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Features

This is a clang plugin that can parse programs with OpenMP directives and generate Json files containing a description about loops.

A. Overview

This tool is an implementation of an external clang plugin. It can parse the source code and extract information about loops with OpenMP directives into Json files.

B. Building the OpenMP Extractor

First, please clone the source code:

git clone https://github.com/gleisonsdm/OpenMP-Extractor.git

Create OpenMP-Extractor build folder.

mkdir OpenMP-Extractor/lib

Enter OpenMP-Extractor build folder.

cd OpenMP-Extractor/lib

Create a makefile using cmake.

CXX=g++ cmake -DLLVM_DIR=${LLVM_INSTALL}/lib/cmake/llvm $HOME/OpenMP-Extractor/clangPlugin/

Then the binaries of this library will be installed to $HOME/OpenMP-Extractor/lib.

make -j4

C. Example Usage

We provide an example file for testing this plugin. Feel free to try other programs.

cat << EOF > test.c
#include <stdlib.h>
#include <stdio.h>
int main (void)
{
  int sum=0;
  #pragma omp parallel for reduction(+:sum)
  for (int i = 0; i < 100; i++)
  {
    sum += 1;
  }
  printf ("sum = %d\n",sum);
  return 0;
}
EOF

Run OpenMP Extractor to run the plugin, you should load the library to run the analysis on clang. To send flags to clang, is necessary to use “-Xclang” before each argument.

-load: Necessary to load libraries on clang.
$HOME/OpenMP-Extractor/lib/ompextractor/libCLANGOMPExtractor.so: This is the library containing the plugin.
-add-plugin: Flag to add a plugin to clang.
-extract-omp: This flag asks clang to run the plugin.
-fopenmp: Turn on OpenMP support in clang’s frontend.
-g: Flag to provide debug information about the source file.
-O0: Disable optimizations from clang.
-c: create an object file only
-fsyntax-only : Prevents the compiler to write an object file. We use this avoid the the creation of an intermediate file, as it is not necessary.

clang -Xclang -load -Xclang $HOME/OpenMP-Extractor/lib/ompextractor/libCLANGOMPExtractor.so -Xclang -add-plugin -Xclang -extract-omp -fopenmp -g -O0 -c -fsyntax-only test.c

Checkout if the Json file was created:

ls

Checkout the output:

cat test.c.json

In the end, the Json file stores loop information extracted by Clang/LLVM.

D. Extending the Plugin

Changing the Plugin

First, let’s open the plugin source file. The plugin was build in one for simplicity.

vim $HOME/OpenMP-Extractor/clangPlugin/ompextractor/ompextractor.cpp

Then, modify the code to recognize the new directives or clauses. This tutorial will show how to add suport to the num_threads clause. To modify the file, go to the line 357 (or before the comment “/Final or If clauses are marked as multiversioned./” in the function “ClassifyClause”) and insert the following code:

      /*NumThreads clause*/
      if (OMPNumThreadsClause *OMPCc = dyn_cast<OMPNumThreadsClause>(clause)) {
        clauseType["num_threads"] = getStrForStmt(OMPCc->getNumThreads());
      }

The next step is to modify the Json builder to write out the information about this new clause. Please, go to the line 480 (or before line with currFile.labels += "\"body\":[" + snippet + "]\n"; in the function “CreateLoopDirectiveNode”) and insert the code as described below.

currFile.labels += "\"num threads\":\"" + ((clauseType.count("num_threads") > 0) ? (clauseType["num_threads"]) : "") + "\",\n";

The last step is to generate similar attribute for loops without openmp directives associated on it. To do that, please, go to the line 164 (or before line with currFile.labels += "\"body\":[" + snippet + "]\n"; in the function “CreateLoopNode”) and insert the following code:

currFile.labels += "\"num threads\":\"\",\n";

Now, it is done. Just close the file typing “:wq”, then type enter.

Testing the New Feature

First, let’s create the example file. We can use the following source file.

cat << EOF > test.c
#include <stdlib.h>
#include <stdio.h>

int main() {
  int i, sum = 0;
  #pragma omp parallel for reduction(+:sum) num_threads(5)
  for (int i = 0; i < 100; i++) {
    sum += 1;
  }

  for (int i = 0; i < 100; i++) {
    sum += 1;
  }

  printf ("sum = %d\n",sum);
  return 0;
}
EOF

Rebuild the binaries of this library within OpenMP-Extractor/lib.

make -j4

To run the example, type the following command line:

clang -Xclang -load -Xclang $HOME/OpenMP-Extractor/lib/ompextractor/libCLANGOMPExtractor.so -Xclang -add-plugin -Xclang -extract-omp -fopenmp -g -O0 -c -fsyntax-only test.c

Checkout if the Json file was created:

ls

The last step is check the output:

cat test.c.json

You can see the num_threads(5) is recognized and encoded into the json file as follows:

25 "num threads":"5",

Well done, you got it.