| /* strrchr (str, ch) -- Return pointer to last occurrence of CH in STR. |
| For Intel 80x86, x>=3. |
| Copyright (C) 1994-2018 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+8 /* space for 2 saved regs */ |
| #define RTN PARMS |
| #define STR RTN |
| #define CHR STR+4 |
| |
| .text |
| ENTRY (strrchr) |
| |
| pushl %edi /* Save callee-safe registers used here. */ |
| cfi_adjust_cfa_offset (4) |
| cfi_rel_offset (edi, 0) |
| pushl %esi |
| cfi_adjust_cfa_offset (4) |
| |
| xorl %eax, %eax |
| movl STR(%esp), %esi |
| cfi_rel_offset (esi, 0) |
| movl CHR(%esp), %ecx |
| |
| /* At the moment %ecx 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 %cl, %ch /* now it is 0|0|c|c */ |
| movl %ecx, %edx |
| shll $16, %ecx /* now it is c|c|0|0 */ |
| movw %dx, %cx /* 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. */ |
| |
| testl $3, %esi /* correctly aligned ? */ |
| jz L(19) /* yes => begin loop */ |
| movb (%esi), %dl /* load byte in question (we need it twice) */ |
| cmpb %dl, %cl /* compare byte */ |
| jne L(11) /* target found => return */ |
| movl %esi, %eax /* remember pointer as possible result */ |
| L(11): orb %dl, %dl /* is NUL? */ |
| jz L(2) /* yes => return NULL */ |
| incl %esi /* increment pointer */ |
| |
| testl $3, %esi /* correctly aligned ? */ |
| jz L(19) /* yes => begin loop */ |
| movb (%esi), %dl /* load byte in question (we need it twice) */ |
| cmpb %dl, %cl /* compare byte */ |
| jne L(12) /* target found => return */ |
| movl %esi, %eax /* remember pointer as result */ |
| L(12): orb %dl, %dl /* is NUL? */ |
| jz L(2) /* yes => return NULL */ |
| incl %esi /* increment pointer */ |
| |
| testl $3, %esi /* correctly aligned ? */ |
| jz L(19) /* yes => begin loop */ |
| movb (%esi), %dl /* load byte in question (we need it twice) */ |
| cmpb %dl, %cl /* compare byte */ |
| jne L(13) /* target found => return */ |
| movl %esi, %eax /* remember pointer as result */ |
| L(13): orb %dl, %dl /* is NUL? */ |
| jz L(2) /* yes => return NULL */ |
| incl %esi /* increment pointer */ |
| |
| /* No we have reached alignment. */ |
| jmp L(19) /* 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. */ |
| |
| /* Jump to here when the character is detected. We chose this |
| way around because the character one is looking for is not |
| as frequent as the rest and taking a conditional jump is more |
| expensive than ignoring it. |
| |
| Some more words to the code below: it might not be obvious why |
| we decrement the source pointer here. In the loop the pointer |
| is not pre-incremented and so it still points before the word |
| we are looking at. But you should take a look at the instruction |
| which gets executed before we get into the loop: `addl $16, %esi'. |
| This makes the following subs into adds. */ |
| |
| /* These fill bytes make the main loop be correctly aligned. |
| We cannot use align because it is not the following instruction |
| which should be aligned. */ |
| .byte 0, 0 |
| #ifndef PROF |
| /* Profiling adds some code and so changes the alignment. */ |
| .byte 0 |
| #endif |
| |
| L(4): subl $4, %esi /* adjust pointer */ |
| L(41): subl $4, %esi |
| L(42): subl $4, %esi |
| L(43): testl $0xff000000, %edx /* is highest byte == C? */ |
| jnz L(33) /* no => try other bytes */ |
| leal 15(%esi), %eax /* store address as result */ |
| jmp L(1) /* and start loop again */ |
| |
| L(3): subl $4, %esi /* adjust pointer */ |
| L(31): subl $4, %esi |
| L(32): subl $4, %esi |
| L(33): testl $0xff0000, %edx /* is C in third byte? */ |
| jnz L(51) /* no => try other bytes */ |
| leal 14(%esi), %eax /* store address as result */ |
| jmp L(1) /* and start loop again */ |
| |
| L(51): |
| /* At this point we know that the byte is in one of the lower bytes. |
| We make a guess and correct it if necessary. This reduces the |
| number of necessary jumps. */ |
| leal 12(%esi), %eax /* guess address of lowest byte as result */ |
| testb %dh, %dh /* is guess correct? */ |
| jnz L(1) /* yes => start loop */ |
| leal 13(%esi), %eax /* correct guess to second byte */ |
| |
| L(1): addl $16, %esi /* increment pointer for full round */ |
| |
| L(19): movl (%esi), %edx /* get word (= 4 bytes) in question */ |
| movl $0xfefefeff, %edi /* magic value */ |
| addl %edx, %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(20) /* found NUL => check last word */ |
| |
| /* We are only interested in carry bits that change due to the |
| previous add, so remove original bits */ |
| xorl %edx, %edi /* (word+magic)^word */ |
| |
