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In the previous tutorial we learnt how to identify or parse a new OpenMP directive in Clang/LLVM. In this tutorial we will cover how to add an AST Node for the new OpenMP directive in Clang/LLVM compiler. The goal of this tutorial is to add an AST Node for a new OpenMP directive – allocate (#pragma omp allocate(A))

Step 1 - Get previous tutorial’s files

First we should get the files that we updated in the previous tutorial. In your local system you may use your own updated files. Or you can checkout the updated code from our alloc branch.

cd $LLVM_SRC/tools/clang
git fetch
git checkout alloc

You may check that all proper code is added in OpenMPKinds.def and ParseOpenMP.cpp from the previous turorial.

Step 2 - Adding New AST Node

First step is to add a Statement Node, which are defined in the file Clang reads this file and generates a file, which is used to define different statement classes and read by several classes to define their node visitor function. We define an OMPAllocateDirective node, which extends the OMPExecutableDirective class - a basic class for representing single OpenMP executable directive. We modify the file and go to the line after the definition of OMPExecutableDirective (line 206)

vim include/clang/Basic/ +206

and add our own definition of OMPAllocateDirective

def OMPAllocateDirective : DStmt<OMPExecutableDirective>;

Next let us define the OMPAllocateDirective class. For this we will modify the StmtOpenMP.h file which defines OpenMP AST classes for executable directives and clauses. Our class extends the OMPExecutableDirective class, so we will keep our definition after the definition of OMPExecutableDirective. Let us open the file StmtOpenMP.h and go to the end of the definition of OMPExecutableDirective (line 266)

vim include/clang/AST/StmtOpenMP.h +266

Now let us add our class OMPAllocateDirective. This class will represent the #pragma omp allocate directive. We will add constructors to build a directive with the given start and end location, and the number of clauses. We will also add an empty constructor. Most importantly we need to add the Create function which creates directive with a list of Clauses. Also a CreateEmpty function which creates an empty directive with the place for N clauses. Finally we also define a classof function which checks if an object is the class of OMPAllocateDirectiveClass.

class OMPAllocateDirective : public OMPExecutableDirective {
  friend class ASTStmtReader;

  OMPAllocateDirective(SourceLocation StartLoc, SourceLocation EndLoc, unsigned NumClauses)
    : OMPExecutableDirective(this, OMPAllocateDirectiveClass, OMPD_allocate,
                            StartLoc, EndLoc, NumClauses, 0) {}

  explicit OMPAllocateDirective(unsigned NumClauses)
    : OMPExecutableDirective(this, OMPAllocateDirectiveClass, OMPD_allocate,
                            SourceLocation(), SourceLocation(), NumClauses,
                            0) {}

  static OMPAllocateDirective *
  Create(const ASTContext &C, SourceLocation StartLoc, SourceLocation EndLoc,
        ArrayRef<OMPClause *> Clauses);

  static OMPAllocateDirective *CreateEmpty(const ASTContext &C,
                                        unsigned NumClauses, EmptyShell);

  static bool classof(const Stmt *T) {
    return T->getStmtClass() == OMPAllocateDirectiveClass;

Next we define the Create and CreateEmpty functions for OMPAllocateDirective class. The definitions of these functions are in the file StmtOpenMP.cpp. We can define our functions anywhere in this file.

vim lib/AST/StmtOpenMP.cpp +19

These functions allocate sufficient memory for the directive and instantiate the required variables. The definition of these functions are self explanatory. You may have to consult the Doxygen documentation of Clang/LLVM APIs to understand the semantics and parameters of functions being used in the code.

