311 lines
9.0 KiB
ArmAsm
311 lines
9.0 KiB
ArmAsm
/* SPDX-License-Identifier: GPL-2.0-only */
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/*
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* Copyright (c) 2013-2022, Arm Limited.
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*
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* Adapted from the original at:
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* https://github.com/ARM-software/optimized-routines/blob/189dfefe37d54c5b/string/aarch64/strncmp.S
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*/
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#include <linux/linkage.h>
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#include <asm/assembler.h>
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/* Assumptions:
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*
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* ARMv8-a, AArch64.
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* MTE compatible.
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*/
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#define L(label) .L ## label
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#define REP8_01 0x0101010101010101
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#define REP8_7f 0x7f7f7f7f7f7f7f7f
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/* Parameters and result. */
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#define src1 x0
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#define src2 x1
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#define limit x2
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#define result x0
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/* Internal variables. */
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#define data1 x3
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#define data1w w3
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#define data2 x4
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#define data2w w4
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#define has_nul x5
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#define diff x6
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#define syndrome x7
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#define tmp1 x8
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#define tmp2 x9
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#define tmp3 x10
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#define zeroones x11
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#define pos x12
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#define mask x13
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#define endloop x14
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#define count mask
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#define offset pos
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#define neg_offset x15
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/* Define endian dependent shift operations.
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On big-endian early bytes are at MSB and on little-endian LSB.
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LS_FW means shifting towards early bytes.
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LS_BK means shifting towards later bytes.
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*/
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#ifdef __AARCH64EB__
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#define LS_FW lsl
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#define LS_BK lsr
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#else
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#define LS_FW lsr
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#define LS_BK lsl
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#endif
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SYM_FUNC_START(__pi_strncmp)
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cbz limit, L(ret0)
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eor tmp1, src1, src2
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mov zeroones, #REP8_01
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tst tmp1, #7
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and count, src1, #7
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b.ne L(misaligned8)
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cbnz count, L(mutual_align)
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/* NUL detection works on the principle that (X - 1) & (~X) & 0x80
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(=> (X - 1) & ~(X | 0x7f)) is non-zero iff a byte is zero, and
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can be done in parallel across the entire word. */
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.p2align 4
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L(loop_aligned):
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ldr data1, [src1], #8
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ldr data2, [src2], #8
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L(start_realigned):
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subs limit, limit, #8
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sub tmp1, data1, zeroones
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orr tmp2, data1, #REP8_7f
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eor diff, data1, data2 /* Non-zero if differences found. */
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csinv endloop, diff, xzr, hi /* Last Dword or differences. */
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bics has_nul, tmp1, tmp2 /* Non-zero if NUL terminator. */
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ccmp endloop, #0, #0, eq
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b.eq L(loop_aligned)
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/* End of main loop */
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L(full_check):
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#ifndef __AARCH64EB__
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orr syndrome, diff, has_nul
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add limit, limit, 8 /* Rewind limit to before last subs. */
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L(syndrome_check):
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/* Limit was reached. Check if the NUL byte or the difference
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is before the limit. */
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rev syndrome, syndrome
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rev data1, data1
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clz pos, syndrome
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rev data2, data2
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lsl data1, data1, pos
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cmp limit, pos, lsr #3
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lsl data2, data2, pos
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/* But we need to zero-extend (char is unsigned) the value and then
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perform a signed 32-bit subtraction. */
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lsr data1, data1, #56
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sub result, data1, data2, lsr #56
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csel result, result, xzr, hi
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ret
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#else
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/* Not reached the limit, must have found the end or a diff. */
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tbz limit, #63, L(not_limit)
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add tmp1, limit, 8
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cbz limit, L(not_limit)
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lsl limit, tmp1, #3 /* Bits -> bytes. */
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mov mask, #~0
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lsr mask, mask, limit
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bic data1, data1, mask
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bic data2, data2, mask
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/* Make sure that the NUL byte is marked in the syndrome. */
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orr has_nul, has_nul, mask
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L(not_limit):
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/* For big-endian we cannot use the trick with the syndrome value
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as carry-propagation can corrupt the upper bits if the trailing
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bytes in the string contain 0x01. */
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/* However, if there is no NUL byte in the dword, we can generate
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the result directly. We can't just subtract the bytes as the
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MSB might be significant. */
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cbnz has_nul, 1f
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cmp data1, data2
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cset result, ne
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cneg result, result, lo
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ret
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1:
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/* Re-compute the NUL-byte detection, using a byte-reversed value. */
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rev tmp3, data1
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sub tmp1, tmp3, zeroones
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orr tmp2, tmp3, #REP8_7f
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bic has_nul, tmp1, tmp2
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rev has_nul, has_nul
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orr syndrome, diff, has_nul
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clz pos, syndrome
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/* The most-significant-non-zero bit of the syndrome marks either the
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first bit that is different, or the top bit of the first zero byte.
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Shifting left now will bring the critical information into the
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top bits. */
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L(end_quick):
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lsl data1, data1, pos
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lsl data2, data2, pos
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/* But we need to zero-extend (char is unsigned) the value and then
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perform a signed 32-bit subtraction. */
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lsr data1, data1, #56
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sub result, data1, data2, lsr #56
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ret
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#endif
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L(mutual_align):
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/* Sources are mutually aligned, but are not currently at an
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alignment boundary. Round down the addresses and then mask off
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the bytes that precede the start point.
