public inbox for glibc-cvs@sourceware.org
help / color / mirror / Atom feed
* [glibc] elf: Optimize _dl_new_hash in dl-new-hash.h
@ 2022-05-23 16:23 Noah Goldstein
0 siblings, 0 replies; only message in thread
From: Noah Goldstein @ 2022-05-23 16:23 UTC (permalink / raw)
To: glibc-cvs
https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=9a421348cd7d0704663e26e6171828bed6e0a2cf
commit 9a421348cd7d0704663e26e6171828bed6e0a2cf
Author: Noah Goldstein <goldstein.w.n@gmail.com>
Date: Thu May 19 17:18:03 2022 -0500
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>
Diff:
---
benchtests/bench-dl-new-hash.c | 3 +-
elf/{dl-new-hash.h => simple-dl-new-hash.h} | 20 ++---
elf/tst-dl-hash.c | 1 +
sysdeps/generic/dl-new-hash.h | 109 ++++++++++++++++++++++++++++
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 3c8a1d5a82..040fa7ce01 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
similarity index 75%
rename from elf/dl-new-hash.h
rename to elf/simple-dl-new-hash.h
index 8641bb4196..1437b1bd36 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 8697eb73a0..b21766c63d 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 0000000000..59bfb0e1de
--- /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 0000000000..ce8fb5a838
--- /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>
^ permalink raw reply [flat|nested] only message in thread
only message in thread, other threads:[~2022-05-23 16:23 UTC | newest]
Thread overview: (only message) (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2022-05-23 16:23 [glibc] elf: Optimize _dl_new_hash in dl-new-hash.h Noah Goldstein
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for read-only IMAP folder(s) and NNTP newsgroup(s).