OMPAllocateDirective *OMPAllocateDirective::Create(
    const ASTContext &C, SourceLocation StartLoc, SourceLocation EndLoc,
    ArrayRef<OMPClause *> Clauses) {
  unsigned Size = llvm::alignTo(sizeof(OMPAllocateDirective), alignof(OMPClause *));
  void *Mem = C.Allocate(Size + sizeof(OMPClause *) * Clauses.size());

  OMPAllocateDirective *Dir = new (Mem) OMPAllocateDirective(StartLoc, EndLoc, Clauses.size());
  return Dir;

OMPAllocateDirective *OMPAllocateDirective::CreateEmpty(const ASTContext &C,
                                                        unsigned NumClauses,
                                                        EmptyShell) {
  unsigned Size = llvm::alignTo(sizeof(OMPAllocateDirective), alignof(OMPClause *));
  void *Mem = C.Allocate(Size + sizeof(OMPClause *) * NumClauses);

  return new (Mem) OMPAllocateDirective(NumClauses);

To define an AST Node, we also need to define all its visitors. The major visitors are defined in

  • RecursiveASTVisitor.h - This file defines the RecursiveASTVisitor interface, which recursively traverses the entire AST.
  • StmtPrinter.cpp - This file implements the Stmt::dumpPretty/Stmt::printPretty methods, which pretty print the AST back out to C code.
  • StmtProfile.cpp - This file implements the Stmt::Profile method, which builds a unique bit representation that identifies a statement/expression.
  • ASTReaderStmt.cpp - Implements Statements and Expression deserialization. This implements the ASTReader::ReadStmt method.
  • ASTWriterStmt.cpp - Implements serialization for Statements and Expressions.

In RecursiveASTVisitor.h file a macro is defined for Stmts to automate iterating over the children defined in children() (every stmt defines these, though sometimes the range is empty). Each individual Traverse method only needs to worry about children other than those.

To define our own traverse method for allocate, we will use this macro. We wil add our traverse method after the definition of the macro DEF_TRAVERSE_STMT (ends at line 2118).

vim include/clang/AST/RecursiveASTVisitor.h +2119

Add the following definition

DEF_TRAVERSE_STMT(OMPAllocateDirective, { TRY_TO(TraverseOMPExecutableDirective(S)); })

In StmtPrinter.cpp we define the VisitOMPAllocateDirective function, which calls the PrintOMPExecutableDirective function. Let us go to the file StmtPrinter.cpp, after the definition of PrintOMPExecutableDirective (line 641)

vim lib/AST/StmtPrinter.cpp +641

And add our definition of VisitOMPAllocateDirective

void StmtPrinter::VisitOMPAllocateDirective(OMPAllocateDirective *Node) {
  Indent() << "#pragma omp allocate";

Similarly in StmtProfile.cpp we add the definition of VisitOMPAllocateDirective after the definition of VisitOMPExecutableDirective.

vim lib/AST/StmtProfile.cpp +768

VisitOMPAllocateDirective simply calls the VisitOMPExecutableDirective function.

void StmtProfiler::VisitOMPAllocateDirective(const OMPAllocateDirective *S) {

Before we define how to read our statement, first we need to create a record for our statement in the enum StmtCode. These constants describe the records that describe statements or expressions. These records occur within type and declarations block, so they begin with record values of 128. Each constant describes a record for a specific statement or expression class in the AST. To add our own record we modify the StmtCode enum in ASTBitCodes.h file. In general we could add our record anywhere in the enum (or where OpenMP directives are defined), but we could not like to upset the current state of the code. So we will add our record at the end of the enum (after line 1977).

vim include/clang/Serialization/ASTBitCodes.h +1977

Add our record after line 1977


In ASTReaderStmt.cpp, expressions are stored in Reverse Polish Notation, with each of the subexpressions preceding the expression they are stored in. Subexpressions are stored from last to first. To evaluate expressions, we continue reading expressions and placing them on the stack, with expressions having operands removing those operands from the stack. Evaluation terminates when we see a STMT_STOP record, and the single remaining expression on the stack is our result.

vim lib/Serialization/ASTReaderStmt.cpp +3396

We modify the function ReadStmtFromStream to read our statement. We go to the end of the switch case (line 3396) and add our own case where we create an empty OMPAllocateDirective statement.

