/*
* memchr - scan memory for a character
*
* Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
* See https://llvm.org/LICENSE.txt for license information.
* SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
*/
/*
Written by Dave Gilbert <[email protected]>
This __memchr_arm routine is optimised on a Cortex-A9 and should work on
all ARMv7 processors. It has a fast past for short sizes, and has
an optimised path for large data sets; the worst case is finding the
match early in a large data set.
*/
@ 2011-02-07 [email protected]
@ Extracted from local git a5b438d861
@ 2011-07-14 [email protected]
@ Import endianness fix from local git ea786f1b
@ 2011-12-07 [email protected]
@ Removed unneeded cbz from align loop
.syntax unified
.arch armv7-a
@ this lets us check a flag in a 00/ff byte easily in either endianness
#ifdef __ARMEB__
#define CHARTSTMASK(c) 1<<(31-(c*8))
#else
#define CHARTSTMASK(c) 1<<(c*8)
#endif
.text
.thumb
@ ---------------------------------------------------------------------------
.thumb_func
.align 2
.p2align 4,,15
.global __memchr_arm
.type __memchr_arm,%function
__memchr_arm:
@ r0 = start of memory to scan
@ r1 = character to look for
@ r2 = length
@ returns r0 = pointer to character or NULL if not found
and r1,r1,#0xff @ Don't think we can trust the caller to actually pass a char
cmp r2,#16 @ If it's short don't bother with anything clever
blt 20f
tst r0, #7 @ If it's already aligned skip the next bit
beq 10f
@ Work up to an aligned point
5:
ldrb r3, [r0],#1
subs r2, r2, #1
cmp r3, r1
beq 50f @ If it matches exit found
tst r0, #7
bne 5b @ If not aligned yet then do next byte
10:
@ At this point, we are aligned, we know we have at least 8 bytes to work with
push {r4,r5,r6,r7}
orr r1, r1, r1, lsl #8 @ expand the match word across to all bytes
orr r1, r1, r1, lsl #16
bic r4, r2, #7 @ Number of double words to work with
mvns r7, #0 @ all F's
movs r3, #0
15:
ldmia r0!,{r5,r6}
subs r4, r4, #8
eor r5,r5, r1 @ Get it so that r5,r6 have 00's where the bytes match the target
eor r6,r6, r1
uadd8 r5, r5, r7 @ Parallel add 0xff - sets the GE bits for anything that wasn't 0
sel r5, r3, r7 @ bytes are 00 for none-00 bytes, or ff for 00 bytes - NOTE INVERSION
uadd8 r6, r6, r7 @ Parallel add 0xff - sets the GE bits for anything that wasn't 0
sel r6, r5, r7 @ chained....bytes are 00 for none-00 bytes, or ff for 00 bytes - NOTE INVERSION
cbnz r6, 60f
bne 15b @ (Flags from the subs above) If not run out of bytes then go around again
pop {r4,r5,r6,r7}
and r1,r1,#0xff @ Get r1 back to a single character from the expansion above
and r2,r2,#7 @ Leave the count remaining as the number after the double words have been done
20:
cbz r2, 40f @ 0 length or hit the end already then not found
21: @ Post aligned section, or just a short call
ldrb r3,[r0],#1
subs r2,r2,#1
eor r3,r3,r1 @ r3 = 0 if match - doesn't break flags from sub
cbz r3, 50f
bne 21b @ on r2 flags
40:
movs r0,#0 @ not found
bx lr
50:
subs r0,r0,#1 @ found
bx lr
60: @ We're here because the fast path found a hit - now we have to track down exactly which word it was
@ r0 points to the start of the double word after the one that was tested
@ r5 has the 00/ff pattern for the first word, r6 has the chained value
cmp r5, #0
itte eq
moveq r5, r6 @ the end is in the 2nd word
subeq r0,r0,#3 @ Points to 2nd byte of 2nd word
subne r0,r0,#7 @ or 2nd byte of 1st word
@ r0 currently points to the 3rd byte of the word containing the hit
tst r5, # CHARTSTMASK(0) @ 1st character
bne 61f
adds r0,r0,#1
tst r5, # CHARTSTMASK(1) @ 2nd character
ittt eq
addeq r0,r0,#1
tsteq r5, # (3<<15) @ 2nd & 3rd character
@ If not the 3rd must be the last one
addeq r0,r0,#1
61:
pop {r4,r5,r6,r7}
subs r0,r0,#1
bx lr
.size __memchr_arm, . - __memchr_arm
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