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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 – allocate. This tutorial is intended to give you some insight into the real-world front end used by LLVM/Clang.

An allocate directive is an executable directive that allocates memory to a given pointer.

The syntax of allocate has the following format:

#pragma omp allocate(A)

Here, A is a pointer defined in the program, and this statement specifies that memory space for the data pointed to by A should be allocated, using malloc internally. To make this task simpler, we assume for this task that the amount of memory to be allocated is always (100*sizeof(int)).

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 token of new directive

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

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 is called).

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

Add the following new line after it:


In our current state we are not dealing with any clause associated with allocate, so we do not need to define OPENMP_ALLOCATE_CLAUSE.

This way we are able to define the token for the new directive #pragma omp allocate.

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_allocate anyweher inside of the body of the switch statement. Here we will print out ALLOCATE is caught and then consume the token.

  case OMPD_allocate: {
    llvm::errs() <<"ALLOCATE 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 -j4 install > /dev/null

This build step may take a few minutes. You might get a couple of warnings about enumeration value 'OMPD_allocate' 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 > allocate.c
int main()
#pragma omp allocate
    return 0;

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

Build this file using your Clang compiler.

clang -fopenmp allocate.c

you should get an output ALLOCATE is caught.

Congratulations you were successfully able to add and identify a new directive to openmp in Clang compiler. Next follow the tutorial – Adding an AST Node for new Directive in OpenMP (Clang).