Messages in this thread | | | Date | Mon, 25 Aug 2014 14:58:37 +0200 | From | Daniel Borkmann <> | Subject | Re: [PATCH/RFC] hash: Let gcc decide how to multiply |
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On 08/25/2014 02:13 PM, Rasmus Villemoes wrote: > 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.
Seems fine by me, but Cc'ing a couple of others (as those you have Cc'ed haven't written that code :)). You might want to let your changes go via Andrew's tree, too, perhaps ...
> 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@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); >
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