| /* memchr (str, chr, len) -- Return pointer to first occurrence of CHR in STR |
| less than LEN. 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.ai.mit.edu> |
| Optimised a little by Alan Modra <Alan@SPRI.Levels.UniSA.Edu.Au> |
| This version is developed using the same algorithm as the fast C |
| version which carries the following introduction: |
| Based on strlen implementation by Torbjorn Granlund (tege@sics.se), |
| with help from Dan Sahlin (dan@sics.se) and |
| commentary by Jim Blandy (jimb@ai.mit.edu); |
| adaptation to memchr suggested by Dick Karpinski (dick@cca.ucsf.edu), |
| and implemented by Roland McGrath (roland@ai.mit.edu). |
| |
| 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+8 /* space for 2 saved regs */ |
| #define RTN PARMS |
| #define STR RTN |
| #define CHR STR+4 |
| #define LEN CHR+4 |
| |
| .text |
| ENTRY (__memchr) |
| |
| /* Save callee-safe registers used in this function. */ |
| pushl %esi |
| cfi_adjust_cfa_offset (4) |
| pushl %edi |
| cfi_adjust_cfa_offset (4) |
| cfi_rel_offset (edi, 0) |
| |
| /* Load parameters into registers. */ |
| movl STR(%esp), %eax /* str: pointer to memory block. */ |
| movl CHR(%esp), %edx /* c: byte we are looking for. */ |
| movl LEN(%esp), %esi /* len: length of memory block. */ |
| cfi_rel_offset (esi, 4) |
| |
| /* If my must not test more than three characters test |
| them one by one. This is especially true for 0. */ |
| cmpl $4, %esi |
| jb L(3) |
| |
| /* 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 c|c|0|0 */ |
| movw %cx, %dx /* And finally c|c|c|c */ |
| |
| /* Better performance can be achieved if the word (32 |
| bit) memory access is aligned on a four-byte-boundary. |
| So process first bytes one by one until boundary is |
| reached. Don't use a loop for better performance. */ |
| |
| testb $3, %al /* correctly aligned ? */ |
| je L(2) /* yes => begin loop */ |
| cmpb %dl, (%eax) /* compare byte */ |
| je L(9) /* target found => return */ |
| incl %eax /* increment source pointer */ |
| decl %esi /* decrement length counter */ |
| je L(4) /* len==0 => return NULL */ |
| |
| testb $3, %al /* correctly aligned ? */ |
| je L(2) /* yes => begin loop */ |
| cmpb %dl, (%eax) /* compare byte */ |
| je L(9) /* target found => return */ |
| incl %eax /* increment source pointer */ |
| decl %esi /* decrement length counter */ |
| je L(4) /* len==0 => return NULL */ |
| |
| testb $3, %al /* correctly aligned ? */ |
| je L(2) /* yes => begin loop */ |
| cmpb %dl, (%eax) /* compare byte */ |
| je L(9) /* target found => return */ |
| incl %eax /* increment source pointer */ |
| decl %esi /* decrement length counter */ |
| /* no test for len==0 here, because this is done in the |
| loop head */ |
| jmp L(2) |
| |
| /* 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): movl (%eax), %ecx /* get word (= 4 bytes) in question */ |
| movl $0xfefefeff, %edi /* magic value */ |
| xorl %edx, %ecx /* XOR with word c|c|c|c => bytes of str == c |
| are now 0 */ |
| addl %ecx, %edi /* add the magic value to the word. We get |
| carry bits reported for each byte which |
| is *not* 0 */ |
| |
| /* 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(8) |
| |
| /* 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(8) /* found it => return pointer */ |
| |
| /* This process is unfolded four times for better performance. |
| we don't increment the source pointer each time. Instead we |
| use offsets and increment by 16 in each run of the loop. But |
| before probing for the matching byte we need some extra code |
| (following LL(13) below). Even the len can be compared with |
| constants instead of decrementing each time. */ |
| |
| movl 4(%eax), %ecx /* get word (= 4 bytes) in question */ |
| movl $0xfefefeff, %edi /* magic value */ |
| xorl %edx, %ecx /* XOR with word c|c|c|c => bytes of str == c |
| are now 0 */ |
| 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 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(7) /* found it => return pointer */ |
| |
| movl 8(%eax), %ecx /* get word (= 4 bytes) in question */ |
| movl $0xfefefeff, %edi /* magic value */ |
| xorl %edx, %ecx /* XOR with word c|c|c|c => bytes of str == c |
| are now 0 */ |
| addl %ecx, %edi /* add the magic value to the word. We get |
| carry bits reported for each byte which |
| is *not* 0 */ |
