root/tools/testing/selftests/powerpc/stringloops/strlen_32.S

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   1 /* SPDX-License-Identifier: GPL-2.0 */
   2 /*
   3  * strlen() for PPC32
   4  *
   5  * Copyright (C) 2018 Christophe Leroy CS Systemes d'Information.
   6  *
   7  * Inspired from glibc implementation
   8  */
   9 #include <asm/ppc_asm.h>
  10 #include <asm/export.h>
  11 #include <asm/cache.h>
  12 
  13         .text
  14 
  15 /*
  16  * Algorithm:
  17  *
  18  * 1) Given a word 'x', we can test to see if it contains any 0 bytes
  19  *    by subtracting 0x01010101, and seeing if any of the high bits of each
  20  *    byte changed from 0 to 1. This works because the least significant
  21  *    0 byte must have had no incoming carry (otherwise it's not the least
  22  *    significant), so it is 0x00 - 0x01 == 0xff. For all other
  23  *    byte values, either they have the high bit set initially, or when
  24  *    1 is subtracted you get a value in the range 0x00-0x7f, none of which
  25  *    have their high bit set. The expression here is
  26  *    (x - 0x01010101) & ~x & 0x80808080), which gives 0x00000000 when
  27  *    there were no 0x00 bytes in the word.  You get 0x80 in bytes that
  28  *    match, but possibly false 0x80 matches in the next more significant
  29  *    byte to a true match due to carries.  For little-endian this is
  30  *    of no consequence since the least significant match is the one
  31  *    we're interested in, but big-endian needs method 2 to find which
  32  *    byte matches.
  33  * 2) Given a word 'x', we can test to see _which_ byte was zero by
  34  *    calculating ~(((x & ~0x80808080) - 0x80808080 - 1) | x | ~0x80808080).
  35  *    This produces 0x80 in each byte that was zero, and 0x00 in all
  36  *    the other bytes. The '| ~0x80808080' clears the low 7 bits in each
  37  *    byte, and the '| x' part ensures that bytes with the high bit set
  38  *    produce 0x00. The addition will carry into the high bit of each byte
  39  *    iff that byte had one of its low 7 bits set. We can then just see
  40  *    which was the most significant bit set and divide by 8 to find how
  41  *    many to add to the index.
  42  *    This is from the book 'The PowerPC Compiler Writer's Guide',
  43  *    by Steve Hoxey, Faraydon Karim, Bill Hay and Hank Warren.
  44  */
  45 
  46 _GLOBAL(strlen)
  47         andi.   r0, r3, 3
  48         lis     r7, 0x0101
  49         addi    r10, r3, -4
  50         addic   r7, r7, 0x0101  /* r7 = 0x01010101 (lomagic) & clear XER[CA] */
  51         rotlwi  r6, r7, 31      /* r6 = 0x80808080 (himagic) */
  52         bne-    3f
  53         .balign IFETCH_ALIGN_BYTES
  54 1:      lwzu    r9, 4(r10)
  55 2:      subf    r8, r7, r9
  56         and.    r8, r8, r6
  57         beq+    1b
  58         andc.   r8, r8, r9
  59         beq+    1b
  60         andc    r8, r9, r6
  61         orc     r9, r9, r6
  62         subfe   r8, r6, r8
  63         nor     r8, r8, r9
  64         cntlzw  r8, r8
  65         subf    r3, r3, r10
  66         srwi    r8, r8, 3
  67         add     r3, r3, r8
  68         blr
  69 
  70         /* Missaligned string: make sure bytes before string are seen not 0 */
  71 3:      xor     r10, r10, r0
  72         orc     r8, r8, r8
  73         lwzu    r9, 4(r10)
  74         slwi    r0, r0, 3
  75         srw     r8, r8, r0
  76         orc     r9, r9, r8
  77         b       2b
  78 EXPORT_SYMBOL(strlen)

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