[AArch64] Add optimized strchr.

Implementation of strchr for AArch64.  Speedups taken from micro-bench
show the improvements relative to the standard C code.

The use of LD1 means we have identical code for both big- and
little-endian systems.

1 files changed, 138 insertions, 0 deletions

diff --git a/sysdeps/aarch64/strchr.S b/sysdeps/aarch64/strchr.S
new file mode 100644
index 0000000..a89725d
--- /dev/null
+++ b/sysdeps/aarch64/strchr.S
@@ -0,0 +1,138 @@
+/* strchr - find a character in a string
+
+   Copyright (C) 2014 Free Software Foundation, Inc.
+
+   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, see
+   <http://www.gnu.org/licenses/>.  */
+
+#include <sysdep.h>
+
+/* Assumptions:
+ *
+ * ARMv8-a, AArch64
+ */
+
+/* Arguments and results.  */
+#define srcin		x0
+#define chrin		w1
+
+#define result		x0
+
+#define src		x2
+#define tmp1		x3
+#define wtmp2		w4
+#define tmp3		x5
+
+#define vrepchr		v0
+#define vdata1		v1
+#define vdata2		v2
+#define vhas_nul1	v3
+#define vhas_nul2	v4
+#define vhas_chr1	v5
+#define vhas_chr2	v6
+#define vrepmask_0	v7
+#define vrepmask_c	v16
+#define vend1		v17
+#define vend2		v18
+
+	/* Core algorithm.
+	   For each 32-byte hunk we calculate a 64-bit syndrome value, with
+	   two bits per byte (LSB is always in bits 0 and 1, for both big
+	   and little-endian systems).  Bit 0 is set iff the relevant byte
+	   matched the requested character.  Bit 1 is set iff the
+	   relevant byte matched the NUL end of string (we trigger off bit0
+	   for the special case of looking for NUL).  Since the bits
+	   in the syndrome reflect exactly the order in which things occur
+	   in the original string a count_trailing_zeros() operation will
+	   identify exactly which byte is causing the termination, and why.  */
+
+/* Locals and temporaries.  */
+
+ENTRY (strchr)
+	mov	wtmp2, #0x0401
+	movk	wtmp2, #0x4010, lsl #16
+	dup	vrepchr.16b, chrin
+	bic	src, srcin, #31
+	dup	vrepmask_c.4s, wtmp2
+	ands	tmp1, srcin, #31
+	add	vrepmask_0.4s, vrepmask_c.4s, vrepmask_c.4s // lsl #1
+	b.eq	L(loop)
+
+	/* Input string is not 32-byte aligned.  Rather than forcing
+	   the padding bytes to a safe value, we calculate the syndrome
+	   for all the bytes, but then mask off those bits of the
+	   syndrome that are related to the padding.  */
+	ld1	{vdata1.16b, vdata2.16b}, [src], #32
+	neg	tmp1, tmp1
+	cmeq	vhas_nul1.16b, vdata1.16b, #0
+	cmeq	vhas_chr1.16b, vdata1.16b, vrepchr.16b
+	cmeq	vhas_nul2.16b, vdata2.16b, #0
+	cmeq	vhas_chr2.16b, vdata2.16b, vrepchr.16b
+	and	vhas_nul1.16b, vhas_nul1.16b, vrepmask_0.16b
+	and	vhas_nul2.16b, vhas_nul2.16b, vrepmask_0.16b
+	and	vhas_chr1.16b, vhas_chr1.16b, vrepmask_c.16b
+	and	vhas_chr2.16b, vhas_chr2.16b, vrepmask_c.16b
+	orr	vend1.16b, vhas_nul1.16b, vhas_chr1.16b
+	orr	vend2.16b, vhas_nul2.16b, vhas_chr2.16b
+	lsl	tmp1, tmp1, #1
+	addp	vend1.16b, vend1.16b, vend2.16b		// 256->128
+	mov	tmp3, #~0
+	addp	vend1.16b, vend1.16b, vend2.16b		// 128->64
+	lsr	tmp1, tmp3, tmp1
+
+	mov	tmp3, vend1.2d[0]
+	bic	tmp1, tmp3, tmp1	// Mask padding bits.
+	cbnz	tmp1, L(tail)
+
+L(loop):
+	ld1	{vdata1.16b, vdata2.16b}, [src], #32
+	cmeq	vhas_nul1.16b, vdata1.16b, #0
+	cmeq	vhas_chr1.16b, vdata1.16b, vrepchr.16b
+	cmeq	vhas_nul2.16b, vdata2.16b, #0
+	cmeq	vhas_chr2.16b, vdata2.16b, vrepchr.16b
+	/* Use a fast check for the termination condition.  */
+	orr	vend1.16b, vhas_nul1.16b, vhas_chr1.16b
+	orr	vend2.16b, vhas_nul2.16b, vhas_chr2.16b
+	orr	vend1.16b, vend1.16b, vend2.16b
+	addp	vend1.2d, vend1.2d, vend1.2d
+	mov	tmp1, vend1.2d[0]
+	cbz	tmp1, L(loop)
+
+	/* Termination condition found.  Now need to establish exactly why
+	   we terminated.  */
+	and	vhas_nul1.16b, vhas_nul1.16b, vrepmask_0.16b
+	and	vhas_nul2.16b, vhas_nul2.16b, vrepmask_0.16b
+	and	vhas_chr1.16b, vhas_chr1.16b, vrepmask_c.16b
+	and	vhas_chr2.16b, vhas_chr2.16b, vrepmask_c.16b
+	orr	vend1.16b, vhas_nul1.16b, vhas_chr1.16b
+	orr	vend2.16b, vhas_nul2.16b, vhas_chr2.16b
+	addp	vend1.16b, vend1.16b, vend2.16b		// 256->128
+	addp	vend1.16b, vend1.16b, vend2.16b		// 128->64
+
+	mov	tmp1, vend1.2d[0]
+L(tail):
+	sub	src, src, #32
+	rbit	tmp1, tmp1
+	clz	tmp1, tmp1
+	/* Tmp1 is even if the target charager was found first.  Otherwise
+	   we've found the end of string and we weren't looking for NUL.  */
+	tst	tmp1, #1
+	add	result, src, tmp1, lsr #1
+	csel	result, result, xzr, eq
+	ret
+END (strchr)
+libc_hidden_builtin_def (strchr)
+weak_alias (strchr, index)