A 9+ years old comment in hash_64 says that gcc can't optimize
multiplication by GOLDEN_RATIO_PRIME_64. Well, compilers get smarter
and CPUs get faster all the time, so it is perhaps about time to
revisit that assumption.
A stupid micro-benchmark [3] on my x86_64 machine shows that letting
gcc generate the imul instruction is ~60% faster than the sequence of
shifts and add/sub. But that is cheating, since the load of the
constant is hoisted out of the loop. A slightly less stupid [1]
micro-benchmark still shows ~55% improvement over the current
version. So let the compiler do its job.
Also, this should reduce the instruction cache footprint of all
callers of the force-inlined hash_64. [2]
While at it, fix the suffixes of GOLDEN_RATIO_PRIME_{32,64} so that
their types are compatible with u32/u64 on all platforms (I'm not sure
what the compiler does on a 32-bit platform when encountering a
too-wide literal with an explicit UL suffix).
[1] It is stupid in another way, since my inline asm skills
suck. Still, I at least get to force the compiler to do the load on
every loop iteration.
[2] Well, it is an overall win: x86_64, defconfig, gcc 4.7.2:
$ scripts/bloat-o-meter /tmp/vmlinux-{master,hash}
add/remove: 0/1 grow/shrink: 17/44 up/down: 622/-2418 (-1796)
[3] Please don't laugh:
/*
$ gcc -Wall -O2 -o hashtest hashtest.c
$ ./hashtest
gcc_hash 2093320 12624
asm_hash 2093320 14264
kernel_hash 2093320 32076
$ echo $((100*12624/32076)), $((100*14264/32076))
39, 44
*/
#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>
#include <rdtsc.h>
#include <fcntl.h>
#include <unistd.h>
#define u64 uint64_t
#define GOLDEN_RATIO_PRIME_64 0x9e37fffffffc0001UL
#ifndef __always_inline
#define __always_inline __inline __attribute__ ((__always_inline__))
#endif
static __always_inline u64 kernel_hash(u64 val, unsigned int bits)
{
u64 hash = val;
/* Sigh, gcc can't optimise this alone like it does for 32 bits. */
u64 n = hash;
n <<= 18;
hash -= n;
n <<= 33;
hash -= n;
n <<= 3;
hash += n;
n <<= 3;
hash -= n;
n <<= 4;
hash += n;
n <<= 2;
hash += n;
/* High bits are more random, so use them. */
return hash >> (64 - bits);
}
static __always_inline u64 gcc_hash(u64 val, unsigned int bits)
{
u64 hash = val * GOLDEN_RATIO_PRIME_64;
return hash >> (64 - bits);
}
static __always_inline u64 asm_hash(u64 val, unsigned int bits)
{
u64 hash;
__asm__("mov %1, %%rax\n\t"
"movabs $0x9e37fffffffc0001,%%rdx\n\t"
"imul %%rdx,%%rax\n\t"
"mov %%rax, %0"
: "=r"(hash)
:"r"(val)
: "%rax", "%rdx");
return hash >> (64 - bits);
}
/* I have 32 KiB of L1 data cache. */
#define N ((1<<15)/sizeof(u64))
#define NBITS 10 /* doesn't seem to affect the outcome */
int main(void)
{
unsigned long start, stop;
u64 buf[N];
int fd = open("/dev/urandom", O_RDONLY);
u64 sum;
int i;
if (fd < 0)
exit(1);
if (read(fd, buf, sizeof(buf)) != sizeof(buf))
exit(2);
close(fd);
#define TEST(f) do { \
sum = 0; \
start = rdtsc(); \
for (i = 0; i < N; ++i) \
sum += f(buf[i], NBITS); \
stop = rdtsc(); \
printf("%s\t%lu\t%lu\n", #f, sum, stop-start); \
} while (0)
TEST(gcc_hash);
TEST(asm_hash);
TEST(kernel_hash);
return 0;
}
Signed-off-by: Rasmus Villemoes <li...@rasmusvillemoes.dk>
---
include/linux/hash.h | 21 +++------------------
1 file changed, 3 insertions(+), 18 deletions(-)
diff --git a/include/linux/hash.h b/include/linux/hash.h
index bd1754c..6a0879a 100644
--- a/include/linux/hash.h
+++ b/include/linux/hash.h
@@ -19,9 +19,9 @@
#include <linux/compiler.h>
/* 2^31 + 2^29 - 2^25 + 2^22 - 2^19 - 2^16 + 1 */
-#define GOLDEN_RATIO_PRIME_32 0x9e370001UL
+#define GOLDEN_RATIO_PRIME_32 0x9e370001U
/* 2^63 + 2^61 - 2^57 + 2^54 - 2^51 - 2^18 + 1 */
-#define GOLDEN_RATIO_PRIME_64 0x9e37fffffffc0001UL
+#define GOLDEN_RATIO_PRIME_64 0x9e37fffffffc0001ULL
#if BITS_PER_LONG == 32
#define GOLDEN_RATIO_PRIME GOLDEN_RATIO_PRIME_32
@@ -35,22 +35,7 @@
static __always_inline u64 hash_64(u64 val, unsigned int bits)
{
- u64 hash = val;
-
- /* Sigh, gcc can't optimise this alone like it does for 32 bits. */
- u64 n = hash;
- n <<= 18;
- hash -= n;
- n <<= 33;
- hash -= n;
- n <<= 3;
- hash += n;
- n <<= 3;
- hash -= n;
- n <<= 4;
- hash += n;
- n <<= 2;
- hash += n;
+ u64 hash = val * GOLDEN_RATIO_PRIME_64;
/* High bits are more random, so use them. */
return hash >> (64 - bits);
--
2.0.4
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