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.
> 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 += …”).
>
> 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
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