diff options
| -rw-r--r-- | benchtests/bench-dl-new-hash.c | 3 | ||||
| -rw-r--r-- | elf/simple-dl-new-hash.h (renamed from elf/dl-new-hash.h) | 20 | ||||
| -rw-r--r-- | elf/tst-dl-hash.c | 1 | ||||
| -rw-r--r-- | sysdeps/generic/dl-new-hash.h | 109 | ||||
| -rw-r--r-- | sysdeps/x86/dl-new-hash.h | 24 |
5 files changed, 144 insertions, 13 deletions
diff --git a/benchtests/bench-dl-new-hash.c b/benchtests/bench-dl-new-hash.c index 3c8a1d5..040fa7c 100644 --- a/benchtests/bench-dl-new-hash.c +++ b/benchtests/bench-dl-new-hash.c @@ -16,7 +16,8 @@ License along with the GNU C Library; if not, see <https://www.gnu.org/licenses/>. */ -#include <elf/dl-new-hash.h> +#include <dl-new-hash.h> +#include <elf/simple-dl-new-hash.h> #define TEST_FUNC(x, y) _dl_new_hash (x) #define SIMPLE_TEST_FUNC(x, y) __simple_dl_new_hash (x) diff --git a/elf/dl-new-hash.h b/elf/simple-dl-new-hash.h index 8641bb4..1437b1b 100644 --- a/elf/dl-new-hash.h +++ b/elf/simple-dl-new-hash.h @@ -1,4 +1,4 @@ -/* _dl_new_hash for elf symbol lookup +/* __simple_dl_new_hash for testing true elf symbol lookup. Copyright (C) 2022 Free Software Foundation, Inc. This file is part of the GNU C Library. @@ -16,16 +16,16 @@ License along with the GNU C Library; if not, see <https://www.gnu.org/licenses/>. */ -#ifndef _DL_NEW_HASH_H -#define _DL_NEW_HASH_H 1 +#ifndef _SIMPLE_DL_NEW_HASH_H +#define _SIMPLE_DL_NEW_HASH_H 1 #include <stdint.h> -/* For __always_inline. */ -#include <sys/cdefs.h> -static __always_inline uint32_t +/* For testing/benchmarking purposes. Real implementation in + sysdeps/generic/dl-new-hash.h. */ +static uint32_t __attribute__ ((unused)) -_dl_new_hash (const char *s) +__simple_dl_new_hash (const char *s) { uint32_t h = 5381; for (unsigned char c = *s; c != '\0'; c = *++s) @@ -33,8 +33,4 @@ _dl_new_hash (const char *s) return h; } -/* For testing/benchmarking purposes. */ -#define __simple_dl_new_hash _dl_new_hash - - -#endif /* dl-new-hash.h */ +#endif /* simple-dl-new-hash.h */ diff --git a/elf/tst-dl-hash.c b/elf/tst-dl-hash.c index 8697eb7..b21766c 100644 --- a/elf/tst-dl-hash.c +++ b/elf/tst-dl-hash.c @@ -18,6 +18,7 @@ #include <simple-dl-hash.h> +#include <simple-dl-new-hash.h> #include <dl-hash.h> #include <dl-new-hash.h> #include <support/support.h> diff --git a/sysdeps/generic/dl-new-hash.h b/sysdeps/generic/dl-new-hash.h new file mode 100644 index 0000000..59bfb0e --- /dev/null +++ b/sysdeps/generic/dl-new-hash.h @@ -0,0 +1,109 @@ +/* _dl_new_hash for elf symbol lookup + Copyright (C) 2022 Free Software Foundation, Inc. + This file is part of the GNU C Library. + + The GNU C Library is free software; you can redistribute it and/or + modify it under the terms of the GNU Lesser General Public + License as published by the Free Software Foundation; either + version 2.1 of the License, or (at your option) any later version. + + The GNU C Library is distributed in the hope that it will be useful, + but WITHOUT ANY WARRANTY; without even the implied warranty of + MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU + Lesser General Public License for more details. + + You should have received a copy of the GNU Lesser General Public + License along with the GNU C Library; if not, see + <https://www.gnu.org/licenses/>. */ + +#ifndef _DL_NEW_HASH_H +#define _DL_NEW_HASH_H 1 + +#include <stdint.h> +/* For __always_inline and __glibc_unlikely. */ +#include <sys/cdefs.h> + +/* The