[src/trunk]: src/common/lib/libc/arch/arm/string strlen implementation for ar...

[matt <matt%NetBSD.org@localhost>]

details:   https://anonhg.NetBSD.org/src/rev/e235a252cb6c
branches:  trunk
changeset: 783542:e235a252cb6c
user:      matt <matt%NetBSD.org@localhost>
date:      Fri Dec 28 07:10:41 2012 +0000

description:
strlen implementation for armv6 and later.  Uses clz and uqadd8 to really
speed the search for NUL.  as fast as normal strlen at about a length of
6 or 7 and 2-3 times faster starting around 10.

diffstat:

 common/lib/libc/arch/arm/string/strlen_armv6.S |  86 ++++++++++++++++++++++++++
 1 files changed, 86 insertions(+), 0 deletions(-)

diffs (90 lines):

diff -r 11f9541c2383 -r e235a252cb6c common/lib/libc/arch/arm/string/strlen_armv6.S
--- /dev/null   Thu Jan 01 00:00:00 1970 +0000
+++ b/common/lib/libc/arch/arm/string/strlen_armv6.S    Fri Dec 28 07:10:41 2012 +0000
@@ -0,0 +1,86 @@
+/*-
+ * Copyright (c) 2012 The NetBSD Foundation, Inc.
+ * All rights reserved.
+ *
+ * This code is derived from software contributed to The NetBSD Foundation
+ * by Matt Thomas of 3am Software Foundry.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ * 1. Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in the
+ *    documentation and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE NETBSD FOUNDATION, INC. AND CONTRIBUTORS
+ * ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED
+ * TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
+ * PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE FOUNDATION OR CONTRIBUTORS
+ * BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
+ * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
+ * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+ * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
+ * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+ * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
+ * POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#include <machine/asm.h>
+
+RCSID("$NetBSD: strlen_armv6.S,v 1.1 2012/12/28 07:10:41 matt Exp $")
+
+       .text
+ENTRY(strlen)
+       ands    r1, r0, #3              /* get misalignment */
+       bic     ip, r0, #3              /* align to word boundary */
+       ldr     r3, [ip], #4            /* load first word */
+       neg     r0, r1                  /* subtract misalignment from length */
+       beq     .Lpre_main_loop         /*   misaligned?  no, go to loop */
+       /*
+        * For misaligned string, we need to make sure that the bytes before
+        * the start of the string will not cause a false match to a NUL.
+        */
+       mvn     r2, #0                  /* create a mask */
+       and     r1, r0, #3              /* find out how many bytes to clear */
+       mov     r1, r1, lsl #3          /* bytes -> bits */
+#ifdef __ARMEL__
+       mov     r2, r2, lsr r1          /* clear relavent bytes */
+#else
+       mov     r2, r2, lsl r1          /* clear relavent bytes */
+#endif
+       orr     r3, r3, r2              /* orr in mask for leading bytes */
+.Lpre_main_loop:
+#ifdef _ARM_ARCH_7
+       movw    r1, #0xfefe             /* magic constant; 254 in each byte */
+#else
+       mov     r1, #0xfe               /* put 254 in low byte */
+       orr     r1, r1, r1, lsl #8      /* move to next byte */
+#endif
+       orr     r1, r1, r1, lsl #16     /* move to next halfword */
+.Lmain_loop:
+       /*
+        * Add 254 to each byte using the UQADD8 (unsigned saturating add 8)
+        * instruction.  For every non-NUL byte, the result for that byte will
+        * become 255.  For NUL, it will be 254.  When we complement the
+        * result, if the result is non-0 then we must have encountered a NUL.
+        */
+       uqadd8  r3, r3, r1              /* magic happens here */
+       mvns    r3, r3                  /* is the complemented result 0? */
+       bne     .Lreturn                /*    no, return # of bytes */
+       add     r0, r0, #4              /* add 4 to the count */
+       ldr     r3, [ip], #4            /* load next word */
+       b       .Lmain_loop             /* and go */
+.Lreturn:
+       /*
+        * We encountered a NUL.  Find out where by doing a CLZ and then
+        * shifting right by 3.  That will be the number of non-NUL bytes.
+        */
+#ifdef __ARMEL__
+       rev     r3, r3                  /* we want this in BE for the CLZ */
+#endif
+       clz     r3, r3                  /* count how many leading zeros */
+       add     r0, r0, r3, lsr #3      /* divide that by 8 and add to count */
+       RET
+END(strlen)