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In this tutorial we will cover how to add a new OpenMP directive in Clang/LLVM compiler. The goal of this tutorial is to add a new OpenMP directive – metadirective (#pragma omp metadirective [clause[[,]clause]...]), defined in OpenMP Specification 5.0, that can specify multiple directive variants of which one may be conditionally selected to replace the metadirective based on the enclosing OpenMP context.

A metadirective is an executable directive that

  • Conditionally resolves to another directive at compile time by selecting from multiple directive variants
  • Based on traits that define an OpenMP condition or context.
  • Improve performance portability by adapting OpenMP pragmas and user code at compile time.

The syntax of a metadirective has the following format:

#pragma omp metadirective [clause[[,]clause]...]new-line

Clause is one of the following

  • when(context-selector-specification: [directive-variant])
  • default(directive-variant)

The following example showing how a metadirective can faciliate compile-time adapation without using the traditional preprocessing directives.

// Traditional way to enable two versions of OpenMP code using #if-#else. 
// The same code is duplicated multiple times.
int v1[N], v2[N], v3[N];
#if defined(nvptx)     
 #pragma omp target teams distribute parallel loop map(to:v1,v2) map(from:v3)
  for (int i= 0; i< N; i++) 
     v3[i] = v1[i] * v2[i];  
 #pragma omp target parallel loop map(to:v1,v2) map(from:v3)
  for (int i= 0; i< N; i++) 
     v3[i] = v1[i] * v2[i];  

Using meatadirective, the same code can be written as follows:

// metadirective supports runtime adaptation using a single copy of code
int v1[N], v2[N], v3[N];
#pragma omp target map(to:v1,v2) map(from:v3)
  #pragma omp metadirective \
     when(device={arch(nvptx)}: target teams distribute parallel loop)\
     default(target parallel loop)
  for (int i= 0; i< N; i++) 
     v3[i] = v1[i] * v2[i];

For more information about metadirective, please consult the OpenMP 5.0 specification.

Step 1 - Locate and go to clang directory

First, let’s enter the LLVM source folder to look around. There are a bunch of files and directories there. For now only interested in the Clang sub-project of the LLVM source code. In this tutorials’s environment, the Clang project is located at $LLVM_SRC/tools/clang. In your machine you should locate the Clang project and switch to that directory.

cd $LLVM_SRC/tools/clang

Step 2 - Define the new directive

The first thing that we should do is let the compiler identify a new directive, which in this tutorial is metadirective.

Now let us update the compiler, such that it just identifies the new directive. For this we need to update two files:

  1. OpenMPKinds.def – which defines the list of supported OpenMP directives and clauses.
  2. ParseOpenMP.cpp – which implements parsing of all OpenMP directives and clauses.

To define the new directive we will modify the file OpenMPKinds.def, located in include/clang/Basic. So open the file using your favorite editor and go to line 237 (or anywhere before #undef OPENMP_DIRECTIVE_EXT is called).

vim include/clang/Basic/OpenMPKinds.def +237

Add the following new line after it:

OPENMP_DIRECTIVE_EXT(metadirective, "metadirective")

In our current state we are not dealing with any clause associated with metadirective, so we do not need to define OPENMP_METADIRECTIVE_CLAUSE. We will learn about adding clause later in the tutorial.

This way we are able to define the new directive #pragma omp metadirective.

Step 3 - Implement parsing

Before parsing the lexer will split the source code into multiple tokens. The parser will read these tokens and give a structural representation to them. To implement the parsing of this new directive we need to modify the file ParseOpenMP.cpp, located in lib/Parse. Open this file and go to the function ParseOpenMPDeclarativeOrExecutableDirective, identify the switch statement (line 997):

vim lib/Parse/ParseOpenMP.cpp +997

Add a new case for OMPD_metadirective anyweher inside of the body of the switch statement. Here we will print out METADIRECTIVE is caught and then consume the token.

  case OMPD_metadirective: {
    llvm::errs() <<"METADIRECTIVE is caught\n";

That’s it for now. Now let us build and test our code.

Step 4 - Building LLVM and testing code

To build LLVM go to the LLVM_BUILD directory and run make. We are redirecting the standard output of make to /dev/null to have a clean output. Warning and error messages will still show up if there are any.

cd $LLVM_BUILD && make -j8 install > /dev/null

This build step may take a few minutes. You might get a couple of warnings about enumeration value 'ompd_metadirective' not handled in switch. Please ignore these warnings for now. we will handle them later. Once the code builds successfully and is installed, its time to test a small program. Let us create a new test file:

cat <<EOF > meta.c
int main()
#pragma omp metadirective 
    for(int i=0; i<10; i++)
    return 0;

Now you have a new test file meta.c which uses the metadirective directive.

Build this file using your Clang compiler.

clang -fopenmp meta.c

you should get an output metadirective is caught.

Congratulations you were successfully able to add and identify a new directive to openmp in Clang compiler.