aarch64: optimize the unaligned case of memcmp

This brings to newlib a performance improvement that we developed in Bionic
libc.  That change has been submitted for review to Bionic libc:
https://android-review.googlesource.com/418279

A similar patch has been submitted for review in glibc:
https://sourceware.org/ml/libc-alpha/2017-06/msg01143.html

Patch written by Vikas Sinha and Sebastian Pop.

The performance was measured on the bionic-benchmarks on a hikey (aarch64 8xA53)
board. There was no performance change to the existing benchmark
and a performance improvement on the new benchmark for memcmp
on the unaligned side. The new benchmark has been submitted for
review at https://android-review.googlesource.com/414860

The overall performance improves by 18% for the small data set 8
and the performance improves by 450% for the large data set 64k.

The base is with the libc from /system/lib64. The bionic libc
with this patch is in /data.

hikey:/data # export LD_LIBRARY_PATH=/system/lib64
hikey:/data # ./bionic-benchmarks --benchmark_filter='BM_string_memcmp*'
Run on (8 X 2.4 MHz CPU s)
Benchmark                                Time           CPU Iterations
----------------------------------------------------------------------
BM_string_memcmp/8                      30 ns         30 ns   22955680    251.07MB/s
BM_string_memcmp/64                     57 ns         57 ns   12349184   1076.99MB/s
BM_string_memcmp/512                   305 ns        305 ns    2297163   1.56496GB/s
BM_string_memcmp/1024                  571 ns        571 ns    1225211   1.66912GB/s
BM_string_memcmp/8k                   4307 ns       4306 ns     162562   1.77177GB/s
BM_string_memcmp/16k                  8676 ns       8675 ns      80676   1.75887GB/s
BM_string_memcmp/32k                 19233 ns      19230 ns      36394   1.58695GB/s
BM_string_memcmp/64k                 36986 ns      36984 ns      18952   1.65029GB/s
BM_string_memcmp_aligned/8             199 ns        199 ns    3519166   38.3336MB/s
BM_string_memcmp_aligned/64            386 ns        386 ns    1810734   158.073MB/s
BM_string_memcmp_aligned/512          1735 ns       1734 ns     403981   281.525MB/s
BM_string_memcmp_aligned/1024         3200 ns       3200 ns     218838   305.151MB/s
BM_string_memcmp_aligned/8k          25084 ns      25080 ns      28180   311.507MB/s
BM_string_memcmp_aligned/16k         51730 ns      51729 ns      13521   302.057MB/s
BM_string_memcmp_aligned/32k        103228 ns     103228 ns       6782   302.727MB/s
BM_string_memcmp_aligned/64k        207117 ns     207087 ns       3450   301.806MB/s
BM_string_memcmp_unaligned/8           339 ns        339 ns    2070998   22.5302MB/s
BM_string_memcmp_unaligned/64         1392 ns       1392 ns     502796   43.8454MB/s
BM_string_memcmp_unaligned/512        9194 ns       9194 ns      76133   53.1104MB/s
BM_string_memcmp_unaligned/1024      18325 ns      18323 ns      38206   53.2963MB/s
BM_string_memcmp_unaligned/8k       148579 ns     148574 ns       4713   52.5831MB/s
BM_string_memcmp_unaligned/16k      298169 ns     298120 ns       2344   52.4118MB/s
BM_string_memcmp_unaligned/32k      598813 ns     598797 ns       1085    52.188MB/s
BM_string_memcmp_unaligned/64k     1196079 ns    1196083 ns        540   52.2539MB/s

