AArch64: Optimized implementations of strcpy and stpcpy.

2 files changed, 346 insertions, 0 deletions

diff --git a/sysdeps/aarch64/stpcpy.S b/sysdeps/aarch64/stpcpy.S
new file mode 100644
index 0000000..6576a75
--- /dev/null
+++ b/sysdeps/aarch64/stpcpy.S
@@ -0,0 +1,20 @@
+/* stpcpy - copy a string returning pointer to end.
+   Copyright (C) 2015 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/>.  */
+
+#define BUILD_STPCPY
+#include "strcpy.S"
diff --git a/sysdeps/aarch64/strcpy.S b/sysdeps/aarch64/strcpy.S
new file mode 100644
index 0000000..28846fb
--- /dev/null
+++ b/sysdeps/aarch64/strcpy.S
@@ -0,0 +1,326 @@
+/* strcpy/stpcpy - copy a string returning pointer to start/end.
+   Copyright (C) 2013-2015 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/>.  */
+
+/* To build as stpcpy, define BUILD_STPCPY before compiling this file.
+
+   To test the page crossing code path more thoroughly, compile with
+   -DSTRCPY_TEST_PAGE_CROSS - this will force all unaligned copies through
+   the slower entry path.  This option is not intended for production use.  */
+
+#include <sysdep.h>
+
+/* Assumptions:
+ *
+ * ARMv8-a, AArch64, unaligned accesses, min page size 4k.
+ */
+
+/* Arguments and results.  */
+#define dstin		x0
+#define srcin		x1
+
+/* Locals and temporaries.  */
+#define src		x2
+#define dst		x3
+#define data1		x4
+#define data1w		w4
+#define data2		x5
+#define data2w		w5
+#define has_nul1	x6
+#define has_nul2	x7
+#define tmp1		x8
+#define tmp2		x9
+#define tmp3		x10
+#define tmp4		x11
+#define zeroones	x12
+#define data1a		x13
+#define data2a		x14
+#define pos		x15
+#define len		x16
+#define to_align	x17
+
+#ifdef BUILD_STPCPY
+#define STRCPY __stpcpy
+#else
+#define STRCPY strcpy
+#endif
+
+	/* NUL detection works on the principle that (X - 1) & (~X) & 0x80
+	   (=> (X - 1) & ~(X | 0x7f)) is non-zero iff a byte is zero, and
+	   can be done in parallel across the entire word.  */
+
+#define REP8_01 0x0101010101010101
+#define REP8_7f 0x7f7f7f7f7f7f7f7f
+#define REP8_80 0x8080808080808080
+
+	/* AArch64 systems have a minimum page size of 4k.  We can do a quick
+	   page size check for crossing this boundary on entry and if we
+	   do not, then we can short-circuit much of the entry code.  We
+	   expect early page-crossing strings to be rare (probability of
+	   16/MIN_PAGE_SIZE ~= 0.4%), so the branch should be quite
+	   predictable, even with random strings.
+
+	   We don't bother checking for larger page sizes, the cost of setting
+	   up the correct page size is just not worth the extra gain from
+	   a small reduction in the cases taking the slow path.  Note that
+	   we only care about whether the first fetch, which may be
+	   misaligned, crosses a page boundary - after that we move to aligned
+	   fetches for the remainder of the string.  */
+
+#ifdef STRCPY_TEST_PAGE_CROSS
+	/* Make everything that isn't Qword aligned look like a page cross.  */
+#define MIN_PAGE_P2 4
+#else
+#define MIN_PAGE_P2 12
+#endif
+
+#define MIN_PAGE_SIZE (1 << MIN_PAGE_P2)
+
+ENTRY_ALIGN (STRCPY, 6)
+	/* For moderately short strings, the fastest way to do the copy is to
+	   calculate the length of the string in the same way as strlen, then
+	   essentially do a memcpy of the result.  This avoids the need for
+	   multiple byte copies and further means that by the time we
