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+/* SPDX-License-Identifier: GPL-2.0-only */
+/*
+ * Copyright (c) 2013-2022, Arm Limited.
+ *
+ * Adapted from the original at:
+ * https://github.com/ARM-software/optimized-routines/blob/189dfefe37d54c5b/string/aarch64/strncmp.S
+ */
+
+#include <linux/linkage.h>
+#include <asm/assembler.h>
+
+/* Assumptions:
+ *
+ * ARMv8-a, AArch64.
+ * MTE compatible.
+ */
+
+#define L(label) .L ## label
+
+#define REP8_01 0x0101010101010101
+#define REP8_7f 0x7f7f7f7f7f7f7f7f
+
+/* Parameters and result. */
+#define src1 x0
+#define src2 x1
+#define limit x2
+#define result x0
+
+/* Internal variables. */
+#define data1 x3
+#define data1w w3
+#define data2 x4
+#define data2w w4
+#define has_nul x5
+#define diff x6
+#define syndrome x7
+#define tmp1 x8
+#define tmp2 x9
+#define tmp3 x10
+#define zeroones x11
+#define pos x12
+#define mask x13
+#define endloop x14
+#define count mask
+#define offset pos
+#define neg_offset x15
+
+/* Define endian dependent shift operations.
+ On big-endian early bytes are at MSB and on little-endian LSB.
+ LS_FW means shifting towards early bytes.
+ LS_BK means shifting towards later bytes.
+ */
+#ifdef __AARCH64EB__
+#define LS_FW lsl
+#define LS_BK lsr
+#else
+#define LS_FW lsr
+#define LS_BK lsl
+#endif
+
+SYM_FUNC_START(__pi_strncmp)
+ cbz limit, L(ret0)
+ eor tmp1, src1, src2
+ mov zeroones, #REP8_01
+ tst tmp1, #7
+ and count, src1, #7
+ b.ne L(misaligned8)
+ cbnz count, L(mutual_align)
+
+ /* 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. */
+ .p2align 4
+L(loop_aligned):
+ ldr data1, [src1], #8
+ ldr data2, [src2], #8
+L(start_realigned):
+ subs limit, limit, #8
+ sub tmp1, data1, zeroones
+ orr tmp2, data1, #REP8_7f
+ eor diff, data1, data2 /* Non-zero if differences found. */
+ csinv endloop, diff, xzr, hi /* Last Dword or differences. */
+ bics has_nul, tmp1, tmp2 /* Non-zero if NUL terminator. */
+ ccmp endloop, #0, #0, eq
+ b.eq L(loop_aligned)
+ /* End of main loop */
+
+L(full_check):
+#ifndef __AARCH64EB__
+ orr syndrome, diff, has_nul
+ add limit, limit, 8 /* Rewind limit to before last subs. */
+L(syndrome_check):
+ /* Limit was reached. Check if the NUL byte or the difference
+ is before the limit. */
+ rev syndrome, syndrome
+ rev data1, data1
+ clz pos, syndrome
+ rev data2, data2
+ lsl data1, data1, pos
+ cmp limit, pos, lsr #3
+ lsl data2, data2, pos
+ /* But we need to zero-extend (char is unsigned) the value and then
+ perform a signed 32-bit subtraction. */
+ lsr data1, data1, #56
+ sub result, data1, data2, lsr #56
+ csel result, result, xzr, hi
+ ret
+#else
+ /* Not reached the limit, must have found the end or a diff. */
+ tbz limit, #63, L(not_limit)
+ add tmp1, limit, 8
+ cbz limit, L(not_limit)
+
+ lsl limit, tmp1, #3 /* Bits -> bytes. */
+ mov mask, #~0
+ lsr mask, mask, limit
+ bic data1, data1, mask
+ bic data2, data2, mask
+
+ /* Make sure that the NUL byte is marked in the syndrome. */
+ orr has_nul, has_nul, mask
+
+L(not_limit):
+ /* For big-endian we cannot use the trick with the syndrome value
+ as carry-propagation can corrupt the upper bits if the trailing
+ bytes in the string contain 0x01. */
+ /* However, if there is no NUL byte in the dword, we can generate
+ the result directly. We can't just subtract the bytes as the
+ MSB might be significant. */
+ cbnz has_nul, 1f
+ cmp data1, data2
+ cset result, ne
+ cneg result, result, lo
+ ret
+1:
+ /* Re-compute the NUL-byte detection, using a byte-reversed value. */
+ rev tmp3, data1
+ sub tmp1, tmp3, zeroones
+ orr tmp2, tmp3, #REP8_7f
+ bic has_nul, tmp1, tmp2
+ rev has_nul, has_nul
+ orr syndrome, diff, has_nul
+ clz pos, syndrome
+ /* The most-significant-non-zero bit of the syndrome marks either the
+ first bit that is different, or the top bit of the first zero byte.
