  39 #define diff      r6
  40 #define tmp1      r7
  41 #define tmp2      r12
  42 #define pos       r8
  43 #define mask      r14
  44
  45     .text
  46     .thumb
  47     .syntax unified
  48     .align  5
  49 ASM_GLOBAL ASM_PFX(InternalMemCompareMem)
  50 ASM_PFX(InternalMemCompareMem):
  51     push    {r4-r8, lr}
  52     eor     tmp1, src1, src2
  53     tst     tmp1, #3
  54     bne     .Lmisaligned4
  55     ands    tmp1, src1, #3
  56     bne     .Lmutual_align
  57     add     limit_wd, limit, #3
  58     nop.w
  59     lsr     limit_wd, limit_wd, #2
  60
  61     // Start of performance-critical section  -- one 32B cache line.
  62 .Lloop_aligned:
  63     ldr     data1, [src1], #4
  64     ldr     data2, [src2], #4
  65 .Lstart_realigned:
  66     subs    limit_wd, limit_wd, #1
  67     eor     diff, data1, data2        // Non-zero if differences found.
  68     cbnz    diff, 0f
  69     bne     .Lloop_aligned
  70     // End of performance-critical section  -- one 32B cache line.
  71
  72     // Not reached the limit, must have found a diff.
  73 0:  cbnz    limit_wd, .Lnot_limit
  74
  75     // Limit % 4 == 0 => all bytes significant.
  76     ands    limit, limit, #3
  77     beq     .Lnot_limit
  78
  79     lsl     limit, limit, #3              // Bits -> bytes.
  80     mov     mask, #~0
  81     lsl     mask, mask, limit
  82     bic     data1, data1, mask
  83     bic     data2, data2, mask
  84
  85     orr     diff, diff, mask
  86
  87 .Lnot_limit:
  88     rev     diff, diff
  89     rev     data1, data1
  90     rev     data2, data2
  91
  92     // The MS-non-zero bit of DIFF marks either the first bit
  93     // that is different, or the end of the significant data.
  94     // Shifting left now will bring the critical information into the
  95     // top bits.
  96     clz     pos, diff
  97     lsl     data1, data1, pos
  98     lsl     data2, data2, pos
  99
 100     // But we need to zero-extend (char is unsigned) the value and then
 101     // perform a signed 32-bit subtraction.
 102     lsr     data1, data1, #28
 103     sub     result, data1, data2, lsr #28
 104     pop     {r4-r8, pc}
 105
 106 .Lmutual_align:
 107     // Sources are mutually aligned, but are not currently at an
 108     // alignment boundary.  Round down the addresses and then mask off
 109     // the bytes that precede the start point.
 110     bic     src1, src1, #3
 111     bic     src2, src2, #3
 112     add     limit, limit, tmp1          // Adjust the limit for the extra.
 113     lsl     tmp1, tmp1, #2              // Bytes beyond alignment -> bits.
 114     ldr     data1, [src1], #4
 115     neg     tmp1, tmp1                  // Bits to alignment -32.
 116     ldr     data2, [src2], #4
 117     mov     tmp2, #~0
 118
 119     // Little-endian.  Early bytes are at LSB.
 120     lsr     tmp2, tmp2, tmp1            // Shift (tmp1 & 31).
 121     add     limit_wd, limit, #3
 122     orr     data1, data1, tmp2
 123     orr     data2, data2, tmp2
 124     lsr     limit_wd, limit_wd, #2
 125     b       .Lstart_realigned
 126
 127 .Lmisaligned4:
 128     sub     limit, limit, #1
 129 1:
 130     // Perhaps we can do better than this.
 131     ldrb    data1, [src1], #1
 132     ldrb    data2, [src2], #1
 133     subs    limit, limit, #1
 134     it      cs
 135     cmpcs   data1, data2
 136     beq     1b
 137     sub     result, data1, data2
 138     pop     {r4-r8, pc}