memcmp.S: optimize for medium to large sizes
This improved memcmp provides a fast path for compares up to 16 bytes and then compares 16 bytes at a time, thus optimizing loads from both sources. The glibc memcmp microbenchmark retains performance (with an error of ~1ns) for smaller compare sizes and reduces up to 31% of execution time for compares up to 4K on the APM Mustang. On Qualcomm Falkor this improves to almost 48%, i.e. it is almost 2x improvement for sizes of 2K and above.
This commit is contained in:
Siddhesh Poyarekar
Corinna Vinschen
parent
f44eee8f1b
commit
2d9f35c2cc
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/* memcmp - compare memory
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Copyright (c) 2018 Linaro Limited
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All rights reserved.
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Redistribution and use in source and binary forms, with or without
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modification, are permitted provided that the following conditions are met:
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* Redistributions of source code must retain the above copyright
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notice, this list of conditions and the following disclaimer.
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* Redistributions in binary form must reproduce the above copyright
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notice, this list of conditions and the following disclaimer in the
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documentation and/or other materials provided with the distribution.
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* Neither the name of the Linaro nor the
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names of its contributors may be used to endorse or promote products
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derived from this software without specific prior written permission.
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THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
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"AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
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LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
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A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
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HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
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SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
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LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
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DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
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THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
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(INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
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OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. */
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/*
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* Copyright (c) 2017 ARM Ltd
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* All rights reserved.
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@ -35,6 +63,8 @@
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* ARMv8-a, AArch64, unaligned accesses.
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*/
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#define L(l) .L ## l
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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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@ -44,9 +74,12 @@
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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 tmp1 x5
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#define data1h x4
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#define data2 x5
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#define data2w w5
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#define data2h x6
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#define tmp1 x7
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#define tmp2 x8
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.macro def_fn f p2align=0
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.text
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@ -56,83 +89,106 @@
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\f:
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.endm
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/* Small inputs of less than 8 bytes are handled separately. This allows the
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main code to be sped up using unaligned loads since there are now at least
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8 bytes to be compared. If the first 8 bytes are equal, align src1.
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This ensures each iteration does at most one unaligned access even if both
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src1 and src2 are unaligned, and mutually aligned inputs behave as if
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aligned. After the main loop, process the last 8 bytes using unaligned
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accesses. */
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def_fn memcmp p2align=6
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subs limit, limit, 8
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b.lo .Lless8
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b.lo L(less8)
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/* Limit >= 8, so check first 8 bytes using unaligned loads. */
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ldr data1, [src1], 8
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ldr data2, [src2], 8
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and tmp1, src1, 7
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add limit, limit, tmp1
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cmp data1, data2
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bne .Lreturn
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b.ne L(return)
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subs limit, limit, 8
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b.gt L(more16)
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ldr data1, [src1, limit]
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ldr data2, [src2, limit]
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b L(return)
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L(more16):
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ldr data1, [src1], 8
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ldr data2, [src2], 8
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cmp data1, data2
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bne L(return)
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/* Jump directly to comparing the last 16 bytes for 32 byte (or less)
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strings. */
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subs limit, limit, 16
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b.ls L(last_bytes)
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/* We overlap loads between 0-32 bytes at either side of SRC1 when we
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try to align, so limit it only to strings larger than 128 bytes. */
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cmp limit, 96
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b.ls L(loop16)
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/* Align src1 and adjust src2 with bytes not yet done. */
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and tmp1, src1, 15
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add limit, limit, tmp1
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sub src1, src1, tmp1
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sub src2, src2, tmp1
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subs limit, limit, 8
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b.ls .Llast_bytes
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/* Loop performing 8 bytes per iteration using aligned src1.
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Limit is pre-decremented by 8 and must be larger than zero.
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Exit if <= 8 bytes left to do or if the data is not equal. */
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/* Loop performing 16 bytes per iteration using aligned src1.
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Limit is pre-decremented by 16 and must be larger than zero.
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Exit if <= 16 bytes left to do or if the data is not equal. */
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.p2align 4
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.Lloop8:
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ldr data1, [src1], 8
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ldr data2, [src2], 8
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subs limit, limit, 8
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ccmp data1, data2, 0, hi /* NZCV = 0b0000. */
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b.eq .Lloop8
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L(loop16):
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ldp data1, data1h, [src1], 16
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ldp data2, data2h, [src2], 16
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subs limit, limit, 16
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ccmp data1, data2, 0, hi
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ccmp data1h, data2h, 0, eq
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b.eq L(loop16)
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cmp data1, data2
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bne .Lreturn
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bne L(return)
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mov data1, data1h
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mov data2, data2h
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cmp data1, data2
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bne L(return)
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/* Compare last 1-8 bytes using unaligned access. */
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.Llast_bytes:
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ldr data1, [src1, limit]
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ldr data2, [src2, limit]
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/* Compare last 1-16 bytes using unaligned access. */
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L(last_bytes):
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add src1, src1, limit
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add src2, src2, limit
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ldp data1, data1h, [src1]
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ldp data2, data2h, [src2]
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cmp data1, data2
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bne L(return)
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mov data1, data1h
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mov data2, data2h
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cmp data1, data2
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/* Compare data bytes and set return value to 0, -1 or 1. */
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.Lreturn:
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L(return):
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#ifndef __AARCH64EB__
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rev data1, data1
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rev data2, data2
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#endif
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cmp data1, data2
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.Lret_eq:
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L(ret_eq):
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cset result, ne
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cneg result, result, lo
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ret
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ret
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.p2align 4
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/* Compare up to 8 bytes. Limit is [-8..-1]. */
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.Lless8:
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L(less8):
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adds limit, limit, 4
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b.lo .Lless4
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b.lo L(less4)
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ldr data1w, [src1], 4
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ldr data2w, [src2], 4
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cmp data1w, data2w
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b.ne .Lreturn
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b.ne L(return)
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sub limit, limit, 4
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.Lless4:
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L(less4):
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adds limit, limit, 4
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beq .Lret_eq
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.Lbyte_loop:
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beq L(ret_eq)
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L(byte_loop):
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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, data2w, 0, ne /* NZCV = 0b0000. */
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b.eq .Lbyte_loop
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b.eq L(byte_loop)
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sub result, data1w, data2w
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ret
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