+	   reach the bulk copy loop we know we can always use DWord
+	   accesses.  We expect strcpy to rarely be called repeatedly
+	   with the same source string, so branch prediction is likely to
+	   always be difficult - we mitigate against this by preferring
+	   conditional select operations over branches whenever this is
+	   feasible.  */
+	and	tmp2, srcin, #(MIN_PAGE_SIZE - 1)
+	mov	zeroones, #REP8_01
+	and	to_align, srcin, #15
+	cmp	tmp2, #(MIN_PAGE_SIZE - 16)
+	neg	tmp1, to_align
+	/* The first fetch will straddle a (possible) page boundary iff
+	   srcin + 15 causes bit[MIN_PAGE_P2] to change value.  A 16-byte
+	   aligned string will never fail the page align check, so will
+	   always take the fast path.  */
+	b.gt	L(page_cross)
+
+L(page_cross_ok):
+	ldp	data1, data2, [srcin]
+#ifdef __AARCH64EB__
+	/* Because we expect the end to be found within 16 characters
+	   (profiling shows this is the most common case), it's worth
+	   swapping the bytes now to save having to recalculate the
+	   termination syndrome later.  We preserve data1 and data2
+	   so that we can re-use the values later on.  */
+	rev	tmp2, data1
+	sub	tmp1, tmp2, zeroones
+	orr	tmp2, tmp2, #REP8_7f
+	bics	has_nul1, tmp1, tmp2
+	b.ne	L(fp_le8)
+	rev	tmp4, data2
+	sub	tmp3, tmp4, zeroones
+	orr	tmp4, tmp4, #REP8_7f
+#else
+	sub	tmp1, data1, zeroones
+	orr	tmp2, data1, #REP8_7f
+	bics	has_nul1, tmp1, tmp2
+	b.ne	L(fp_le8)
+	sub	tmp3, data2, zeroones
+	orr	tmp4, data2, #REP8_7f
+#endif
+	bics	has_nul2, tmp3, tmp4
+	b.eq	L(bulk_entry)
+
+	/* The string is short (<=16 bytes).  We don't know exactly how
+	   short though, yet.  Work out the exact length so that we can
+	   quickly select the optimal copy strategy.  */
+L(fp_gt8):
+	rev	has_nul2, has_nul2
+	clz	pos, has_nul2
+	mov	tmp2, #56
+	add	dst, dstin, pos, lsr #3		/* Bits to bytes.  */
+	sub	pos, tmp2, pos
+#ifdef __AARCH64EB__
+	lsr	data2, data2, pos
+#else
+	lsl	data2, data2, pos
+#endif
+	str	data2, [dst, #1]
+	str	data1, [dstin]
+#ifdef BUILD_STPCPY
+	add	dstin, dst, #8
+#endif
+	ret
+
+L(fp_le8):
+	rev	has_nul1, has_nul1
+	clz	pos, has_nul1
+	add	dst, dstin, pos, lsr #3		/* Bits to bytes.  */
+	subs	tmp2, pos, #24			/* Pos in bits. */
+	b.lt	L(fp_lt4)
+#ifdef __AARCH64EB__
+	mov	tmp2, #56
+	sub	pos, tmp2, pos
+	lsr	data2, data1, pos
+	lsr	data1, data1, #32
+#else
+	lsr	data2, data1, tmp2
+#endif
+	/* 4->7 bytes to copy.  */
+	str	data2w, [dst, #-3]
+	str	data1w, [dstin]
+#ifdef BUILD_STPCPY
+	mov	dstin, dst
+#endif
+	ret
+L(fp_lt4):
+	cbz	pos, L(fp_lt2)
+	/* 2->3 bytes to copy.  */
+#ifdef __AARCH64EB__
+	lsr	data1, data1, #48
+#endif
+	strh	data1w, [dstin]
+	/* Fall-through, one byte (max) to go.  */
+L(fp_lt2):
+	/* Null-terminated string.  Last character must be zero!  */
+	strb	wzr, [dst]
+#ifdef BUILD_STPCPY
+	mov	dstin, dst
+#endif
+	ret
+
+	.p2align 6
+	/* Aligning here ensures that the entry code and main loop all lies
+	   within one 64-byte cache line.  */
+L(bulk_entry):
+	sub	to_align, to_align, #16
+	stp	data1, data2, [dstin]
+	sub	src, srcin, to_align
+	sub	dst, dstin, to_align
+	b	L(entry_no_page_cross)
+
+	/* The inner loop deals with two Dwords at a time.  This has a
+	   slightly higher start-up cost, but we should win quite quickly,
+	   especially on cores with a high number of issue slots per