+ Shifting left now will bring the critical information into the
+ top bits. */
+L(end_quick):
+ lsl data1, data1, pos
+ lsl data2, data2, pos
+ /* But we need to zero-extend (char is unsigned) the value and then
+ perform a signed 32-bit subtraction. */
+ lsr data1, data1, #56
+ sub result, data1, data2, lsr #56
+ ret
+#endif
+
+L(mutual_align):
+ /* Sources are mutually aligned, but are not currently at an
+ alignment boundary. Round down the addresses and then mask off
+ the bytes that precede the start point.
+ We also need to adjust the limit calculations, but without
+ overflowing if the limit is near ULONG_MAX. */
+ bic src1, src1, #7
+ bic src2, src2, #7
+ ldr data1, [src1], #8
+ neg tmp3, count, lsl #3 /* 64 - bits(bytes beyond align). */
+ ldr data2, [src2], #8
+ mov tmp2, #~0
+ LS_FW tmp2, tmp2, tmp3 /* Shift (count & 63). */
+ /* Adjust the limit and ensure it doesn't overflow. */
+ adds limit, limit, count
+ csinv limit, limit, xzr, lo
+ orr data1, data1, tmp2
+ orr data2, data2, tmp2
+ b L(start_realigned)
+
+ .p2align 4
+ /* Don't bother with dwords for up to 16 bytes. */
+L(misaligned8):
+ cmp limit, #16
+ b.hs L(try_misaligned_words)
+
+L(byte_loop):
+ /* Perhaps we can do better than this. */
+ ldrb data1w, [src1], #1
+ ldrb data2w, [src2], #1
+ subs limit, limit, #1
+ ccmp data1w, #1, #0, hi /* NZCV = 0b0000. */
+ ccmp data1w, data2w, #0, cs /* NZCV = 0b0000. */
+ b.eq L(byte_loop)
+L(done):
+ sub result, data1, data2
+ ret
+ /* Align the SRC1 to a dword by doing a bytewise compare and then do
+ the dword loop. */
+L(try_misaligned_words):
+ cbz count, L(src1_aligned)
+
+ neg count, count
+ and count, count, #7
+ sub limit, limit, count
+
+L(page_end_loop):
+ ldrb data1w, [src1], #1
+ ldrb data2w, [src2], #1
+ cmp data1w, #1
+ ccmp data1w, data2w, #0, cs /* NZCV = 0b0000. */
+ b.ne L(done)
+ subs count, count, #1
+ b.hi L(page_end_loop)
+
+ /* The following diagram explains the comparison of misaligned strings.
+ The bytes are shown in natural order. For little-endian, it is
+ reversed in the registers. The "x" bytes are before the string.
+ The "|" separates data that is loaded at one time.
+ src1 | a a a a a a a a | b b b c c c c c | . . .
+ src2 | x x x x x a a a a a a a a b b b | c c c c c . . .
+
+ After shifting in each step, the data looks like this:
+ STEP_A STEP_B STEP_C
+ data1 a a a a a a a a b b b c c c c c b b b c c c c c
+ data2 a a a a a a a a b b b 0 0 0 0 0 0 0 0 c c c c c
+
+ The bytes with "0" are eliminated from the syndrome via mask.
