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314 lines
12 KiB
ArmAsm
314 lines
12 KiB
ArmAsm
/* memchr (str, chr, len) -- Return pointer to first occurrence of CHR in STR
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less than LEN. For Intel 80x86, x>=3.
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Copyright (C) 1994-2024 Free Software Foundation, Inc.
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This file is part of the GNU C Library.
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The GNU C Library is free software; you can redistribute it and/or
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modify it under the terms of the GNU Lesser General Public
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License as published by the Free Software Foundation; either
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version 2.1 of the License, or (at your option) any later version.
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The GNU C Library is distributed in the hope that it will be useful,
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but WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
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Lesser General Public License for more details.
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You should have received a copy of the GNU Lesser General Public
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License along with the GNU C Library; if not, see
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<https://www.gnu.org/licenses/>. */
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#include <sysdep.h>
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#include "asm-syntax.h"
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#define PARMS 4+8 /* space for 2 saved regs */
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#define RTN PARMS
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#define STR RTN
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#define CHR STR+4
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#define LEN CHR+4
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.text
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ENTRY (__memchr)
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/* Save callee-safe registers used in this function. */
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pushl %esi
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cfi_adjust_cfa_offset (4)
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pushl %edi
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cfi_adjust_cfa_offset (4)
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cfi_rel_offset (edi, 0)
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/* Load parameters into registers. */
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movl STR(%esp), %eax /* str: pointer to memory block. */
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movl CHR(%esp), %edx /* c: byte we are looking for. */
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movl LEN(%esp), %esi /* len: length of memory block. */
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cfi_rel_offset (esi, 4)
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/* If my must not test more than three characters test
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them one by one. This is especially true for 0. */
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cmpl $4, %esi
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jb L(3)
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/* At the moment %edx contains CHR. What we need for the
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algorithm is CHR in all bytes of the dword. Avoid
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operations on 16 bit words because these require an
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prefix byte (and one more cycle). */
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movb %dl, %dh /* Now it is 0|0|c|c */
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movl %edx, %ecx
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shll $16, %edx /* Now c|c|0|0 */
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movw %cx, %dx /* And finally c|c|c|c */
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/* Better performance can be achieved if the word (32
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bit) memory access is aligned on a four-byte-boundary.
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So process first bytes one by one until boundary is
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reached. Don't use a loop for better performance. */
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testb $3, %al /* correctly aligned ? */
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je L(2) /* yes => begin loop */
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cmpb %dl, (%eax) /* compare byte */
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je L(9) /* target found => return */
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incl %eax /* increment source pointer */
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decl %esi /* decrement length counter */
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je L(4) /* len==0 => return NULL */
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testb $3, %al /* correctly aligned ? */
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je L(2) /* yes => begin loop */
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cmpb %dl, (%eax) /* compare byte */
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je L(9) /* target found => return */
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incl %eax /* increment source pointer */
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decl %esi /* decrement length counter */
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je L(4) /* len==0 => return NULL */
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testb $3, %al /* correctly aligned ? */
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je L(2) /* yes => begin loop */
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cmpb %dl, (%eax) /* compare byte */
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je L(9) /* target found => return */
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incl %eax /* increment source pointer */
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decl %esi /* decrement length counter */
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/* no test for len==0 here, because this is done in the
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loop head */
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jmp L(2)
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/* We exit the loop if adding MAGIC_BITS to LONGWORD fails to
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change any of the hole bits of LONGWORD.
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1) Is this safe? Will it catch all the zero bytes?
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Suppose there is a byte with all zeros. Any carry bits
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propagating from its left will fall into the hole at its
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least significant bit and stop. Since there will be no
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carry from its most significant bit, the LSB of the
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byte to the left will be unchanged, and the zero will be
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detected.
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2) Is this worthwhile? Will it ignore everything except
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zero bytes? Suppose every byte of LONGWORD has a bit set
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somewhere. There will be a carry into bit 8. If bit 8
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is set, this will carry into bit 16. If bit 8 is clear,
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one of bits 9-15 must be set, so there will be a carry
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into bit 16. Similarly, there will be a carry into bit
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24. If one of bits 24-31 is set, there will be a carry
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into bit 32 (=carry flag), so all of the hole bits will
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be changed.
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3) But wait! Aren't we looking for CHR, not zero?
