[PATCH v2 3/8] arm64: Import updated version of Cortex Strings' strlen
Robin Murphy
robin.murphy at arm.com
Thu May 27 08:34:43 PDT 2021
From: Sam Tebbs <sam.tebbs at arm.com>
Import an updated version of the former Cortex Strings - now Arm
Optimized Routines - strcmp function. The latest version introduces
Advanced SIMD usage which rules it out for our purposes, but we can
still pick an intermediate improvement from the previous version,
namely string/aarch64/strlen.S at commit 98e4d6a from
https://github.com/ARM-software/optimized-routines
Note that for simplicity Arm have chosen to contribute this code
to Linux under GPLv2 rather than the original MIT license.
Signed-off-by: Sam Tebbs <sam.tebbs at arm.com>
[ rm: update attribution and commit message ]
Signed-off-by: Robin Murphy <robin.murphy at arm.com>
---
arch/arm64/lib/strlen.S | 262 +++++++++++++++++++++++++++-------------
1 file changed, 175 insertions(+), 87 deletions(-)
diff --git a/arch/arm64/lib/strlen.S b/arch/arm64/lib/strlen.S
index ee3ed882dd79..b557185b54a5 100644
--- a/arch/arm64/lib/strlen.S
+++ b/arch/arm64/lib/strlen.S
@@ -1,115 +1,203 @@
/* SPDX-License-Identifier: GPL-2.0-only */
/*
- * Copyright (C) 2013 ARM Ltd.
- * Copyright (C) 2013 Linaro.
+ * Copyright (c) 2013, Arm Limited.
*
- * This code is based on glibc cortex strings work originally authored by Linaro
- * be found @
- *
- * http://bazaar.launchpad.net/~linaro-toolchain-dev/cortex-strings/trunk/
- * files/head:/src/aarch64/
+ * Adapted from the original at:
+ * https://github.com/ARM-software/optimized-routines/blob/master/string/aarch64/strlen.S
*/
#include <linux/linkage.h>
#include <asm/assembler.h>
-/*
- * calculate the length of a string
+/* Assumptions:
*
- * Parameters:
- * x0 - const string pointer
- * Returns:
- * x0 - the return length of specific string
+ * ARMv8-a, AArch64, unaligned accesses, min page size 4k.
*/
+#define L(label) .L ## label
+
/* Arguments and results. */
-srcin .req x0
-len .req x0
+#define srcin x0
+#define len x0
/* Locals and temporaries. */
-src .req x1
-data1 .req x2
-data2 .req x3
-data2a .req x4
-has_nul1 .req x5
-has_nul2 .req x6
-tmp1 .req x7
-tmp2 .req x8
-tmp3 .req x9
-tmp4 .req x10
-zeroones .req x11
-pos .req x12
+#define src x1
+#define data1 x2
+#define data2 x3
+#define has_nul1 x4
+#define has_nul2 x5
+#define tmp1 x4
+#define tmp2 x5
+#define tmp3 x6
+#define tmp4 x7
+#define zeroones x8
+
+ /* 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. A faster check
+ (X - 1) & 0x80 is zero for non-NUL ASCII characters, but gives
+ false hits for characters 129..255. */
#define REP8_01 0x0101010101010101
#define REP8_7f 0x7f7f7f7f7f7f7f7f
#define REP8_80 0x8080808080808080
+#define MIN_PAGE_SIZE 4096
+
+ /* Since strings are short on average, we check the first 16 bytes
+ of the string for a NUL character. In order to do an unaligned ldp
+ safely we have to do a page cross check first. If there is a NUL
+ byte we calculate the length from the 2 8-byte words using
+ conditional select to reduce branch mispredictions (it is unlikely
+ strlen will be repeatedly called on strings with the same length).
+
+ If the string is longer than 16 bytes, we align src so don't need
+ further page cross checks, and process 32 bytes per iteration
+ using the fast NUL check. If we encounter non-ASCII characters,
+ fallback to a second loop using the full NUL check.
+
+ If the page cross check fails, we read 16 bytes from an aligned
+ address, remove any characters before the string, and continue
+ in the main loop using aligned loads. Since strings crossing a
+ page in the first 16 bytes are rare (probability of
+ 16/MIN_PAGE_SIZE ~= 0.4%), this case does not need to be optimized.
+
+ AArch64 systems have a minimum page size of 4k. 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. */
+
SYM_FUNC_START_WEAK_PI(strlen)
- mov zeroones, #REP8_01
- bic src, srcin, #15
- ands tmp1, srcin, #15
- b.ne .Lmisaligned
- /*
- * 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.
- */
- /*
- * 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.
- */
-.Lloop:
- ldp data1, data2, [src], #16
-.Lrealigned:
+ and tmp1, srcin, MIN_PAGE_SIZE - 1
+ mov zeroones, REP8_01
+ cmp tmp1, MIN_PAGE_SIZE - 16
+ b.gt L(page_cross)
+ ldp data1, data2, [srcin]
+#ifdef __AARCH64EB__
+ /* For big-endian, carry propagation (if the final byte in the
+ string is 0x01) means we cannot use has_nul1/2 directly.
+ Since we expect strings to be small and early-exit,
+ byte-swap the data now so has_null1/2 will be correct. */
+ rev data1, data1
+ rev data2, data2
+#endif
sub tmp1, data1, zeroones
- orr tmp2, data1, #REP8_7f
+ 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 .Lloop
+ orr tmp4, data2, REP8_7f
+ bics has_nul1, tmp1, tmp2
+ bic has_nul2, tmp3, tmp4
+ ccmp has_nul2, 0, 0, eq
+ beq L(main_loop_entry)
- sub len, src, srcin
- cbz has_nul1, .Lnul_in_data2
-CPU_BE( mov data2, data1 ) /*prepare data to re-calculate the syndrome*/
- sub len, len, #8
- mov has_nul2, has_nul1
-.Lnul_in_data2:
- /*
- * 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.
