Re: [ovs-dev] [PATCH v4 2/5] bitmap: add bitmap_count1 function

[Alexander Wu]

On 10/12/2013 04:50, Jarno Rajahalme wrote:

On Dec 8, 2013, at 7:34 PM, Alexander Wu <alexander...@huawei.com> wrote:

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.

From the numbers below it seems that performance is better with -O2 (1063893 < 
1317450), so I’m not sure what you mean here.

I mean: (-O2)1063893/293463 > (no -O2)1317450/991438, so it seems worse.

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.

You could prevent optimizations by adding instead of simply assigning, (i.e., 
“last_bits += …”).

Thanks, it works.

So do you think it's enough to represent __builtin_popcount is not
suitable for __core2?

Seems so, and it also makes sense as Core2 does not have the popcnt instruction.

   Jarno

.

Thanks!

Best regards,
Alexander Wu


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