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elf: Optimize _dl_new_hash in dl-new-hash.h 

Unroll slightly and enforce good instruction scheduling. This improves
performance on out-of-order machines. The unrolling allows for
pipelined multiplies.

As well, as an optional sysdep, reorder the operations and prevent
reassosiation for better scheduling and higher ILP. This commit
only adds the barrier for x86, although it should be either no
change or a win for any architecture.

Unrolling further started to induce slowdowns for sizes [0, 4]
but can help the loop so if larger sizes are the target further
unrolling can be beneficial.

Results for _dl_new_hash
Benchmarked on Tigerlake: 11th Gen Intel(R) Core(TM) i7-1165G7 @ 2.80GHz

Time as Geometric Mean of N=30 runs
Geometric of all benchmark New / Old: 0.674
  type, length, New Time, Old Time, New Time / Old Time
 fixed,      0,    2.865,     2.72,               1.053
 fixed,      1,    3.567,    2.489,               1.433
 fixed,      2,    2.577,    3.649,               0.706
 fixed,      3,    3.644,    5.983,               0.609
 fixed,      4,    4.211,    6.833,               0.616
 fixed,      5,    4.741,    9.372,               0.506
 fixed,      6,    5.415,    9.561,               0.566
 fixed,      7,    6.649,   10.789,               0.616
 fixed,      8,    8.081,   11.808,               0.684
 fixed,      9,    8.427,   12.935,               0.651
 fixed,     10,    8.673,   14.134,               0.614
 fixed,     11,    10.69,   15.408,               0.694
 fixed,     12,   10.789,   16.982,               0.635
 fixed,     13,   12.169,   18.411,               0.661
 fixed,     14,   12.659,   19.914,               0.636
 fixed,     15,   13.526,   21.541,               0.628
 fixed,     16,   14.211,   23.088,               0.616
 fixed,     32,   29.412,   52.722,               0.558
 fixed,     64,    65.41,  142.351,               0.459
 fixed,    128,  138.505,  295.625,               0.469
 fixed,    256,  291.707,  601.983,               0.485
random,      2,   12.698,   12.849,               0.988
random,      4,   16.065,   15.857,               1.013
random,      8,   19.564,   21.105,               0.927
random,     16,   23.919,   26.823,               0.892
random,     32,   31.987,   39.591,               0.808
random,     64,   49.282,   71.487,               0.689
random,    128,    82.23,  145.364,               0.566
random,    256,  152.209,  298.434,                0.51

Co-authored-by: Alexander Monakov <amonakov@ispras.ru>
Reviewed-by: Siddhesh Poyarekar <siddhesh@sourceware.org>
This commit is contained in:
Noah Goldstein 2022-05-19 17:18:03 -05:00
parent 3d155d4b6c
commit 9a421348cd
5 changed files with 144 additions and 13 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

3
benchtests/bench-dl-new-hash.c
View File

@ -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)

20
elf/dl-new-hash.h → elf/simple-dl-new-hash.h
View File

@ -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 */

1
elf/tst-dl-hash.c
View File

@ -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>

109
sysdeps/generic/dl-new-hash.h Normal file
View File

@ -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 */

24
sysdeps/x86/dl-new-hash.h Normal file
View File

@ -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>