[PATCH/RFC] hash: Let gcc decide how to multiply

[Rasmus Villemoes]

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 <linux@xxxxxxxxxxxxxxxxxx>
---
 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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