+	   cycle, as we get much better parallelism out of the operations.  */
+L(main_loop):
+	stp	data1, data2, [dst], #16
+L(entry_no_page_cross):
+	ldp	data1, data2, [src], #16
+	sub	tmp1, data1, zeroones
+	orr	tmp2, data1, #REP8_7f
+	sub	tmp3, data2, zeroones
+	orr	tmp4, data2, #REP8_7f
+	bic	has_nul1, tmp1, tmp2
+	bics	has_nul2, tmp3, tmp4
+	ccmp	has_nul1, #0, #0, eq	/* NZCV = 0000  */
+	b.eq	L(main_loop)
+
+	/* Since we know we are copying at least 16 bytes, the fastest way
+	   to deal with the tail is to determine the location of the
+	   trailing NUL, then (re)copy the 16 bytes leading up to that.  */
+	cmp	has_nul1, #0
+#ifdef __AARCH64EB__
+	/* For big-endian, carry propagation (if the final byte in the
+	   string is 0x01) means we cannot use has_nul directly.  The
+	   easiest way to get the correct byte is to byte-swap the data
+	   and calculate the syndrome a second time.  */
+	csel	data1, data1, data2, ne
+	rev	data1, data1
+	sub	tmp1, data1, zeroones
+	orr	tmp2, data1, #REP8_7f
+	bic	has_nul1, tmp1, tmp2
+#else
+	csel	has_nul1, has_nul1, has_nul2, ne
+#endif
+	rev	has_nul1, has_nul1
+	clz	pos, has_nul1
+	add	tmp1, pos, #72
+	add	pos, pos, #8
+	csel	pos, pos, tmp1, ne
+	add	src, src, pos, lsr #3
+	add	dst, dst, pos, lsr #3
+	ldp	data1, data2, [src, #-32]
+	stp	data1, data2, [dst, #-16]
+#ifdef BUILD_STPCPY
+	sub	dstin, dst, #1
+#endif
+	ret
+
+L(page_cross):
+	bic	src, srcin, #15
+	/* Start by loading two words at [srcin & ~15], then forcing the
+	   bytes that precede srcin to 0xff.  This means they never look
+	   like termination bytes.  */
+	ldp	data1, data2, [src]
+	lsl	tmp1, tmp1, #3	/* Bytes beyond alignment -> bits.  */
+	tst	to_align, #7
+	csetm	tmp2, ne
+#ifdef __AARCH64EB__
+	lsl	tmp2, tmp2, tmp1	/* Shift (tmp1 & 63).  */
+#else
+	lsr	tmp2, tmp2, tmp1	/* Shift (tmp1 & 63).  */
+#endif
+	orr	data1, data1, tmp2
+	orr	data2a, data2, tmp2
+	cmp	to_align, #8
+	csinv	data1, data1, xzr, lt
+	csel	data2, data2, data2a, lt
+	sub	tmp1, data1, zeroones
+	orr	tmp2, data1, #REP8_7f
+	sub	tmp3, data2, zeroones
+	orr	tmp4, data2, #REP8_7f
+	bic	has_nul1, tmp1, tmp2
+	bics	has_nul2, tmp3, tmp4
+	ccmp	has_nul1, #0, #0, eq	/* NZCV = 0000  */
+	b.eq	L(page_cross_ok)
+	/* We now need to make data1 and data2 look like they've been
+	   loaded directly from srcin.  Do a rotate on the 128-bit value.  */
+	lsl	tmp1, to_align, #3	/* Bytes->bits.  */
+	neg	tmp2, to_align, lsl #3
+#ifdef __AARCH64EB__
+	lsl	data1a, data1, tmp1
+	lsr	tmp4, data2, tmp2
+	lsl	data2, data2, tmp1
+	orr	tmp4, tmp4, data1a
+	cmp	to_align, #8
+	csel	data1, tmp4, data2, lt
+	rev	tmp2, data1
+	rev	tmp4, data2
+	sub	tmp1, tmp2, zeroones
+	orr	tmp2, tmp2, #REP8_7f
+	sub	tmp3, tmp4, zeroones
+	orr	tmp4, tmp4, #REP8_7f
+#else
+	lsr	data1a, data1, tmp1
+	lsl	tmp4, data2, tmp2
+	lsr	data2, data2, tmp1
+	orr	tmp4, tmp4, data1a
+	cmp	to_align, #8
+	csel	data1, tmp4, data2, lt
+	sub	tmp1, data1, zeroones
+	orr	tmp2, data1, #REP8_7f
+	sub	tmp3, data2, zeroones
+	orr	tmp4, data2, #REP8_7f
+#endif
+	bic	has_nul1, tmp1, tmp2
+	cbnz	has_nul1, L(fp_le8)
+	bic	has_nul2, tmp3, tmp4
+	b	L(fp_gt8)
+END (STRCPY)
+
+#ifdef BUILD_STPCPY
+weak_alias (__stpcpy, stpcpy)
+libc_hidden_def (__stpcpy)
+libc_hidden_builtin_def (stpcpy)
+#else
+libc_hidden_builtin_def (strcpy)
+#endif