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aarch64: Optimized implementation of strnlen

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Optimize the strlen implementation by using vector operations and
loop unrooling in main loop. Compared to aarch64/strnlen.S, it
reduces latency of cases in bench-strnlen by 11%~24% when the length
of src is greater than 64 bytes, with gains throughout the benchmark.

Checked on aarch64-linux-gnu.

Reviewed-by: Wilco Dijkstra <Wilco.Dijkstra@arm.com>
This commit is contained in:
Xuelei Zhang 2019-12-19 13:49:46 +00:00 committed by Adhemerval Zanella
parent 0237b61526
commit 2911cb68ed
1 changed files with 51 additions and 1 deletions
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52
sysdeps/aarch64/strnlen.S
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@ -45,6 +45,11 @@
#define pos x13
#define limit_wd x14
#define dataq q2
#define datav v2
#define datab2 b3
#define dataq2 q3
#define datav2 v3
#define REP8_01 0x0101010101010101
#define REP8_7f 0x7f7f7f7f7f7f7f7f
#define REP8_80 0x8080808080808080
@ -71,7 +76,7 @@ ENTRY_ALIGN_AND_PAD (__strnlen, 6, 9)
cycle, as we get much better parallelism out of the operations. */
/* Start of critial section -- keep to one 64Byte cache line. */
L(loop):
ldp data1, data2, [src], #16
L(realigned):
sub tmp1, data1, zeroones
@ -119,6 +124,51 @@ L(nul_in_data2):
csel len, len, limit, ls /* Return the lower value. */
RET
L(loop):
ldr dataq, [src], #16
uminv datab2, datav.16b
mov tmp1, datav2.d[0]
subs limit_wd, limit_wd, #1
ccmp tmp1, #0, #4, pl /* NZCV = 0000 */
b.eq L(loop_end)
ldr dataq, [src], #16
uminv datab2, datav.16b
mov tmp1, datav2.d[0]
subs limit_wd, limit_wd, #1
ccmp tmp1, #0, #4, pl /* NZCV = 0000 */
b.ne L(loop)
L(loop_end):
/* End of critical section -- keep to one 64Byte cache line. */
cbnz tmp1, L(hit_limit) /* No null in final Qword. */
/* We know there's a null in the final Qword. The easiest thing
to do now is work out the length of the string and return
MIN (len, limit). */
#ifdef __AARCH64EB__
rev64 datav.16b, datav.16b
#endif
/* Set te NULL byte as 0xff and the rest as 0x00, move the data into a
pair of scalars and then compute the length from the earliest NULL
byte. */
cmeq datav.16b, datav.16b, #0
mov data1, datav.d[0]
mov data2, datav.d[1]
cmp data1, 0
csel data1, data1, data2, ne
sub len, src, srcin
sub len, len, #16
rev data1, data1
add tmp2, len, 8
clz tmp1, data1
csel len, len, tmp2, ne
add len, len, tmp1, lsr 3
cmp len, limit
csel len, len, limit, ls /* Return the lower value. */
RET
L(misaligned):
/* Deal with a partial first word.
We're doing two things in parallel here;