[llvm-dev] Automatic Insertion of OpenACC/OpenMP directives

[Fernando Magno Quintao Pereira via llvm-dev]

Dear Mehdi,

    I've changed your example a little bit:

float saxpy(float a, float *x, float *y, int n) {
 int j = 0;
 for (int i = 0; i < n; ++i) {
   y[j] = a*x[i] + y[I]; // Change 'I' into 'j'?
   ++j;
 }
}

I get this code below, once I replace 'I' with 'j'. We are copying n
positions of both arrays, 'x' and 'y':

float saxpy(float a, float *x, float *y, int n) {
  int j = 0;

  long long int AI1[6];
  AI1[0] = n + -1;
  AI1[1] = 4 * AI1[0];
  AI1[2] = AI1[1] + 4;
  AI1[3] = AI1[2] / 4;
  AI1[4] = (AI1[3] > 0);
  AI1[5] = (AI1[4] ? AI1[3] : 0);
  #pragma acc data pcopy(x[0:AI1[5]],y[0:AI1[5]])
  #pragma acc kernels
  for (int i = 0; i < n; ++i) {
    y[j] = a * x[i] + y[j];
    ++j;
  }
}

Regards,

Fernando

On Sat, Dec 31, 2016 at 3:58 PM, Mehdi Amini <mehdi.amini at apple.com> wrote:
> Hi,
>
>> On Dec 31, 2016, at 8:33 AM, Fernando Magno Quintao Pereira via llvm-dev <llvm-dev at lists.llvm.org> wrote:
>>
>> Dear LLVMers,
>>
>>    we have released a tool that uses LLVM to insert OpenACC or OpenMP
>> 4.0 directives in programs. You can use the tool online here:
>> http://cuda.dcc.ufmg.br/dawn/. Our tool, dawn-cc, analyzes the LLVM IR
>> to infer the sizes of memory chunks, and to find dependences within
>> loops. After that, we use debug information to translate the low-level
>> information back into annotations that we insert into C/C++ programs.
>> For instance, if we take a program like this one below:
>>
>> void saxpy(float a, float *x, float *y, int n) {
>>  for (int i = 0; i < n; ++i)
>>    y[i] = a*x[i] + y[i];
>> }
>>
>> Then dawn-cc produces the code below:
>>
>> void saxpy(float a, float *x, float *y, int n) {
>>  long long int AI1[6];
>>  AI1[0] = n - 1;
>>  AI1[1] = 4 * AI1[0];
>>  AI1[2] = AI1[1] + 4;
>>  AI1[3] = AI1[2] / 4;
>>  AI1[4] = (AI1[3] > 0);
>>  AI1[5] = (AI1[4] ? AI1[3] : 0);
>>  #pragma acc data pcopy(x[0:AI1[5]],y[0:AI1[5]])
>>  #pragma acc kernels
>>  for (int i = 0; i < n; ++i)
>>    y[i] = a * x[i] + y[i];
>> }
>
> That’s nice!
>
> I’m wondering about how would you handle the issue that retro-fitting the pragma on the original code from the optimized can be fragile. For example if I use a different variable for indexing into the output array:
>
>
> float saxpy(float a, float *x, float *y, int n) {
>  int j = 0;
>
>  for (int i = 0; i < n; ++i) {
>    y[j] = a*x[i] + y[I];
>    ++j;
>  }
> }
>
> The optimized LLVM IR is likely identical to the original code, but simply adding the pragma wouldn’t be correct.
>
> —
> Mehdi
>
>
>
>>
>> I was wondering if we could add a link to dawn-cc in the LLVM's
>> project page (http://llvm.org/ProjectsWithLLVM/). There are a number
>> of papers that describe what dawn-cc does. The main publication is
>> this paper:
>>
>> * Automatic Insertion of Copy Annotation in Data-Parallel Programs -
>> SBAC-PAD 2016
>>
>> The array size inference analysis comes from this work:
>>
>> * Runtime Pointer Disambiguation - OOPSLA 2015
>>
>> The source code of dawn-cc, including all the static analyses, is available at:
>>
>> * https://github.com/gleisonsdm/DawnCC-Compiler
>>
>> And, as I've mentioned, you can try it through an online interface:
>>
>> * cuda.dcc.ufmg.br/dawn
>>
>> Feel free to report bugs, or send us questions.
>>
>> Fernando
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>