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>

1 files changed, 109 insertions, 0 deletions

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 */