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Good point. So what we do is XOR LONGWORD with a longword,
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each of whose bytes is CHR. This turns each byte that is CHR
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into a zero. */
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/* Each round the main loop processes 16 bytes. */
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ALIGN (4)
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L(1): movl (%eax), %ecx /* get word (= 4 bytes) in question */
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movl $0xfefefeff, %edi /* magic value */
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xorl %edx, %ecx /* XOR with word c|c|c|c => bytes of str == c
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are now 0 */
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addl %ecx, %edi /* add the magic value to the word. We get
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carry bits reported for each byte which
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is *not* 0 */
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/* According to the algorithm we had to reverse the effect of the
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XOR first and then test the overflow bits. But because the
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following XOR would destroy the carry flag and it would (in a
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representation with more than 32 bits) not alter then last
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overflow, we can now test this condition. If no carry is signaled
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no overflow must have occurred in the last byte => it was 0. */
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jnc L(8)
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/* We are only interested in carry bits that change due to the
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previous add, so remove original bits */
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xorl %ecx, %edi /* ((word^charmask)+magic)^(word^charmask) */
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/* Now test for the other three overflow bits. */
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orl $0xfefefeff, %edi /* set all non-carry bits */
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incl %edi /* add 1: if one carry bit was *not* set
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the addition will not result in 0. */
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/* If at least one byte of the word is CHR we don't get 0 in %edi. */
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jnz L(8) /* found it => return pointer */
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/* This process is unfolded four times for better performance.
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we don't increment the source pointer each time. Instead we
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use offsets and increment by 16 in each run of the loop. But
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before probing for the matching byte we need some extra code
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(following LL(13) below). Even the len can be compared with
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constants instead of decrementing each time. */
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movl 4(%eax), %ecx /* get word (= 4 bytes) in question */
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movl $0xfefefeff, %edi /* magic value */
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xorl %edx, %ecx /* XOR with word c|c|c|c => bytes of str == c
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are now 0 */
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addl %ecx, %edi /* add the magic value to the word. We get
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carry bits reported for each byte which
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is *not* 0 */
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jnc L(7) /* highest byte is CHR => return pointer */
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xorl %ecx, %edi /* ((word^charmask)+magic)^(word^charmask) */
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orl $0xfefefeff, %edi /* set all non-carry bits */
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incl %edi /* add 1: if one carry bit was *not* set
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the addition will not result in 0. */
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jnz L(7) /* found it => return pointer */
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movl 8(%eax), %ecx /* get word (= 4 bytes) in question */
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movl $0xfefefeff, %edi /* magic value */
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xorl %edx, %ecx /* XOR with word c|c|c|c => bytes of str == c
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are now 0 */
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addl %ecx, %edi /* add the magic value to the word. We get
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carry bits reported for each byte which
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is *not* 0 */
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jnc L(6) /* highest byte is CHR => return pointer */
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xorl %ecx, %edi /* ((word^charmask)+magic)^(word^charmask) */
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orl $0xfefefeff, %edi /* set all non-carry bits */
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incl %edi /* add 1: if one carry bit was *not* set
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the addition will not result in 0. */
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jnz L(6) /* found it => return pointer */
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movl 12(%eax), %ecx /* get word (= 4 bytes) in question */
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movl $0xfefefeff, %edi /* magic value */
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xorl %edx, %ecx /* XOR with word c|c|c|c => bytes of str == c
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are now 0 */
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addl %ecx, %edi /* add the magic value to the word. We get
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carry bits reported for each byte which
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is *not* 0 */
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jnc L(5) /* highest byte is CHR => return pointer */
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xorl %ecx, %edi /* ((word^charmask)+magic)^(word^charmask) */
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orl $0xfefefeff, %edi /* set all non-carry bits */
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incl %edi /* add 1: if one carry bit was *not* set
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the addition will not result in 0. */
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jnz L(5) /* found it => return pointer */
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/* Adjust both counters for a full round, i.e. 16 bytes. */
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addl $16, %eax
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L(2): subl $16, %esi
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jae L(1) /* Still more than 16 bytes remaining */
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/* Process remaining bytes separately. */
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cmpl $4-16, %esi /* rest < 4 bytes? */
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jb L(3) /* yes, than test byte by byte */
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movl (%eax), %ecx /* get word (= 4 bytes) in question */
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movl $0xfefefeff, %edi /* magic value */
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xorl %edx, %ecx /* XOR with word c|c|c|c => bytes of str == c