simplest implementation of _dl_new_hash is: + + _dl_new_hash (const char *s) + { + uint32_t h = 5381; + for (unsigned char c = *s; c != '\0'; c = *++s) + h = h * 33 + c; + return h; + } + + We can get better performance by slightly unrolling the loop to + pipeline the multiples, which gcc cannot easily do due to + dependencies across iterations. + + As well, as an architecture specific option we add asm statements + to explicitly specify order of operations and prevent reassociation + of instructions that lengthens the loop carried dependency. This + may have no affect as the compiler may have ordered instructions + the same way without it but in testing this has not been the case + for GCC. Improving GCC to reliably schedule instructions ideally + cannot be easily done. + + Architecture(s) that use the reassociation barriers are: + x86 + + Note it is very unlikely the reassociation barriers would + de-optimize performance on any architecture and with an imperfect + compiler it may help performance, especially on out-of-order cpus, + so it is suggested that the respective maintainers add them. + + Architecture maintainers are encouraged to benchmark this with + __asm_reassociation_barrier defined to __asm__ like it is in x86. +*/ + + +#ifndef __asm_reassociation_barrier +# define __asm_reassociation_barrier(...) +#endif + +static __always_inline uint32_t +__attribute__ ((unused)) +_dl_new_hash (const char *str) +{ + const unsigned char *s = (const unsigned char *) str; + unsigned int h = 5381; + unsigned int c0, c1; + for (;;) + { + c0 = s[0]; + /* Since hashed string is normally not empty, this is unlikely on the + first iteration of the loop. */ + if (__glibc_unlikely (c0 == 0)) + return h; + + c1 = s[1]; + if (c1 == 0) + { + /* Ideal computational order is: + c0 += h; + h *= 32; + h += c0; */ + c0 += h; + __asm_reassociation_barrier("" : "+r"(h) : "r"(c0)); + h = h * 32 + c0; + return h; + } + + /* Ideal computational order is: + c1 += c0; + h *= 33 * 33; + c0 *= 32; + c1 += c0; + h += c1; */ + c1 += c0; + __asm_reassociation_barrier("" : "+r"(c1), "+r"(c0)); + h *= 33 * 33; + c1 += c0 * 32; + __asm_reassociation_barrier("" : "+r"(c1)); + h += c1; + s += 2; + } +} + +#endif /* dl-new-hash.h */ diff --git a/sysdeps/x86/dl-new-hash.h b/sysdeps/x86/dl-new-hash.h new file mode 100644 index 0000000..ce8fb5a --- /dev/null +++ b/sysdeps/x86/dl-new-hash.h @@ -0,0 +1,24 @@ +/* _dl_new_hash for elf symbol lookup + Copyright (C) 2022 Free Software Foundation, Inc. + This file is part of the GNU C Library. + + The GNU C Library is free software; you can redistribute it and/or + modify it under the terms of the GNU Lesser General Public + License as published by the Free Software Foundation; either + version 2.1 of the License, or (at your option) any later version. + + The GNU C Library is distributed in the hope that it will be useful, + but WITHOUT ANY WARRANTY; without even the implied warranty of + MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU + Lesser General Public License for more details. + + You should have received a copy of the GNU Lesser General Public + License along with the GNU C Library; if not, see + <https://www.gnu.org/licenses/>. */ + +#ifdef __asm_reassociation_barrier +# error "__asm_reassociation_barrier should never already be defined." +#endif + +#define __asm_reassociation_barrier __asm__ +#include <sysdeps/generic/dl-new-hash.h> |