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We also need to adjust the limit calculations, but without
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overflowing if the limit is near ULONG_MAX. */
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bic src1, src1, #7
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bic src2, src2, #7
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ldr data1, [src1], #8
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neg tmp3, count, lsl #3 /* 64 - bits(bytes beyond align). */
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ldr data2, [src2], #8
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mov tmp2, #~0
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LS_FW tmp2, tmp2, tmp3 /* Shift (count & 63). */
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/* Adjust the limit and ensure it doesn't overflow. */
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adds limit, limit, count
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csinv limit, limit, xzr, lo
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orr data1, data1, tmp2
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orr data2, data2, tmp2
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b L(start_realigned)
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.p2align 4
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/* Don't bother with dwords for up to 16 bytes. */
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L(misaligned8):
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cmp limit, #16
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b.hs L(try_misaligned_words)
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L(byte_loop):
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/* Perhaps we can do better than this. */
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ldrb data1w, [src1], #1
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ldrb data2w, [src2], #1
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subs limit, limit, #1
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ccmp data1w, #1, #0, hi /* NZCV = 0b0000. */
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ccmp data1w, data2w, #0, cs /* NZCV = 0b0000. */
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b.eq L(byte_loop)
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L(done):
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sub result, data1, data2
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ret
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/* Align the SRC1 to a dword by doing a bytewise compare and then do
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the dword loop. */
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L(try_misaligned_words):
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cbz count, L(src1_aligned)
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neg count, count
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and count, count, #7
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sub limit, limit, count
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L(page_end_loop):
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ldrb data1w, [src1], #1
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ldrb data2w, [src2], #1
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cmp data1w, #1
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ccmp data1w, data2w, #0, cs /* NZCV = 0b0000. */
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b.ne L(done)
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subs count, count, #1
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b.hi L(page_end_loop)
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/* The following diagram explains the comparison of misaligned strings.
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The bytes are shown in natural order. For little-endian, it is
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reversed in the registers. The "x" bytes are before the string.
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The "|" separates data that is loaded at one time.
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src1 | a a a a a a a a | b b b c c c c c | . . .
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src2 | x x x x x a a a a a a a a b b b | c c c c c . . .
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After shifting in each step, the data looks like this:
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STEP_A STEP_B STEP_C
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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
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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
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The bytes with "0" are eliminated from the syndrome via mask.
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Align SRC2 down to 16 bytes. This way we can read 16 bytes at a
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time from SRC2. The comparison happens in 3 steps. After each step
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the loop can exit, or read from SRC1 or SRC2. */
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L(src1_aligned):
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/* Calculate offset from 8 byte alignment to string start in bits. No
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need to mask offset since shifts are ignoring upper bits. */
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lsl offset, src2, #3
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bic src2, src2, #0xf
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mov mask, -1
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neg neg_offset, offset
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ldr data1, [src1], #8
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ldp tmp1, tmp2, [src2], #16
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LS_BK mask, mask, neg_offset
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and neg_offset, neg_offset, #63 /* Need actual value for cmp later. */
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/* Skip the first compare if data in tmp1 is irrelevant. */
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tbnz offset, 6, L(misaligned_mid_loop)
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L(loop_misaligned):
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/* STEP_A: Compare full 8 bytes when there is enough data from SRC2.*/
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LS_FW data2, tmp1, offset
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LS_BK tmp1, tmp2, neg_offset
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subs limit, limit, #8
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orr data2, data2, tmp1 /* 8 bytes from SRC2 combined from two regs.*/
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sub has_nul, data1, zeroones
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eor diff, data1, data2 /* Non-zero if differences found. */
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orr tmp3, data1, #REP8_7f
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csinv endloop, diff, xzr, hi /* If limit, set to all ones. */
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bic has_nul, has_nul, tmp3 /* Non-zero if NUL byte found in SRC1. */
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orr tmp3, endloop, has_nul
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cbnz tmp3, L(full_check)
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ldr data1, [src1], #8
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L(misaligned_mid_loop):
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/* STEP_B: Compare first part of data1 to second part of tmp2. */
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LS_FW data2, tmp2, offset
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#ifdef __AARCH64EB__
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/* For big-endian we do a byte reverse to avoid carry-propagation
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problem described above. This way we can reuse the has_nul in the
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next step and also use syndrome value trick at the end. */
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rev tmp3, data1
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#define data1_fixed tmp3
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#else
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#define data1_fixed data1
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#endif
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sub has_nul, data1_fixed, zeroones
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orr tmp3, data1_fixed, #REP8_7f
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eor diff, data2, data1 /* Non-zero if differences found. */
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bic has_nul, has_nul, tmp3 /* Non-zero if NUL terminator. */
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#ifdef __AARCH64EB__
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rev has_nul, has_nul
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#endif
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cmp limit, neg_offset, lsr #3
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orr syndrome, diff, has_nul
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bic syndrome, syndrome, mask /* Ignore later bytes. */
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csinv tmp3, syndrome, xzr, hi /* If limit, set to all ones. */
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cbnz tmp3, L(syndrome_check)
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/* STEP_C: Compare second part of data1 to first part of tmp1. */
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ldp tmp1, tmp2, [src2], #16
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cmp limit, #8
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LS_BK data2, tmp1, neg_offset
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eor diff, data2, data1 /* Non-zero if differences found. */
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orr syndrome, diff, has_nul
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and syndrome, syndrome, mask /* Ignore earlier bytes. */
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csinv tmp3, syndrome, xzr, hi /* If limit, set to all ones. */
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cbnz tmp3, L(syndrome_check)
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ldr data1, [src1], #8
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sub limit, limit, #8
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b L(loop_misaligned)
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#ifdef __AARCH64EB__
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L(syndrome_check):
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clz pos, syndrome
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cmp pos, limit, lsl #3
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b.lo L(end_quick)
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#endif
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L(ret0):
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mov result, #0
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ret
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SYM_FUNC_END(__pi_strncmp)
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SYM_FUNC_ALIAS_WEAK(strncmp, __pi_strncmp)
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EXPORT_SYMBOL_NOKASAN(strncmp)
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