Improve strncmp for mutually misaligned inputs
The mutually misaligned inputs on aarch64 are compared with a simple byte copy, which is not very efficient. Enhance the comparison similar to strcmp by loading a double-word at a time. The peak performance improvement (i.e. 4k maxlen comparisons) due to this on the strncmp microbenchmark in glibc is as follows: falkor: 3.5x (up to 72% time reduction) cortex-a73: 3.5x (up to 71% time reduction) cortex-a53: 3.5x (up to 71% time reduction) All mutually misaligned inputs from 16 bytes maxlen onwards show upwards of 15% improvement and there is no measurable effect on the performance of aligned/mutually aligned inputs.
This commit is contained in:
Siddhesh Poyarekar
Corinna Vinschen
parent
5ed8f2354f
commit
f44eee8f1b
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@ -1,4 +1,4 @@
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/* Copyright (c) 2013, Linaro Limited
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/* Copyright (c) 2013, 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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@ -67,6 +67,7 @@
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#define limit_wd x13
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#define mask x14
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#define endloop x15
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#define count mask
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.text
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.p2align 6
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@ -78,9 +79,9 @@ def_fn strncmp
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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 .Lmisaligned8
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ands tmp1, src1, #7
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b.ne .Lmutual_align
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cbnz count, .Lmutual_align
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/* Calculate the number of full and partial words -1. */
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sub limit_wd, limit, #1 /* limit != 0, so no underflow. */
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lsr limit_wd, limit_wd, #3 /* Convert to Dwords. */
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@ -185,44 +186,105 @@ def_fn strncmp
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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, tmp1, lsl #3 /* 64 - bits(bytes beyond align). */
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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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sub limit_wd, limit, #1 /* limit != 0, so no underflow. */
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#ifdef __AARCH64EB__
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/* Big-endian. Early bytes are at MSB. */
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lsl tmp2, tmp2, tmp3 /* Shift (tmp1 & 63). */
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lsl tmp2, tmp2, tmp3 /* Shift (count & 63). */
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#else
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/* Little-endian. Early bytes are at LSB. */
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lsr tmp2, tmp2, tmp3 /* Shift (tmp1 & 63). */
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lsr tmp2, tmp2, tmp3 /* Shift (count & 63). */
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#endif
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and tmp3, limit_wd, #7
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lsr limit_wd, limit_wd, #3
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/* Adjust the limit. Only low 3 bits used, so overflow irrelevant. */
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add limit, limit, tmp1
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add tmp3, tmp3, tmp1
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add limit, limit, count
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add tmp3, tmp3, count
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orr data1, data1, tmp2
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orr data2, data2, tmp2
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add limit_wd, limit_wd, tmp3, lsr #3
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b .Lstart_realigned
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.Lret0:
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mov result, #0
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ret
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.p2align 6
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/* Don't bother with dwords for up to 16 bytes. */
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.Lmisaligned8:
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sub limit, limit, #1
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1:
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cmp limit, #16
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b.hs .Ltry_misaligned_words
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.Lbyte_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, cs /* NZCV = 0b0000. */
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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 1b
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b.eq .Lbyte_loop
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.Ldone:
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sub result, data1, data2
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ret
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.size strncmp, . - strncmp
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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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.Ltry_misaligned_words:
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lsr limit_wd, limit, #3
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cbz count, .Ldo_misaligned
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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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lsr limit_wd, limit, #3
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.Lpage_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 .Ldone
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subs count, count, #1
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b.hi .Lpage_end_loop
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.Ldo_misaligned:
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/* Prepare ourselves for the next page crossing. Unlike the aligned
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loop, we fetch 1 less dword because we risk crossing bounds on
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SRC2. */
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mov count, #8
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subs limit_wd, limit_wd, #1
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b.lo .Ldone_loop
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.Lloop_misaligned:
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and tmp2, src2, #0xff8
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eor tmp2, tmp2, #0xff8
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cbz tmp2, .Lpage_end_loop
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ldr data1, [src1], #8
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ldr data2, [src2], #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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bics has_nul, tmp1, tmp2 /* Non-zero if NUL terminator. */
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ccmp diff, #0, #0, eq
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b.ne .Lnot_limit
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subs limit_wd, limit_wd, #1
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b.pl .Lloop_misaligned
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.Ldone_loop:
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/* We found a difference or a NULL before the limit was reached. */
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and limit, limit, #7
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cbz limit, .Lnot_limit
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/* Read the last word. */
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sub src1, src1, 8
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sub src2, src2, 8
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ldr data1, [src1, limit]
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ldr data2, [src2, limit]
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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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bics has_nul, tmp1, tmp2 /* Non-zero if NUL terminator. */
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ccmp diff, #0, #0, eq
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b.ne .Lnot_limit
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.Lret0:
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mov result, #0
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ret
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.size strncmp, . - strncmp
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#endif
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