Hi Ruoyao,
I see. Thanks very much for your time and detailed explanation!
Best Regards
Nan Xiao
On Tue, Nov 21, 2017 at 4:56 PM, Xi Ruoyao <ryxi@xxxxxxxxxxxxxxxxx> wrote:
> On 2017-11-21 16:50 +0800, Xi Ruoyao wrote:
>> On 2017-11-21 15:49 +0800, Nan Xiao wrote:
>> >
>> > #include <omp.h>
>> > #include <stdio.h>
>> >
>> > int main(void) {
>> > #pragma omp parallel for
>> > for (auto i = 0; i < 10; i++) {
>> > int sum = 0;
>> > #pragma omp taskloop shared(sum)
>> > for (auto j = 0; j < 1000000; j++) {
>> > sum += j;
>> > }
>> > printf("%d\n", sum);
>> > }
>> > return 0;
>> > }
>>
>> There are two bugs in your code. First, signed overflow is an undefined
>> behaviour and may generate arbitary result. Second, the access to shared
>> variable sum is racing, the result may vary with scheduling.
>
> Fix:
>
> #pragma omp parallel for
> for (auto i = 0; i < 10; i++) {
> long long sum = 0;
> #pragma omp taskloop shared(sum)
> for (auto j = 0; j < 1000000; j++) {
> __atomic_add_fetch(&sum, j, __ATOMIC_RELAXED);
> }
> printf("%lld\n", sum);
> }
>
> This would generate "lock addq" instruction for "sum", instead of loading
> it into a register.
> --
> Xi Ruoyao <ryxi@xxxxxxxxxxxxxxxxx>
> School of Aerospace Science and Technology, Xidian University