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are now 0 */
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addl %ecx, %edi /* add the magic value to the word. We get
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carry bits reported for each byte which
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is *not* 0 */
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jnc L(8) /* highest byte is CHR => return pointer */
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xorl %ecx, %edi /* ((word^charmask)+magic)^(word^charmask) */
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orl $0xfefefeff, %edi /* set all non-carry bits */
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incl %edi /* add 1: if one carry bit was *not* set
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the addition will not result in 0. */
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jne L(8) /* found it => return pointer */
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addl $4, %eax /* adjust source pointer */
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cmpl $8-16, %esi /* rest < 8 bytes? */
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jb L(3) /* yes, than test byte by byte */
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movl (%eax), %ecx /* get word (= 4 bytes) in question */
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movl $0xfefefeff, %edi /* magic value */
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xorl %edx, %ecx /* XOR with word c|c|c|c => bytes of str == c
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are now 0 */
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addl %ecx, %edi /* add the magic value to the word. We get
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carry bits reported for each byte which
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is *not* 0 */
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jnc L(8) /* highest byte is CHR => return pointer */
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xorl %ecx, %edi /* ((word^charmask)+magic)^(word^charmask) */
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orl $0xfefefeff, %edi /* set all non-carry bits */
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incl %edi /* add 1: if one carry bit was *not* set
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the addition will not result in 0. */
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jne L(8) /* found it => return pointer */
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addl $4, %eax /* adjust source pointer */
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cmpl $12-16, %esi /* rest < 12 bytes? */
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jb L(3) /* yes, than test byte by byte */
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movl (%eax), %ecx /* get word (= 4 bytes) in question */
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movl $0xfefefeff, %edi /* magic value */
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xorl %edx, %ecx /* XOR with word c|c|c|c => bytes of str == c
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are now 0 */
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addl %ecx, %edi /* add the magic value to the word. We get
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carry bits reported for each byte which
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is *not* 0 */
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jnc L(8) /* highest byte is CHR => return pointer */
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xorl %ecx, %edi /* ((word^charmask)+magic)^(word^charmask) */
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orl $0xfefefeff, %edi /* set all non-carry bits */
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incl %edi /* add 1: if one carry bit was *not* set
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the addition will not result in 0. */
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jne L(8) /* found it => return pointer */
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addl $4, %eax /* adjust source pointer */
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/* Check the remaining bytes one by one. */
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L(3): andl $3, %esi /* mask out uninteresting bytes */
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jz L(4) /* no remaining bytes => return NULL */
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cmpb %dl, (%eax) /* compare byte with CHR */
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je L(9) /* equal, than return pointer */
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incl %eax /* increment source pointer */
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decl %esi /* decrement length */
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jz L(4) /* no remaining bytes => return NULL */
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cmpb %dl, (%eax) /* compare byte with CHR */
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je L(9) /* equal, than return pointer */
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incl %eax /* increment source pointer */
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decl %esi /* decrement length */
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jz L(4) /* no remaining bytes => return NULL */
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cmpb %dl, (%eax) /* compare byte with CHR */
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je L(9) /* equal, than return pointer */
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L(4): /* no byte found => return NULL */
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xorl %eax, %eax
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jmp L(9)
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/* add missing source pointer increments */
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L(5): addl $4, %eax
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L(6): addl $4, %eax
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L(7): addl $4, %eax
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/* Test for the matching byte in the word. %ecx contains a NUL
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char in the byte which originally was the byte we are looking
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at. */
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L(8): testb %cl, %cl /* test first byte in dword */
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jz L(9) /* if zero => return pointer */
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incl %eax /* increment source pointer */
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testb %ch, %ch /* test second byte in dword */
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jz L(9) /* if zero => return pointer */
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incl %eax /* increment source pointer */
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testl $0xff0000, %ecx /* test third byte in dword */
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jz L(9) /* if zero => return pointer */
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incl %eax /* increment source pointer */
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/* No further test needed we we know it is one of the four bytes. */
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L(9): popl %edi /* pop saved registers */
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cfi_adjust_cfa_offset (-4)
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cfi_restore (edi)
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popl %esi
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cfi_adjust_cfa_offset (-4)
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cfi_restore (esi)
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ret
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END (__memchr)
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weak_alias (__memchr, memchr)
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libc_hidden_builtin_def (memchr)
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