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Diffstat (limited to 'REORG.TODO/sysdeps/i386/strchr.S')
| -rw-r--r-- | REORG.TODO/sysdeps/i386/strchr.S | 290 |
1 files changed, 290 insertions, 0 deletions
diff --git a/REORG.TODO/sysdeps/i386/strchr.S b/REORG.TODO/sysdeps/i386/strchr.S new file mode 100644 index 0000000000..6075e77882 --- /dev/null +++ b/REORG.TODO/sysdeps/i386/strchr.S @@ -0,0 +1,290 @@ +/* strchr (str, ch) -- Return pointer to first occurrence of CH in STR. + For Intel 80x86, x>=3. + Copyright (C) 1994-2017 Free Software Foundation, Inc. + This file is part of the GNU C Library. + Contributed by Ulrich Drepper <drepper@gnu.ai.mit.edu> + 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 (strchr) + + 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 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 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(2) /* 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(2) /* 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(2) /* 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 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) + +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* 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 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 C 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(2) /* 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(2) /* 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 L(71) /* 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 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(2) /* 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(2) /* 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 L(72) /* 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 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(2) /* 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(2) /* 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 L(73) /* 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 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(2) /* 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(2): /* Return NULL. */ + xorl %eax, %eax + popl %edi /* restore saved register content */ + cfi_adjust_cfa_offset (-4) + cfi_restore (edi) + + ret + + cfi_adjust_cfa_offset (4) + cfi_rel_offset (edi, 0) +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. Note that we XORed %ecx + with the byte we're looking for, therefore the tests below look + reversed. */ + +L(7): testb %cl, %cl /* is first byte C? */ + jz L(6) /* yes => return pointer */ + cmpb %dl, %cl /* is first byte NUL? */ + je L(2) /* yes => return NULL */ + incl %eax /* it's not in the first byte */ + + testb %ch, %ch /* is second byte C? */ + jz L(6) /* yes => return pointer */ + cmpb %dl, %ch /* is second byte NUL? */ + je L(2) /* 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 L(6) /* yes => return pointer */ + cmpb %dl, %cl /* is third byte NUL? */ + je L(2) /* 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 (strchr) + +weak_alias (strchr, index) +libc_hidden_builtin_def (strchr) |