[llvm-dev] Issues with omp simd

[hameeza ahmed via llvm-dev]

i changed my code to following;
   #pragma omp simd
        for (int i=0; i<size; ++i)
        {
             a[i]=2; b[i]=3; c[i]=4;
            c[i]= a[i] + b[i];
    printf("c value %f",c[i]);
        }

still no effect of omp simd?

On Sun, Dec 31, 2017 at 12:26 AM, Craig Topper <craig.topper at gmail.com>
wrote:

> The for loop has no effect on the observable behavior of the program. If
> you run the program there is no way to tell if the numbers were added or
> not. So the loop doesn’t matter, so the compiler deleted the code. You need
> to print the contents of c or something so that it matters.
>
>
>
> On Sat, Dec 30, 2017 at 11:19 AM hameeza ahmed <hahmed2305 at gmail.com>
> wrote:
>
>> I even tried following;
>>
>> int main(int argc, char **argv)
>> {
>>     const int size = 1000000;
>>
>>
>>     float a[size], b[size],c[size];
>>
>>
>>
>>         #pragma omp simd
>>         for (int i=0; i<size; ++i)
>>         {
>>              a[i]=2; b[i]=3; c[i]=4;
>>             c[i]= a[i] + b[i];
>>         }
>>
>>
>>     return 0;
>> }
>> but the output with and without openmp simd is same. why is that so?
>>
>> On Sun, Dec 31, 2017 at 12:01 AM, Craig Topper <craig.topper at gmail.com>
>> wrote:
>>
>>> I didn’t look at the assembly, but you didn’t initialize your arrays.
>>> Your program isn’t meaningful.
>>>
>>> On Sat, Dec 30, 2017 at 9:08 AM hameeza ahmed <hahmed2305 at gmail.com>
>>> wrote:
>>>
>>>> hello,
>>>>
>>>> i am trying to optimize omp simd loop as follows
>>>>
>>>> int main(int argc, char **argv)
>>>> {
>>>>     const int size = 1000000;
>>>>
>>>>
>>>>     float a[size], b[size],c[size];
>>>>
>>>>
>>>>
>>>>         #pragma omp simd
>>>>         for (int i=0; i<size; ++i)
>>>>         {
>>>>             c[i]= a[i] + b[i];
>>>>         }
>>>>
>>>>
>>>>     return 0;
>>>> }
>>>>
>>>> i run it using the following command;
>>>>
>>>> g++ -O0 --std=c++14 -fopenmp-simd lab.cpp  -Iinclude -S -o lab.s
>>>>
>>>> when i compared it with code as follows without simd
>>>>
>>>>
>>>> int main(int argc, char **argv)
>>>> {
>>>>     const int size = 1000000;
>>>>
>>>>
>>>>     float a[size], b[size],c[size];
>>>>
>>>>
>>>>
>>>>         for (int i=0; i<size; ++i)
>>>>         {
>>>>             c[i]= a[i] + b[i];
>>>>         }
>>>>
>>>>
>>>>     return 0;
>>>> }
>>>>
>>>> using following command
>>>>
>>>> g++ -O0 --std=c++14  lab.cpp -Iinclude -S -o lab.s
>>>>
>>>> I am getting the same assembly. Why is that so? why not omp simd is
>>>> more efficient?
>>>>
>>>> Please help
>>>>
>>>>
>>>> --
>>> ~Craig
>>>
>>
>> --
> ~Craig
>
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