Reduction

Reporting

Reduction Loops are reported in the following format:

Reduction at: 1:2
Start line: 1:7
End line: 1:9
pragma: "#pragma omp parallel for"
private: []
shared: []
first private: []
reduction: []
last private: []

Interpretation

The reported values shall be interpreted as follows:

• Reduction at: <file_id>:<cu_id>, where the respective parent file can be looked up in the FileMapping.txt using file_id and cu_id can be used for a look up in Data.xml
• Start line: <file_id>:<line_num>, where line_num refers to the source code line of the parallelizable loop.
• End line: <file_id>:<line_num>, where line_num refers to the last line of the parallelizable loop.
• pragma:shows which type of OpenMP pragma shall be inserted before the target loop in order to parallelize it.
• private: [<vars>] lists a set of variables which have been identified as thread-private
• The same interpretation applies to the following values aswell:
  • shared
  • first_private
  • last_private
• reduction: [<operation>:<var>] specifies a set of identified reduction operations and variables.

Implementation

In order to implement a suggestion, first open the source code file corresponding to file_id and navigate to line Start line -> <line_num>. Insert pragma before the loop begins. In order to ensure a valid parallelization, you need to add the following clauses to the OpenMP pragma, if the respective lists are not empty:

• private -> clause: private(<vars>)
• shared -> clause: shared(<vars>)
• first_private -> clause: firstprivate(<vars>)
• last_private -> clause: lastprivate(<vars>)
• reduction-> clause: reduction(<operation>:<vars>)

Example

As an example, we will analyze the following code snippet for parallelization potential. All location and meta data will be ignored for the sake of simplicity.

for (int i = 0; i < N; i++) {
    local_var *= global_array[i];
}

Analyzing this code snippet results in the following parallelization suggestion:

pragma: "#pragma omp parallel for"
private: ["i"]
shared: []
first private: ["global_array"]
reduction: ["*:local_var"]
last private: []

After interpreting and implementing the suggestion, the resulting, now parallel, source code could look as follows:

#pragma omp parallel for private(i) firstprivate(global_array) reduction(*:local_var)
for (int i = 0; i < N; i++) {
    local_var *= global_array[i];
}