+
+ Align SRC2 down to 16 bytes. This way we can read 16 bytes at a
+ time from SRC2. The comparison happens in 3 steps. After each step
+ the loop can exit, or read from SRC1 or SRC2. */
+L(src1_aligned):
+ /* Calculate offset from 8 byte alignment to string start in bits. No
+ need to mask offset since shifts are ignoring upper bits. */
+ lsl offset, src2, #3
+ bic src2, src2, #0xf
+ mov mask, -1
+ neg neg_offset, offset
+ ldr data1, [src1], #8
+ ldp tmp1, tmp2, [src2], #16
+ LS_BK mask, mask, neg_offset
+ and neg_offset, neg_offset, #63 /* Need actual value for cmp later. */
+ /* Skip the first compare if data in tmp1 is irrelevant. */
+ tbnz offset, 6, L(misaligned_mid_loop)
+
+L(loop_misaligned):
+ /* STEP_A: Compare full 8 bytes when there is enough data from SRC2.*/
+ LS_FW data2, tmp1, offset
+ LS_BK tmp1, tmp2, neg_offset
+ subs limit, limit, #8
+ orr data2, data2, tmp1 /* 8 bytes from SRC2 combined from two regs.*/
+ sub has_nul, data1, zeroones
+ eor diff, data1, data2 /* Non-zero if differences found. */
+ orr tmp3, data1, #REP8_7f
+ csinv endloop, diff, xzr, hi /* If limit, set to all ones. */
+ bic has_nul, has_nul, tmp3 /* Non-zero if NUL byte found in SRC1. */
+ orr tmp3, endloop, has_nul
+ cbnz tmp3, L(full_check)
+
+ ldr data1, [src1], #8
+L(misaligned_mid_loop):
+ /* STEP_B: Compare first part of data1 to second part of tmp2. */
+ LS_FW data2, tmp2, offset
+#ifdef __AARCH64EB__
+ /* For big-endian we do a byte reverse to avoid carry-propagation
+ problem described above. This way we can reuse the has_nul in the
+ next step and also use syndrome value trick at the end. */
+ rev tmp3, data1
+ #define data1_fixed tmp3
+#else
+ #define data1_fixed data1
+#endif
+ sub has_nul, data1_fixed, zeroones
+ orr tmp3, data1_fixed, #REP8_7f
+ eor diff, data2, data1 /* Non-zero if differences found. */
+ bic has_nul, has_nul, tmp3 /* Non-zero if NUL terminator. */
+#ifdef __AARCH64EB__
+ rev has_nul, has_nul
+#endif
+ cmp limit, neg_offset, lsr #3
+ orr syndrome, diff, has_nul
+ bic syndrome, syndrome, mask /* Ignore later bytes. */
+ csinv tmp3, syndrome, xzr, hi /* If limit, set to all ones. */
+ cbnz tmp3, L(syndrome_check)
+
+ /* STEP_C: Compare second part of data1 to first part of tmp1. */
+ ldp tmp1, tmp2, [src2], #16
+ cmp limit, #8
+ LS_BK data2, tmp1, neg_offset
+ eor diff, data2, data1 /* Non-zero if differences found. */
+ orr syndrome, diff, has_nul
+ and syndrome, syndrome, mask /* Ignore earlier bytes. */
+ csinv tmp3, syndrome, xzr, hi /* If limit, set to all ones. */
+ cbnz tmp3, L(syndrome_check)
+
+ ldr data1, [src1], #8
+ sub limit, limit, #8
+ b L(loop_misaligned)
+
+#ifdef __AARCH64EB__
+L(syndrome_check):
+ clz pos, syndrome
+ cmp pos, limit, lsl #3
+ b.lo L(end_quick)
+#endif
+
+L(ret0):
+ mov result, #0
+ ret
+SYM_FUNC_END(__pi_strncmp)
+SYM_FUNC_ALIAS_WEAK(strncmp, __pi_strncmp)
+EXPORT_SYMBOL_NOKASAN(strncmp)