| jnc L(6) /* 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(6) /* found it => return pointer */ |
| |
| movl 12(%eax), %ecx /* get word (= 4 bytes) in question */ |
| movl $0xfefefeff, %edi /* magic value */ |
| xorl %edx, %ecx /* XOR with word c|c|c|c => bytes of str == c |
| are now 0 */ |
| addl %ecx, %edi /* add the magic value to the word. We get |
| carry bits reported for each byte which |
| is *not* 0 */ |
| jnc L(5) /* 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(5) /* found it => return pointer */ |
| |
| /* Adjust both counters for a full round, i.e. 16 bytes. */ |
| addl $16, %eax |
| L(2): subl $16, %esi |
| jae L(1) /* Still more than 16 bytes remaining */ |
| |
| /* Process remaining bytes separately. */ |
| cmpl $4-16, %esi /* rest < 4 bytes? */ |
| jb L(3) /* yes, than test byte by byte */ |
| |
| movl (%eax), %ecx /* get word (= 4 bytes) in question */ |
| movl $0xfefefeff, %edi /* magic value */ |
| xorl %edx, %ecx /* XOR with word c|c|c|c => bytes of str == c |
| are now 0 */ |
| addl %ecx, %edi /* add the magic value to the word. We get |
| carry bits reported for each byte which |
| is *not* 0 */ |
| jnc L(8) /* 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. */ |
| jne L(8) /* found it => return pointer */ |
| addl $4, %eax /* adjust source pointer */ |
| |
| cmpl $8-16, %esi /* rest < 8 bytes? */ |
| jb L(3) /* yes, than test byte by byte */ |
| |
| movl (%eax), %ecx /* get word (= 4 bytes) in question */ |
| movl $0xfefefeff, %edi /* magic value */ |
| xorl %edx, %ecx /* XOR with word c|c|c|c => bytes of str == c |
| are now 0 */ |
| addl %ecx, %edi /* add the magic value to the word. We get |
| carry bits reported for each byte which |
| is *not* 0 */ |
| jnc L(8) /* 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. */ |
| jne L(8) /* found it => return pointer */ |
| addl $4, %eax /* adjust source pointer */ |
| |
| cmpl $12-16, %esi /* rest < 12 bytes? */ |
| jb L(3) /* yes, than test byte by byte */ |
| |
| movl (%eax), %ecx /* get word (= 4 bytes) in question */ |
| movl $0xfefefeff, %edi /* magic value */ |
| xorl %edx, %ecx /* XOR with word c|c|c|c => bytes of str == c |
| are now 0 */ |
| addl %ecx, %edi /* add the magic value to the word. We get |
| carry bits reported for each byte which |
| is *not* 0 */ |
| jnc L(8) /* 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. */ |
| jne L(8) /* found it => return pointer */ |
| addl $4, %eax /* adjust source pointer */ |
| |
| /* Check the remaining bytes one by one. */ |
| L(3): andl $3, %esi /* mask out uninteresting bytes */ |
| jz L(4) /* no remaining bytes => return NULL */ |
| |
| cmpb %dl, (%eax) /* compare byte with CHR */ |
| je L(9) /* equal, than return pointer */ |
| incl %eax /* increment source pointer */ |
| decl %esi /* decrement length */ |
| jz L(4) /* no remaining bytes => return NULL */ |
| |
| cmpb %dl, (%eax) /* compare byte with CHR */ |
| je L(9) /* equal, than return pointer */ |
| incl %eax /* increment source pointer */ |
| decl %esi /* decrement length */ |
| jz L(4) /* no remaining bytes => return NULL */ |
| |
| cmpb %dl, (%eax) /* compare byte with CHR */ |
| je L(9) /* equal, than return pointer */ |
| |
| L(4): /* no byte found => return NULL */ |
| xorl %eax, %eax |
| jmp L(9) |
| |
| /* add missing source pointer increments */ |
| L(5): addl $4, %eax |
| L(6): addl $4, %eax |
| L(7): addl $4, %eax |
| |
| /* Test for the matching byte in the word. %ecx contains a NUL |
| char in the byte which originally was the byte we are looking |
| at. */ |
| L(8): testb %cl, %cl /* test first byte in dword */ |
| jz L(9) /* if zero => return pointer */ |
| incl %eax /* increment source pointer */ |
| |
| testb %ch, %ch /* test second byte in dword */ |
| jz L(9) /* if zero => return pointer */ |
| incl %eax /* increment source pointer */ |
| |
| testl $0xff0000, %ecx /* test third byte in dword */ |
| jz L(9) /* if zero => return pointer */ |
| incl %eax /* increment source pointer */ |
| |
| /* No further test needed we we know it is one of the four bytes. */ |
| L(9): popl %edi /* pop saved registers */ |
| cfi_adjust_cfa_offset (-4) |
| cfi_restore (edi) |
| popl %esi |
| cfi_adjust_cfa_offset (-4) |
| cfi_restore (esi) |
| |
| ret |
| END (__memchr) |
| |
| weak_alias (__memchr, memchr) |
| libc_hidden_builtin_def (memchr) |