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+/* _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 */