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-rw-r--r--libc/string/x86_64/strlen.S135
1 files changed, 135 insertions, 0 deletions
diff --git a/libc/string/x86_64/strlen.S b/libc/string/x86_64/strlen.S
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+++ b/libc/string/x86_64/strlen.S
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+/* strlen(str) -- determine the length of the string STR.
+ Copyright (C) 2002, 2003 Free Software Foundation, Inc.
+ Based on i486 version contributed by Ulrich Drepper <drepper@redhat.com>.
+ 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 "_glibc_inc.h"
+
+
+ .text
+ENTRY (strlen)
+ movq %rdi, %rcx /* Duplicate source pointer. */
+ andl $7, %ecx /* mask alignment bits */
+ movq %rdi, %rax /* duplicate destination. */
+ jz 1f /* aligned => start loop */
+
+ neg %ecx /* We need to align to 8 bytes. */
+ addl $8,%ecx
+ /* Search the first bytes directly. */
+0: cmpb $0x0,(%rax) /* is byte NUL? */
+ je 2f /* yes => return */
+ incq %rax /* increment pointer */
+ decl %ecx
+ jnz 0b
+
+1: movq $0xfefefefefefefeff,%r8 /* Save magic. */
+
+ .p2align 4 /* Align loop. */
+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 */
+ 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 NUL => return pointer */
+
+ /* 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 */
+ 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 NUL => return pointer */
+
+ /* 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 */
+ 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 NUL => return pointer */
+
+ /* 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 */
+ 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. */
+ jz 4b /* no NUL found => continue loop */
+
+ .p2align 4 /* Align, it's a jump target. */
+3: subq $8,%rax /* correct pointer increment. */
+
+ testb %cl, %cl /* is first byte NUL? */
+ jz 2f /* yes => return */
+ incq %rax /* increment pointer */
+
+ testb %ch, %ch /* is second byte NUL? */
+ jz 2f /* yes => return */
+ incq %rax /* increment pointer */
+
+ testl $0x00ff0000, %ecx /* is third byte NUL? */
+ jz 2f /* yes => return pointer */
+ incq %rax /* increment pointer */
+
+ testl $0xff000000, %ecx /* is fourth byte NUL? */
+ jz 2f /* yes => return pointer */
+ incq %rax /* increment pointer */
+
+ shrq $32, %rcx /* look at other half. */
+
+ testb %cl, %cl /* is first byte NUL? */
+ jz 2f /* yes => return */
+ incq %rax /* increment pointer */
+
+ testb %ch, %ch /* is second byte NUL? */
+ jz 2f /* yes => return */
+ incq %rax /* increment pointer */
+
+ testl $0xff0000, %ecx /* is third byte NUL? */
+ jz 2f /* yes => return pointer */
+ incq %rax /* increment pointer */
+2:
+ subq %rdi, %rax /* compute difference to string start */
+ ret
+END (strlen)