hikey:/data # export LD_LIBRARY_PATH=/data
hikey:/data # ./bionic-benchmarks --benchmark_filter='BM_string_memcmp*'
Run on (8 X 2.4 MHz CPU s)
Benchmark                                Time           CPU Iterations
----------------------------------------------------------------------
BM_string_memcmp/8                      30 ns         30 ns   23209918   252.802MB/s
BM_string_memcmp/64                     57 ns         57 ns   12348447   1076.95MB/s
BM_string_memcmp/512                   305 ns        305 ns    2296878   1.56471GB/s
BM_string_memcmp/1024                  572 ns        571 ns    1224426    1.6689GB/s
BM_string_memcmp/8k                   4309 ns       4308 ns     162491   1.77109GB/s
BM_string_memcmp/16k                  9348 ns       9345 ns      74894   1.63285GB/s
BM_string_memcmp/32k                 18329 ns      18322 ns      38249    1.6656GB/s
BM_string_memcmp/64k                 36992 ns      36981 ns      18952   1.65045GB/s
BM_string_memcmp_aligned/8             199 ns        199 ns    3513925   38.3162MB/s
BM_string_memcmp_aligned/64            386 ns        386 ns    1814038   158.192MB/s
BM_string_memcmp_aligned/512          1735 ns       1735 ns     402279   281.502MB/s
BM_string_memcmp_aligned/1024         3204 ns       3202 ns     218761   304.941MB/s
BM_string_memcmp_aligned/8k          25577 ns      25569 ns      27406   305.548MB/s
BM_string_memcmp_aligned/16k         52143 ns      52123 ns      13522   299.769MB/s
BM_string_memcmp_aligned/32k        105169 ns     105127 ns       6637    297.26MB/s
BM_string_memcmp_aligned/64k        206508 ns     206383 ns       3417   302.835MB/s
BM_string_memcmp_unaligned/8           282 ns        282 ns    2482953    27.062MB/s
BM_string_memcmp_unaligned/64          542 ns        541 ns    1298317    112.77MB/s
BM_string_memcmp_unaligned/512        2152 ns       2152 ns     325267   226.915MB/s
BM_string_memcmp_unaligned/1024       4025 ns       4025 ns     173904   242.622MB/s
BM_string_memcmp_unaligned/8k        32276 ns      32271 ns      21818    242.09MB/s
BM_string_memcmp_unaligned/16k       65970 ns      65970 ns      10554   236.851MB/s
BM_string_memcmp_unaligned/32k      131241 ns     131242 ns       5129    238.11MB/s
BM_string_memcmp_unaligned/64k      266159 ns     266160 ns       2661   234.821MB/s

1 files changed, 55 insertions, 2 deletions

diff --git a/newlib/libc/machine/aarch64/memcmp.S b/newlib/libc/machine/aarch64/memcmp.S
index 860384cc8..09be4c344 100644
--- a/newlib/libc/machine/aarch64/memcmp.S
+++ b/newlib/libc/machine/aarch64/memcmp.S
@@ -1,6 +1,7 @@
 /* memcmp - compare memory
 
    Copyright (c) 2013, Linaro Limited
+   Copyright (c) 2017, Samsung Austin R&D Center
    All rights reserved.
 
    Redistribution and use in source and binary forms, with or without
@@ -152,9 +153,61 @@ def_fn memcmp p2align=6
 
 	.p2align 6
 .Lmisaligned8:
+
+	cmp	limit, #8
+	b.lo	.LmisalignedLt8
+
+.LunalignedGe8 :
+
+	/* Load the first dword with both src potentially unaligned. */
+	ldr	data1, [src1]
+	ldr	data2, [src2]
+
+	eor	diff, data1, data2	/* Non-zero if differences found. */
+	cbnz	diff, .Lnot_limit
+
+	/* Sources are not aligned: align one of the sources. */
+
+	and	tmp1, src1, #0x7
+	orr	tmp3, xzr, #0x8
+	sub	pos, tmp3, tmp1
+
+	/* Increment SRC pointers by POS so SRC1 is word-aligned. */
+	add	src1, src1, pos
+	add	src2, src2, pos
+
+	sub	limit, limit, pos
+	lsr	limit_wd, limit, #3
+
+	cmp limit_wd, #0
+
+	/* save #bytes to go back to be able to read 8byte at end
+	   pos=negative offset position to read 8 bytes when len%8 != 0 */
+	and	limit, limit, #7
+	sub	pos, limit, #8
+
+	b	.Lstart_part_realigned
+
+	.p2align 5
+.Lloop_part_aligned:
+	ldr	data1, [src1], #8
+	ldr	data2, [src2], #8
+	subs	limit_wd, limit_wd, #1
+.Lstart_part_realigned:
+	eor	diff, data1, data2	/* Non-zero if differences found. */
+	cbnz	diff, .Lnot_limit
+	b.ne	.Lloop_part_aligned
+
+	/* process leftover bytes: read the leftover bytes, starting with
+	   negative offset - so we can load 8 bytes. */
+	ldr	data1, [src1, pos]
+	ldr	data2, [src2, pos]
+	eor	diff, data1, data2	/* Non-zero if differences found.  */
+	b .Lnot_limit
+
+.LmisalignedLt8:
 	sub	limit, limit, #1
 1:
-	/* Perhaps we can do better than this.  */
 	ldrb	data1w, [src1], #1
 	ldrb	data2w, [src2], #1
 	subs	limit, limit, #1
@@ -164,4 +217,4 @@ def_fn memcmp p2align=6
 	ret
 	.size memcmp, . - memcmp
 
-#endif
\ No newline at end of file
+#endif