diff options
Diffstat (limited to 'libc/sysdeps/x86_64/strchr.S')
| -rw-r--r-- | libc/sysdeps/x86_64/strchr.S | 291 |
1 files changed, 291 insertions, 0 deletions
diff --git a/libc/sysdeps/x86_64/strchr.S b/libc/sysdeps/x86_64/strchr.S new file mode 100644 index 000000000..893469797 --- /dev/null +++ b/libc/sysdeps/x86_64/strchr.S @@ -0,0 +1,291 @@ +/* strchr (str, ch) -- Return pointer to first occurrence of CH in STR. + For AMD x86-64. + Copyright (C) 2002, 2005 Free Software Foundation, Inc. + 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 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, write to the Free + Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA + 02111-1307 USA. */ + +#include <sysdep.h> +#include "asm-syntax.h" +#include "bp-sym.h" +#include "bp-asm.h" + + + .text +ENTRY (BP_SYM (strchr)) + + /* Before we start with the main loop we process single bytes + until the source pointer is aligned. This has two reasons: + 1. aligned 64-bit memory access is faster + and (more important) + 2. we process in the main loop 64 bit in one step although + we don't know the end of the string. But accessing at + 8-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 8). */ + + movq %rdi, %rdx + andl $7, %edx /* Mask alignment bits */ + movq %rdi, %rax /* duplicate destination. */ + jz 1f /* aligned => start loop */ + neg %edx + addl $8, %edx /* Align to 8 bytes. */ + + /* Search the first bytes directly. */ +0: movb (%rax), %cl /* load byte */ + cmpb %cl,%sil /* compare byte. */ + je 6f /* target found */ + testb %cl,%cl /* is byte NUL? */ + je 7f /* yes => return NULL */ + incq %rax /* increment pointer */ + decl %edx + jnz 0b + + +1: + /* At the moment %rsi contains C. What we need for the + algorithm is C in all bytes of the register. Avoid + operations on 16 bit words because these require an + prefix byte (and one more cycle). */ + /* Populate 8 bit data to full 64-bit. */ + movabs $0x0101010101010101,%r9 + movzbl %sil,%edx + imul %rdx,%r9 + + movq $0xfefefefefefefeff, %r8 /* Save magic. */ + + /* 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 QUARDWORD 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 tec.. If one of bits 54-63 is set, there will be a carry + into bit 64 (=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. */ + + .p2align 4 +4: + /* Main Loop is unrolled 4 times. */ + /* First unroll. */ + movq (%rax), %rcx /* get double word (= 8 bytes) in question */ + addq $8,%rax /* adjust pointer for next word */ + movq %r8, %rdx /* magic value */ + xorq %r9, %rcx /* XOR with qword c|...|c => bytes of str == c + are now 0 */ + addq %rcx, %rdx /* add the magic value to the word. We get + carry bits reported for each byte which + is *not* 0 */ + jnc 3f /* highest byte is NUL => return pointer */ + xorq %rcx, %rdx /* (word+magic)^word */ + orq %r8, %rdx /* set all non-carry bits */ + incq %rdx /* add 1: if one carry bit was *not* set + the addition will not result in 0. */ + jnz 3f /* found c => return pointer */ + + /* The quadword we looked at does not contain the value we're looking + for. Let's search now whether we have reached the end of the + string. */ + xorq %r9, %rcx /* restore original dword without reload */ + movq %r8, %rdx /* magic value */ + addq %rcx, %rdx /* add the magic value to the word. We get + carry bits reported for each byte which + is *not* 0 */ + jnc 7f /* highest byte is NUL => return NULL */ + xorq %rcx, %rdx /* (word+magic)^word */ + orq %r8, %rdx /* set all non-carry bits */ + incq %rdx /* add 1: if one carry bit was *not* set + the addition will not result in 0. */ + jnz 7f /* found NUL => return NULL */ + + /* Second unroll. */ + movq (%rax), %rcx /* get double word (= 8 bytes) in question */ + addq $8,%rax /* adjust pointer for next word */ + movq %r8, %rdx /* magic value */ + xorq %r9, %rcx /* XOR with qword c|...|c => bytes of str == c + are now 0 */ + addq %rcx, %rdx /* add the magic value to the word. We get + carry bits reported for each byte which + is *not* 0 */ + jnc 3f /* highest byte is NUL => return pointer */ + xorq %rcx, %rdx /* (word+magic)^word */ + orq %r8, %rdx /* set all non-carry bits */ + incq %rdx /* add 1: if one carry bit was *not* set + the addition will not result in 0. */ + jnz 3f /* found c => return pointer */ + + /* The quadword we looked at does not contain the value we're looking + for. Let's search now whether we have reached the end of the + string. */ + xorq %r9, %rcx /* restore original dword without reload */ + movq %r8, %rdx /* magic value */ + addq %rcx, %rdx /* add the magic value to the word. We get + carry bits reported for each byte which + is *not* 0 */ + jnc 7f /* highest byte is NUL => return NULL */ + xorq %rcx, %rdx /* (word+magic)^word */ + orq %r8, %rdx /* set all non-carry bits */ + incq %rdx /* add 1: if one carry bit was *not* set + the addition will not result in 0. */ + jnz 7f /* found NUL => return NULL */ + /* Third unroll. */ + movq (%rax), %rcx /* get double word (= 8 bytes) in question */ + addq $8,%rax /* adjust pointer for next word */ + movq %r8, %rdx /* magic value */ + xorq %r9, %rcx /* XOR with qword c|...