| /* strchrnul (str, chr) -- Return pointer to first occurrence of CHR in STR |
| or the final NUL byte. |
| For Intel 80x86, x>=3. |
| Copyright (C) 1994-2016 Free Software Foundation, Inc. |
| This file is part of the GNU C Library. |
| Contributed by Ulrich Drepper <drepper@gnu.org> |
| Some optimisations by Alan Modra <Alan@SPRI.Levels.UniSA.Edu.Au> |
| |
| The GNU C Library is free software; you can redistribute it and/or |
| modify it under the terms of the GNU Lesser General Public |
| License as published by the Free Software Foundation; either |
| version 2.1 of the License, or (at your option) any later version. |
| |
| The GNU C Library is distributed in the hope that it will be useful, |
| but WITHOUT ANY WARRANTY; without even the implied warranty of |
| MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU |
| Lesser General Public License for more details. |
| |
| You should have received a copy of the GNU Lesser General Public |
| License along with the GNU C Library; if not, see |
| <http://www.gnu.org/licenses/>. */ |
| |
| #include <sysdep.h> |
| #include "asm-syntax.h" |
| |
| #define PARMS 4+4 /* space for 1 saved reg */ |
| #define RTN PARMS |
| #define STR RTN |
| #define CHR STR+4 |
| |
| .text |
| ENTRY (__strchrnul) |
| |
| pushl %edi /* Save callee-safe registers used here. */ |
| cfi_adjust_cfa_offset (4) |
| cfi_rel_offset (edi, 0) |
| |
| movl STR(%esp), %eax |
| movl CHR(%esp), %edx |
| |
| /* At the moment %edx contains CHR. What we need for the |
| algorithm is CHR in all bytes of the dword. Avoid |
| operations on 16 bit words because these require an |
| prefix byte (and one more cycle). */ |
| movb %dl, %dh /* now it is 0|0|c|c */ |
| movl %edx, %ecx |
| shll $16, %edx /* now it is c|c|0|0 */ |
| movw %cx, %dx /* and finally c|c|c|c */ |
| |
| /* Before we start with the main loop we process single bytes |
| until the source pointer is aligned. This has two reasons: |
| 1. aligned 32-bit memory access is faster |
| and (more important) |
| 2. we process in the main loop 32 bit in one step although |
| we don't know the end of the string. But accessing at |
| 4-byte alignment guarantees that we never access illegal |
| memory if this would not also be done by the trivial |
| implementation (this is because all processor inherent |
| boundaries are multiples of 4. */ |
| |
| testb $3, %al /* correctly aligned ? */ |
| jz L(11) /* yes => begin loop */ |
| movb (%eax), %cl /* load byte in question (we need it twice) */ |
| cmpb %cl, %dl /* compare byte */ |
| je L(6) /* target found => return */ |
| testb %cl, %cl /* is NUL? */ |
| jz L(6) /* yes => return NULL */ |
| incl %eax /* increment pointer */ |
| |
| testb $3, %al /* correctly aligned ? */ |
| jz L(11) /* yes => begin loop */ |
| movb (%eax), %cl /* load byte in question (we need it twice) */ |
| cmpb %cl, %dl /* compare byte */ |
| je L(6) /* target found => return */ |
| testb %cl, %cl /* is NUL? */ |
| jz L(6) /* yes => return NULL */ |
| incl %eax /* increment pointer */ |
| |
| testb $3, %al /* correctly aligned ? */ |
| jz L(11) /* yes => begin loop */ |
| movb (%eax), %cl /* load byte in question (we need it twice) */ |
| cmpb %cl, %dl /* compare byte */ |
| je L(6) /* target found => return */ |
| testb %cl, %cl /* is NUL? */ |
| jz L(6) /* yes => return NULL */ |
| incl %eax /* increment pointer */ |
| |
| /* No we have reached alignment. */ |
| jmp L(11) /* begin loop */ |
| |
| /* We exit the loop if adding MAGIC_BITS to LONGWORD fails to |
| change any of the hole bits of LONGWORD. |
| |
| 1) Is this safe? Will it catch all the zero bytes? |
| Suppose there is a byte with all zeros. Any carry bits |
| propagating from its left will fall into the hole at its |
| least significant bit and stop. Since there will be no |
| carry from its most significant bit, the LSB of the |