| /* 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(20) /* found NUL => check last word */ |
| |
| /* 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 %ecx, %edx /* XOR with word c|c|c|c => bytes of str == c |
| are now 0 */ |
| movl $0xfefefeff, %edi /* magic value */ |
| addl %edx, %edi /* add the magic value to the word. We get |
| carry bits reported for each byte which |
| is *not* 0 */ |
| jnc L(4) /* highest byte is C => examine dword */ |
| xorl %edx, %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(3) /* C is detected in the word => examine it */ |
| |
| movl 4(%esi), %edx /* get word (= 4 bytes) in question */ |
| movl $0xfefefeff, %edi /* magic value */ |
| addl %edx, %edi /* add the magic value to the word. We get |
| carry bits reported for each byte which |
| is *not* 0 */ |
| jnc L(21) /* found NUL => check last word */ |
| xorl %edx, %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(21) /* found NUL => check last word */ |
| xorl %ecx, %edx /* XOR with word c|c|c|c => bytes of str == c |
| are now 0 */ |
| movl $0xfefefeff, %edi /* magic value */ |
| addl %edx, %edi /* add the magic value to the word. We get |
| carry bits reported for each byte which |
| is *not* 0 */ |
| jnc L(41) /* highest byte is C => examine dword */ |
| xorl %edx, %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(31) /* C is detected in the word => examine it */ |
| |
| movl 8(%esi), %edx /* get word (= 4 bytes) in question */ |
| movl $0xfefefeff, %edi /* magic value */ |
| addl %edx, %edi /* add the magic value to the word. We get |
| carry bits reported for each byte which |
| is *not* 0 */ |
| jnc L(22) /* found NUL => check last word */ |
| xorl %edx, %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(22) /* found NUL => check last word */ |
| xorl %ecx, %edx /* XOR with word c|c|c|c => bytes of str == c |
| are now 0 */ |
| movl $0xfefefeff, %edi /* magic value */ |
| addl %edx, %edi /* add the magic value to the word. We get |
| carry bits reported for each byte which |
| is *not* 0 */ |
| jnc L(42) /* highest byte is C => examine dword */ |
| xorl %edx, %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(32) /* C is detected in the word => examine it */ |
| |
| movl 12(%esi), %edx /* get word (= 4 bytes) in question */ |
| movl $0xfefefeff, %edi /* magic value */ |
| addl %edx, %edi /* add the magic value to the word. We get |
| carry bits reported for each byte which |
| is *not* 0 */ |
| jnc L(23) /* found NUL => check last word */ |
| xorl %edx, %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(23) /* found NUL => check last word */ |
| xorl %ecx, %edx /* XOR with word c|c|c|c => bytes of str == c |
| are now 0 */ |
| movl $0xfefefeff, %edi /* magic value */ |
| addl %edx, %edi /* add the magic value to the word. We get |
| carry bits reported for each byte which |
| is *not* 0 */ |
| jnc L(43) /* highest byte is C => examine dword */ |
| xorl %edx, %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. */ |
| jz L(1) /* C is not detected => restart loop */ |
| jmp L(33) /* examine word */ |
| |
| L(23): addl $4, %esi /* adjust pointer */ |
| L(22): addl $4, %esi |
| L(21): addl $4, %esi |
| |
| /* What remains to do is to test which byte the NUL char is and |
| whether the searched character appears in one of the bytes |
| before. A special case is that the searched byte maybe NUL. |
| In this case a pointer to the terminating NUL char has to be |
| returned. */ |
| |
| L(20): cmpb %cl, %dl /* is first byte == C? */ |
| jne L(24) /* no => skip */ |
| movl %esi, %eax /* store address as result */ |
| L(24): testb %dl, %dl /* is first byte == NUL? */ |
| jz L(2) /* yes => return */ |
| |
| cmpb %cl, %dh /* is second byte == C? */ |
| jne L(25) /* no => skip */ |
| leal 1(%esi), %eax /* store address as result */ |
| L(25): testb %dh, %dh /* is second byte == NUL? */ |
| jz L(2) /* yes => return */ |
| |
| shrl $16,%edx /* make upper bytes accessible */ |
| cmpb %cl, %dl /* is third byte == C */ |
| jne L(26) /* no => skip */ |
| leal 2(%esi), %eax /* store address as result */ |
| L(26): testb %dl, %dl /* is third byte == NUL */ |
| jz L(2) /* yes => return */ |
| |
| cmpb %cl, %dh /* is fourth byte == C */ |
| jne L(2) /* no => skip */ |
| leal 3(%esi), %eax /* store address as result */ |
| |
| L(2): popl %esi /* restore saved register content */ |
| cfi_adjust_cfa_offset (-4) |
| cfi_restore (esi) |
| popl %edi |
| cfi_adjust_cfa_offset (-4) |
| cfi_restore (edi) |
| |
| ret |
| END (strrchr) |
| |
| weak_alias (strrchr, rindex) |
| libc_hidden_builtin_def (strrchr) |