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

[Mehdi Amini via llvm-dev]

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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