| byte to the left will be unchanged, and the zero will be |
| detected. |
| |
| 2) Is this worthwhile? Will it ignore everything except |
| zero bytes? Suppose every byte of LONGWORD has a bit set |
| somewhere. There will be a carry into bit 8. If bit 8 |
| is set, this will carry into bit 16. If bit 8 is clear, |
| one of bits 9-15 must be set, so there will be a carry |
| into bit 16. Similarly, there will be a carry into bit |
| 24. If one of bits 24-31 is set, there will be a carry |
| into bit 32 (=carry flag), so all of the hole bits will |
| be changed. |
| |
| 3) But wait! Aren't we looking for CHR, not zero? |
| Good point. So what we do is XOR LONGWORD with a longword, |
| each of whose bytes is CHR. This turns each byte that is CHR |
| into a zero. */ |
| |
| /* Each round the main loop processes 16 bytes. */ |
| |
| ALIGN(4) |
| |
| L(1): addl $16, %eax /* adjust pointer for whole round */ |
| |
| L(11): movl (%eax), %ecx /* get word (= 4 bytes) in question */ |
| xorl %edx, %ecx /* XOR with word c|c|c|c => bytes of str == c |
| are now 0 */ |
| movl $0xfefefeff, %edi /* magic value */ |
| addl %ecx, %edi /* add the magic value to the word. We get |
| carry bits reported for each byte which |
| is *not* CHR */ |
| |
| /* According to the algorithm we had to reverse the effect of the |
| XOR first and then test the overflow bits. But because the |
| following XOR would destroy the carry flag and it would (in a |
| representation with more than 32 bits) not alter then last |
| overflow, we can now test this condition. If no carry is signaled |
| no overflow must have occurred in the last byte => it was 0. */ |
| jnc L(7) |
| |
| /* We are only interested in carry bits that change due to the |
| previous add, so remove original bits */ |
| xorl %ecx, %edi /* ((word^charmask)+magic)^(word^charmask) */ |
| |
| /* Now test for the other three overflow bits. */ |
| orl $0xfefefeff, %edi /* set all non-carry bits */ |
| incl %edi /* add 1: if one carry bit was *not* set |
| the addition will not result in 0. */ |
| |
| /* If at least one byte of the word is CHR we don't get 0 in %edi. */ |
| jnz L(7) /* found it => return pointer */ |
| |
| /* Now we made sure the dword does not contain the character we are |
| looking for. But because we deal with strings we have to check |
| for the end of string before testing the next dword. */ |
| |
| xorl %edx, %ecx /* restore original dword without reload */ |
| movl $0xfefefeff, %edi /* magic value */ |
| addl %ecx, %edi /* add the magic value to the word. We get |
| carry bits reported for each byte which |
| is *not* 0 */ |
| jnc L(7) /* highest byte is NUL => return NULL */ |
| xorl %ecx, %edi /* (word+magic)^word */ |
| orl $0xfefefeff, %edi /* set all non-carry bits */ |
| incl %edi /* add 1: if one carry bit was *not* set |
| the addition will not result in 0. */ |
| jnz L(7) /* found NUL => return NULL */ |
| |
| movl 4(%eax), %ecx /* get word (= 4 bytes) in question */ |
| xorl %edx, %ecx /* XOR with word c|c|c|c => bytes of str == c |
| are now 0 */ |
| movl $0xfefefeff, %edi /* magic value */ |
| addl %ecx, %edi /* add the magic value to the word. We get |
| carry bits reported for each byte which |
| is *not* CHR */ |
| jnc L(71) /* highest byte is CHR => return pointer */ |
| xorl %ecx, %edi /* ((word^charmask)+magic)^(word^charmask) */ |
| orl $0xfefefeff, %edi /* set all non-carry bits */ |
| incl %edi /* add 1: if one carry bit was *not* set |
| the addition will not result in 0. */ |
| jnz L(71) /* found it => return pointer */ |
| xorl %edx, %ecx /* restore original dword without reload */ |
| movl $0xfefefeff, %edi /* magic value */ |
| addl %ecx, %edi /* add the magic value to the word. We get |
| carry bits reported for each byte which |
| is *not* 0 */ |
