/*
* strcpy/stpcpy - copy a string returning pointer to start/end.
*
* 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
*/
/* Assumptions:
*
* ARMv8-a, AArch64, unaligned accesses, min page size 4k.
*/
#include "../asmdefs.h"
/* 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 copies through the slower
entry path. This option is not intended for production use. */
/* 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_aarch64
#else
#define STRCPY __strcpy_aarch64
#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 (STRCPY)
/* 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_aarch64 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)
Codebase Browser
llvm