|c => bytes of str == c + are now 0 */ + addq %rcx, %rdx /* add the magic value to the word. We get + carry bits reported for each byte which + is *not* 0 */ + jnc 3f /* highest byte is NUL => return pointer */ + xorq %rcx, %rdx /* (word+magic)^word */ + orq %r8, %rdx /* set all non-carry bits */ + incq %rdx /* add 1: if one carry bit was *not* set + the addition will not result in 0. */ + jnz 3f /* found c => return pointer */ + + /* The quadword we looked at does not contain the value we're looking + for. Let's search now whether we have reached the end of the + string. */ + xorq %r9, %rcx /* restore original dword without reload */ + movq %r8, %rdx /* magic value */ + addq %rcx, %rdx /* add the magic value to the word. We get + carry bits reported for each byte which + is *not* 0 */ + jnc 7f /* highest byte is NUL => return NULL */ + xorq %rcx, %rdx /* (word+magic)^word */ + orq %r8, %rdx /* set all non-carry bits */ + incq %rdx /* add 1: if one carry bit was *not* set + the addition will not result in 0. */ + jnz 7f /* found NUL => return NULL */ + /* Fourth unroll. */ + movq (%rax), %rcx /* get double word (= 8 bytes) in question */ + addq $8,%rax /* adjust pointer for next word */ + movq %r8, %rdx /* magic value */ + xorq %r9, %rcx /* XOR with qword c|...|c => bytes of str == c + are now 0 */ + addq %rcx, %rdx /* add the magic value to the word. We get + carry bits reported for each byte which + is *not* 0 */ + jnc 3f /* highest byte is NUL => return pointer */ + xorq %rcx, %rdx /* (word+magic)^word */ + orq %r8, %rdx /* set all non-carry bits */ + incq %rdx /* add 1: if one carry bit was *not* set + the addition will not result in 0. */ + jnz 3f /* found c => return pointer */ + + /* The quadword we looked at does not contain the value we're looking + for. Let's search now whether we have reached the end of the + string. */ + xorq %r9, %rcx /* restore original dword without reload */ + movq %r8, %rdx /* magic value */ + addq %rcx, %rdx /* add the magic value to the word. We get + carry bits reported for each byte which + is *not* 0 */ + jnc 7f /* highest byte is NUL => return NULL */ + xorq %rcx, %rdx /* (word+magic)^word */ + orq %r8, %rdx /* set all non-carry bits */ + incq %rdx /* add 1: if one carry bit was *not* set + the addition will not result in 0. */ + jz 4b /* no NUL found => restart loop */ + + +7: /* Return NULL. */ + xorl %eax, %eax + retq + + + /* 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 %rcx + with the byte we're looking for, therefore the tests below look + reversed. */ + + + .p2align 4 /* Align, it's a jump target. */ +3: movq %r9,%rdx /* move to %rdx so that we can access bytes */ + subq $8,%rax /* correct pointer increment. */ + testb %cl, %cl /* is first byte C? */ + jz 6f /* yes => return pointer */ + cmpb %dl, %cl /* is first byte NUL? */ + je 7b /* yes => return NULL */ + incq %rax /* increment pointer */ + + testb %ch, %ch /* is second byte C? */ + jz 6f /* yes => return pointer */ + cmpb %dl, %ch /* is second byte NUL? */ + je 7b /* yes => return NULL? */ + incq %rax /* increment pointer */ + + shrq $16, %rcx /* make upper bytes accessible */ + testb %cl, %cl /* is third byte C? */ + jz 6f /* yes => return pointer */ + cmpb %dl, %cl /* is third byte NUL? */ + je 7b /* yes => return NULL */ + incq %rax /* increment pointer */ + + testb %ch, %ch /* is fourth byte C? */ + jz 6f /* yes => return pointer */ + cmpb %dl, %ch /* is fourth byte NUL? */ + je 7b /* yes => return NULL? */ + incq %rax /* increment pointer */ + + shrq $16, %rcx /* make upper bytes accessible */ + testb %cl, %cl /* is fifth byte C? */ + jz 6f /* yes => return pointer */ + cmpb %dl, %cl /* is fifth byte NUL? */ + je 7b /* yes => return NULL */ + incq %rax /* increment pointer */ + + testb %ch, %ch /* is sixth byte C? */ + jz 6f /* yes => return pointer */ + cmpb %dl, %ch /* is sixth byte NUL? */ + je 7b /* yes => return NULL? */ + incq %rax /* increment pointer */ + + shrq $16, %rcx /* make upper bytes accessible */ + testb %cl, %cl /* is seventh byte C? */ + jz 6f /* yes => return pointer */ + cmpb %dl, %cl /* is seventh byte NUL? */ + je 7b /* yes => return NULL */ + + /* It must be in the eigth byte and it cannot be NUL. */ + incq %rax + +6: + nop + retq +END (BP_SYM (strchr)) + +weak_alias (BP_SYM (strchr), BP_SYM (index)) +libc_hidden_builtin_def (strchr) |