| jnc L(71) /* highest byte is NUL => return NULL */ |
| xorl %ecx, %edi /* (word+magic)^word */ |
| orl $0xfefefeff, %edi /* set all non-carry bits */ |
| incl %edi /* add 1: if one carry bit was *not* set |
| the addition will not result in 0. */ |
| jnz L(71) /* found NUL => return NULL */ |
| |
| movl 8(%eax), %ecx /* get word (= 4 bytes) in question */ |
| xorl %edx, %ecx /* XOR with word c|c|c|c => bytes of str == c |
| are now 0 */ |
| movl $0xfefefeff, %edi /* magic value */ |
| addl %ecx, %edi /* add the magic value to the word. We get |
| carry bits reported for each byte which |
| is *not* CHR */ |
| jnc L(72) /* highest byte is CHR => return pointer */ |
| xorl %ecx, %edi /* ((word^charmask)+magic)^(word^charmask) */ |
| orl $0xfefefeff, %edi /* set all non-carry bits */ |
| incl %edi /* add 1: if one carry bit was *not* set |
| the addition will not result in 0. */ |
| jnz L(72) /* found it => return pointer */ |
| xorl %edx, %ecx /* restore original dword without reload */ |
| movl $0xfefefeff, %edi /* magic value */ |
| addl %ecx, %edi /* add the magic value to the word. We get |
| carry bits reported for each byte which |
| is *not* 0 */ |
| jnc L(72) /* highest byte is NUL => return NULL */ |
| xorl %ecx, %edi /* (word+magic)^word */ |
| orl $0xfefefeff, %edi /* set all non-carry bits */ |
| incl %edi /* add 1: if one carry bit was *not* set |
| the addition will not result in 0. */ |
| jnz L(72) /* found NUL => return NULL */ |
| |
| movl 12(%eax), %ecx /* get word (= 4 bytes) in question */ |
| xorl %edx, %ecx /* XOR with word c|c|c|c => bytes of str == c |
| are now 0 */ |
| movl $0xfefefeff, %edi /* magic value */ |
| addl %ecx, %edi /* add the magic value to the word. We get |
| carry bits reported for each byte which |
| is *not* CHR */ |
| jnc L(73) /* highest byte is CHR => return pointer */ |
| xorl %ecx, %edi /* ((word^charmask)+magic)^(word^charmask) */ |
| orl $0xfefefeff, %edi /* set all non-carry bits */ |
| incl %edi /* add 1: if one carry bit was *not* set |
| the addition will not result in 0. */ |
| jnz L(73) /* found it => return pointer */ |
| xorl %edx, %ecx /* restore original dword without reload */ |
| movl $0xfefefeff, %edi /* magic value */ |
| addl %ecx, %edi /* add the magic value to the word. We get |
| carry bits reported for each byte which |
| is *not* 0 */ |
| jnc L(73) /* highest byte is NUL => return NULL */ |
| xorl %ecx, %edi /* (word+magic)^word */ |
| orl $0xfefefeff, %edi /* set all non-carry bits */ |
| incl %edi /* add 1: if one carry bit was *not* set |
| the addition will not result in 0. */ |
| jz L(1) /* no NUL found => restart loop */ |
| |
| L(73): addl $4, %eax /* adjust pointer */ |
| L(72): addl $4, %eax |
| L(71): addl $4, %eax |
| |
| /* We now scan for the byte in which the character was matched. |
| But we have to take care of the case that a NUL char is |
| found before this in the dword. */ |
| |
| L(7): testb %cl, %cl /* is first byte CHR? */ |
| jz L(6) /* yes => return pointer */ |
| cmpb %dl, %cl /* is first byte NUL? */ |
| je L(6) /* yes => return NULL */ |
| incl %eax /* it's not in the first byte */ |
| |
| testb %ch, %ch /* is second byte CHR? */ |
| jz L(6) /* yes => return pointer */ |
| cmpb %dl, %ch /* is second byte NUL? */ |
| je L(6) /* yes => return NULL? */ |
| incl %eax /* it's not in the second byte */ |
| |
| shrl $16, %ecx /* make upper byte accessible */ |
| testb %cl, %cl /* is third byte CHR? */ |
| jz L(6) /* yes => return pointer */ |
| cmpb %dl, %cl /* is third byte NUL? */ |
| je L(6) /* yes => return NULL */ |
| |
| /* It must be in the fourth byte and it cannot be NUL. */ |
| incl %eax |
| |
| L(6): popl %edi /* restore saved register content */ |
| cfi_adjust_cfa_offset (-4) |
| cfi_restore (edi) |
| |
| ret |
| END (__strchrnul) |
| |
| weak_alias (__strchrnul, strchrnul) |