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721314c980
As indicated in a recent thread, this it is a simple brute-force
algorithm that checks the whole needle at a matching character pair
(and does so 1 byte at a time after the first 64 bytes of a needle).
Also it never skips ahead and thus can match at every haystack
position after trying to match all of the needle, which generic
implementation avoids.
As indicated by Wilco, a 4x larger needle and 16x larger haystack gives
a clear 65x slowdown both basic_strstr and __strstr_avx512:
"ifuncs": ["basic_strstr", "twoway_strstr", "__strstr_avx512",
"__strstr_sse2_unaligned", "__strstr_generic"],
{
"len_haystack": 65536,
"len_needle": 1024,
"align_haystack": 0,
"align_needle": 0,
"fail": 1,
"desc": "Difficult bruteforce needle",
"timings": [4.0948e+07, 15094.5, 3.20818e+07, 108558, 10839.2]
},
{
"len_haystack": 1048576,
"len_needle": 4096,
"align_haystack": 0,
"align_needle": 0,
"fail": 1,
"desc": "Difficult bruteforce needle",
"timings": [2.69767e+09, 100797, 2.08535e+09, 495706, 82666.9]
}
PS: I don't have an AVX512 capable machine to verify this issues, but
skimming through the code it does seems to follow what Wilco has
described.
Reviewed-by: Noah Goldstein <goldstein.w.n@gmail.com>
50 lines
1.8 KiB
C
50 lines
1.8 KiB
C
/* Multiple versions of strstr.
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All versions must be listed in ifunc-impl-list.c.
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Copyright (C) 2012-2024 Free Software Foundation, Inc.
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This file is part of the GNU C Library.
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The GNU C Library is free software; you can redistribute it and/or
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modify it under the terms of the GNU Lesser General Public
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License as published by the Free Software Foundation; either
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version 2.1 of the License, or (at your option) any later version.
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The GNU C Library is distributed in the hope that it will be useful,
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but WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
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Lesser General Public License for more details.
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You should have received a copy of the GNU Lesser General Public
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License along with the GNU C Library; if not, see
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<https://www.gnu.org/licenses/>. */
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/* Redefine strstr so that the compiler won't complain about the type
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mismatch with the IFUNC selector in strong_alias, below. */
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#undef strstr
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#define strstr __redirect_strstr
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#include <string.h>
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#undef strstr
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#define STRSTR __strstr_generic
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#ifdef SHARED
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# undef libc_hidden_builtin_def
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# define libc_hidden_builtin_def(name) \
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__hidden_ver1 (__strstr_generic, __GI_strstr, __strstr_generic);
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#endif
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#include "string/strstr.c"
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extern __typeof (__redirect_strstr) __strstr_sse2_unaligned attribute_hidden;
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extern __typeof (__redirect_strstr) __strstr_generic attribute_hidden;
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#include "init-arch.h"
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/* Avoid DWARF definition DIE on ifunc symbol so that GDB can handle
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ifunc symbol properly. */
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extern __typeof (__redirect_strstr) __libc_strstr;
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libc_ifunc (__libc_strstr,
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HAS_ARCH_FEATURE (Fast_Unaligned_Load)
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? __strstr_sse2_unaligned
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: __strstr_generic)
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#undef strstr
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strong_alias (__libc_strstr, strstr)
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