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Diffstat (limited to 'sysdeps/i386/strchr.S')
| -rw-r--r-- | sysdeps/i386/strchr.S | 278 |
1 files changed, 278 insertions, 0 deletions
diff --git a/sysdeps/i386/strchr.S b/sysdeps/i386/strchr.S new file mode 100644 index 0000000000..de947cd252 --- /dev/null +++ b/sysdeps/i386/strchr.S @@ -0,0 +1,278 @@ +/* strchr (str, ch) -- Return pointer to first occurrence of CH in STR. +For Intel 80x86, x>=3. +Copyright (C) 1994, 1995 Free Software Foundation, Inc. +Contributed by Ulrich Drepper <drepper@gnu.ai.mit.edu> +Some optimisations by Alan Modra <Alan@SPRI.Levels.UniSA.Edu.Au> +This file is part of the GNU C Library. + +The GNU C Library is free software; you can redistribute it and/or +modify it under the terms of the GNU Library General Public License as +published by the Free Software Foundation; either version 2 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 +Library General Public License for more details. + +You should have received a copy of the GNU Library General Public +License along with the GNU C Library; see the file COPYING.LIB. If +not, write to the Free Software Foundation, Inc., 59 Temple Place - Suite 330, +Boston, MA 02111-1307, USA. */ + +#include <sysdep.h> +#include "asm-syntax.h" + +/* + INPUT PARAMETERS: + str (sp + 4) + ch (sp + 8) +*/ + + .text +ENTRY (strchr) + pushl %edi /* Save callee-safe registers used here. */ + + movl 8(%esp), %eax /* get string pointer */ + movl 12(%esp), %edx /* get character we are looking for */ + + /* At the moment %edx contains C. What we need for the + algorithm is C 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 inherant + boundaries are multiples of 4. */ + + testb $3, %eax /* correctly aligned ? */ + jz L11 /* yes => begin loop */ + movb (%eax), %cl /* load byte in question (we need it twice) */ + cmpb %cl, %dl /* compare byte */ + je L6 /* target found => return */ + testb %cl, %cl /* is NUL? */ + jz L2 /* yes => return NULL */ + incl %eax /* increment pointer */ + + testb $3, %eax /* correctly aligned ? */ + jz L11 /* yes => begin loop */ + movb (%eax), %cl /* load byte in question (we need it twice) */ + cmpb %cl, %dl /* compare byte */ + je L6 /* target found => return */ + testb %cl, %cl /* is NUL? */ + jz L2 /* yes => return NULL */ + incl %eax /* increment pointer */ + + testb $3, %eax /* correctly aligned ? */ + jz L11 /* yes => begin loop */ + movb (%eax), %cl /* load byte in question (we need it twice) */ + cmpb %cl, %dl /* compare byte */ + je L6 /* target found => return */ + testb %cl, %cl /* is NUL? */ + jz L2 /* yes => return NULL */ + incl %eax /* increment pointer */ + + /* No we have reached alignment. */ + jmp L11 /* 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 C, not zero? + Good point. So what we do is XOR LONGWORD with a longword, + each of whose bytes is C. This turns each byte that is C + into a zero. */ + + /* Each round the main loop processes 16 bytes. */ + + ALIGN(4) + +L1: addl $16, %eax /* adjust pointer for whole round */ + +L11: 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* C */ + + /* 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 occured in the last byte => it was 0. */ + jnc L7 + + /* 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 C we don't get 0 in %edi. */ + jnz L7 /* 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 L2 /* 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 L2 /* 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* C */ + jnc L71 /* highest byte is C => 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 L71 /* 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 L2 /* 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 L2 /* 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* C */ + jnc L72 /* highest byte is C => 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 L72 /* 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 L2 /* 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 L2 /* 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* C */ + jnc L73 /* highest byte is C => 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 L73 /* 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 L2 /* 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 L1 /* no NUL found => restart loop */ + +L2: /* Return NULL. */ + xorl %eax, %eax /* load NULL in return value register */ + popl %edi /* restore saved register content */ + ret + +L73: addl $4, %eax /* adjust pointer */ +L72: addl $4, %eax +L71: 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. */ + +L7: testb %cl, %cl /* is first byte C? */ + jz L6 /* yes => return pointer */ + cmpb %dl, %cl /* is first byte NUL? */ + je L2 /* yes => return NULL */ + incl %eax /* it's not in the first byte */ + + testb %ch, %ch /* is second byte C? */ + jz L6 /* yes => return pointer */ + cmpb %dl, %ch /* is second byte NUL? */ + je L2 /* 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 C? */ + jz L6 /* yes => return pointer */ + cmpb %dl, %cl /* is third byte NUL? */ + je L2 /* yes => return NULL */ + + /* It must be in the fourth byte and it cannot be NUL. */ + incl %eax + +L6: popl %edi /* restore saved register content */ + + ret + +weak_alias (strchr, index) |