On 07/12/2013 00:13, Jarno Rajahalme wrote:
On Dec 6, 2013, at 1:18 AM, Alexander Wu <alexander...@huawei.com> wrote:
Hi Jarno,
I've read your patch "better count1_bits", and I test the gcc
builtins separately.
Call __builtin_popcount|__builtin_popcountl|__builtin_popcountll 10 million
times
--------------------------------------
suse-kvm-of13:/test # time ./bit4
real 0m0.034s
user 0m0.032s
sys 0m0.000s
Call count1_bits 10 million times
--------------------------------------
suse-kvm-of13:/test # time ./bit1
real 0m0.080s
user 0m0.076s
sys 0m0.000s
Looks good, but I've a problem below.
My cpuinfo: 16U * Intel(R) Xeon(R) CPU E5620 @ 2.40GHz. (westmere)
I've read gcc source, find M_INTEL_COREI7_WESTMERE, it seems
to say westmere is corei7, but the following code doesn't work:
#if defined(__corei7)
int i;
for (i = 0; i < 10000000; i++)
__builtin_popcount(i);
#endif
You need to tell gcc to compile for your processor:
$ echo | gcc -dM -E - | grep core
$ echo | gcc -march=native -dM -E - | grep core
#define __corei7 1
#define __tune_corei7__ 1
#define __corei7__ 1
$
Also, you need to be careful to both allow the compiler to optimize as we do
with building OVS (-O2), but make sure the test cases are not optimized away.
I believe there're some particuler cpus which the buildin_popcount
is suitable for, any way to represent them?
I think it is trial and error, since the builtin popcount is kind of bad
without direct CPU support.
On 06/12/2013 12:26, Ben Pfaff wrote:
But I'm inclined to believe that a 65536-byte array wastes too much
memory.
I’m inclined to agree that it might waste too much (L1 cache) memory.
Jarno
Hi Jarno,
I get my gcc predefined __core2. But its performance seems to be worse when
I add '-O2'. Not sure if it's the reality.
Here are part of my test code, compile command and its result.
Code:
uint32_t i, last_bits;
struct timespec start = {0};
struct timespec end = {0};
srand(time(NULL));
int r = rand();
#define N_LOOP 100000
int random_array[N_LOOP];
srand(time(NULL));
for (i = 0; i < N_LOOP; i++) {
r = rand();
random_array[i] = r;
}
//__builtin_popcount
clock_gettime(CLOCK_THREAD_CPUTIME_ID, &start);
for (i = 0; i < N_LOOP; i++) {
last_bits = __builtin_popcount(random_array[i]);
}
clock_gettime(CLOCK_THREAD_CPUTIME_ID, &end);
printf("time-diff:%ld\n", end.tv_nsec - start.tv_nsec);
printf("last-bits:%d\n", last_bits);
//original ovs count_1bits_32
clock_gettime(CLOCK_THREAD_CPUTIME_ID, &start);
for (i = 0; i < N_LOOP; i++) {
last_bits = count_1bits_32(random_array[i]);
}
clock_gettime(CLOCK_THREAD_CPUTIME_ID, &end);
printf("time-diff:%ld\n", end.tv_nsec - start.tv_nsec);
printf("last-bits:%d\n", last_bits);
//simple foo function, to count '=' and function time.
clock_gettime(CLOCK_THREAD_CPUTIME_ID, &start);
for (i = 0; i < N_LOOP; i++) {
last_bits = foo();
}
clock_gettime(CLOCK_THREAD_CPUTIME_ID, &end);
printf("time-diff:%ld\n", end.tv_nsec - start.tv_nsec);
printf("last-bits:%d\n", last_bits);
Compile:
gcc bit1.c -o bit1 -march=native -mtune=native -lrt -O2 && ./bit1
Result:
time-diff:1063893 //__builtin_popcount
last-bits:10
time-diff:293463 //original ovs count_1bits_32
last-bits:10
time-diff:188 //simple foo function, to count '=' and function
time.(maybe it has been optimized out)
last-bits:99999
Result without -O2:
time-diff:1317450
last-bits:10
time-diff:991438
last-bits:10
time-diff:416265
last-bits:99999
Note I use last_bits to restore the return value, and when I use it,
performance of __builtin_popcount seems to decrease, I guess compiler
optimize __builtin_popcount as its wish like -O2.
So do you think it's enough to represent __builtin_popcount is not
suitable for __core2?
Best regards,
Alexander Wu
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