- */
-CPU_BE( rev data2, data2 )
-CPU_BE( sub tmp1, data2, zeroones )
-CPU_BE( orr tmp2, data2, #REP8_7f )
-CPU_BE( bic has_nul2, tmp1, tmp2 )
-
- sub len, len, #8
- rev has_nul2, has_nul2
- clz pos, has_nul2
- add len, len, pos, lsr #3 /* Bits to bytes. */
+ /* Enter with C = has_nul1 == 0. */
+ csel has_nul1, has_nul1, has_nul2, cc
+ mov len, 8
+ rev has_nul1, has_nul1
+ clz tmp1, has_nul1
+ csel len, xzr, len, cc
+ add len, len, tmp1, lsr 3
ret
-.Lmisaligned:
- cmp tmp1, #8
- neg tmp1, tmp1
- ldp data1, data2, [src], #16
- lsl tmp1, tmp1, #3 /* Bytes beyond alignment -> bits. */
- mov tmp2, #~0
- /* Big-endian. Early bytes are at MSB. */
-CPU_BE( lsl tmp2, tmp2, tmp1 ) /* Shift (tmp1 & 63). */
- /* Little-endian. Early bytes are at LSB. */
-CPU_LE( lsr tmp2, tmp2, tmp1 ) /* Shift (tmp1 & 63). */
+ /* The inner loop processes 32 bytes per iteration and uses the fast
+ NUL check. If we encounter non-ASCII characters, use a second
+ loop with the accurate NUL check. */
+ .p2align 4
+L(main_loop_entry):
+ bic src, srcin, 15
+ sub src, src, 16
+L(main_loop):
+ ldp data1, data2, [src, 32]!
+L(page_cross_entry):
+ sub tmp1, data1, zeroones
+ sub tmp3, data2, zeroones
+ orr tmp2, tmp1, tmp3
+ tst tmp2, zeroones, lsl 7
+ bne 1f
+ ldp data1, data2, [src, 16]
+ sub tmp1, data1, zeroones
+ sub tmp3, data2, zeroones
+ orr tmp2, tmp1, tmp3
+ tst tmp2, zeroones, lsl 7
+ beq L(main_loop)
+ add src, src, 16
+1:
+ /* The fast check failed, so do the slower, accurate NUL check. */
+ orr tmp2, data1, REP8_7f
+ orr tmp4, data2, REP8_7f
+ bics has_nul1, tmp1, tmp2
+ bic has_nul2, tmp3, tmp4
+ ccmp has_nul2, 0, 0, eq
+ beq L(nonascii_loop)
+
+ /* Enter with C = has_nul1 == 0. */
+L(tail):
+#ifdef __AARCH64EB__
+ /* For big-endian, carry propagation (if the final byte in the
+ string is 0x01) means we cannot use has_nul1/2 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, cc
+ 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, cc
+#endif
+ sub len, src, srcin
+ rev has_nul1, has_nul1
+ add tmp2, len, 8
+ clz tmp1, has_nul1
+ csel len, len, tmp2, cc
+ add len, len, tmp1, lsr 3
+ ret
+
+L(nonascii_loop):
+ ldp data1, data2, [src, 16]!
+ sub tmp1, data1, zeroones
+ orr tmp2, data1, REP8_7f
+ sub tmp3, data2, zeroones
+ orr tmp4, data2, REP8_7f
+ bics has_nul1, tmp1, tmp2
+ bic has_nul2, tmp3, tmp4
+ ccmp has_nul2, 0, 0, eq
+ bne L(tail)
+ ldp data1, data2, [src, 16]!
+ sub tmp1, data1, zeroones
+ orr tmp2, data1, REP8_7f
+ sub tmp3, data2, zeroones
+ orr tmp4, data2, REP8_7f
+ bics has_nul1, tmp1, tmp2
+ bic has_nul2, tmp3, tmp4
+ ccmp has_nul2, 0, 0, eq
+ beq L(nonascii_loop)
+ b L(tail)
+
+ /* Load 16 bytes from [srcin & ~15] and force the bytes that precede
+ srcin to 0x7f, so we ignore any NUL bytes before the string.
+ Then continue in the aligned loop. */
+L(page_cross):
+ bic src, srcin, 15
+ ldp data1, data2, [src]
+ lsl tmp1, srcin, 3
+ mov tmp4, -1
+#ifdef __AARCH64EB__
+ /* Big-endian. Early bytes are at MSB. */
+ lsr tmp1, tmp4, tmp1 /* Shift (tmp1 & 63). */
+#else
+ /* Little-endian. Early bytes are at LSB. */
+ lsl tmp1, tmp4, tmp1 /* Shift (tmp1 & 63). */
+#endif
+ orr tmp1, tmp1, REP8_80
+ orn data1, data1, tmp1
+ orn tmp2, data2, tmp1
+ tst srcin, 8
+ csel data1, data1, tmp4, eq
+ csel data2, data2, tmp2, eq
+ b L(page_cross_entry)
- orr data1, data1, tmp2
- orr data2a, data2, tmp2
- csinv data1, data1, xzr, le
- csel data2, data2, data2a, le
- b .Lrealigned
SYM_FUNC_END_PI(strlen)
EXPORT_SYMBOL_NOKASAN(strlen)
--
2.21.0.dirty
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