      S = OMPAllocateDirective::CreateEmpty(Context,

We also add a definition for the function VisitOMPAllocateDirective, anywhere in the file after the definition of the class ASTStmtReader and the function VisitOMPExecutableDirective (line 1920)

void ASTStmtReader::VisitOMPAllocateDirective(OMPAllocateDirective *D) {

In ASTWriterStmt.cpp we define the function VisitOMPAllocateDirective preferably after the definition of VisitOMPExecutableDirective (line 1889)

vim lib/Serialization/ASTWriterStmt.cpp +1889

We assign the code as our recently defined STMT_OMP_ALLOCATE_DIRECTIVE

void ASTStmtWriter::VisitOMPAllocateDirective(OMPAllocateDirective *D) {
  Code = serialization::STMT_OMP_ALLOCATE_DIRECTIVE;

We also need to define a tree transformation in TreeTransform.h - This file implements a semantic tree transformation that takes a given AST and rebuilds it, possibly transforming some nodes in the process. Using, this class will already declare the TransformOMPAllocateDirective function. Here we need to define it, after the definition of TransformOMPExecutableDirective. So let us open the file TreeTransform.h

vim lib/Sema/TreeTransform.h +7756

and define the function as

template <typename Derived>
TreeTransform<Derived>::TransformOMPAllocateDirective(OMPAllocateDirective *D) {
  DeclarationNameInfo DirName;
  getDerived().getSema().StartOpenMPDSABlock(OMPD_allocate, DirName, nullptr,
  StmtResult Res = getDerived().TransformOMPExecutableDirective(D);
  return Res;

Once our statement is defined, it is time to parse our new directive, this time creating an AST Node for the directive. We will again modify the ParseOpenMP.cpp file to modify the case we added in the previous tutorial.

vim lib/Parse/ParseOpenMP.cpp +998

In the last tutorial, we simply consumed the token and break out of the switch case. Now we will instantiate the scope of the directive, get the associated statement and create the Directive.

  case OMPD_allocate: {
    llvm::errs() <<"ALLOCATE is caught\n";
    ParseScope OMPDirectiveScope(this, ScopeFlags);
    Actions.StartOpenMPDSABlock(DKind, DirName, Actions.getCurScope(), Loc);
    Directive = Actions.ActOnOpenMPExecutableDirective(
        DKind, DirName, CancelRegion, Clauses, nullptr, Loc,

    // Exit scope.

In Clang Parser class, Actions is an instance of the class Sema, which implements semantic analysis and AST building for C. For allocate directive, we should not have any captured region. So if getOpenMPCapturedRegions is called, it should return an error.

This function is defined in OpenMPKinds.cpp. We modify this file as

vim lib/Basic/OpenMPKinds.cpp +1005

and add a case in the switch statement (towards the end) of getOpenMPCaptureRegions

  case OMPD_allocate:

Going back to our parsing (ParseOpenMP.cpp), you can see that to create the Directive we used the function ActOnOpenMPExecutableDirective. This function (defined in the file SemaOpenMP.cpp), tells how to act on an OpenMP executable directive. We will modify this function to handle our case for allocate. First we need to define how to act on OpenMP allocate directive. We declare our function ActOnOpenMPAllocateDirective in the header file Sema.h as part of the class Sema and provide its definition in SemaOpenMP.cpp.

vim include/clang/Sema/Sema.h +8849

The function ActOnOpenMPAllocateDirective is called on well-formed ‘#pragma omp allocate’ after parsing of the associated statement.

  StmtResult ActOnOpenMPAllocateDirective(ArrayRef<OMPClause *> Clauses,
                                      SourceLocation StartLoc,
                                      SourceLocation EndLoc);

Now let us define the function. For that we open the SemaOpenMP.cpp file

vim lib/Sema/SemaOpenMP.cpp +3681

In this function we create the OMPAllocateDirective.

StmtResult Sema::ActOnOpenMPAllocateDirective(ArrayRef<OMPClause *> Clauses,
                                          SourceLocation StartLoc,
                                          SourceLocation EndLoc) {
  assert(Clauses.size() < 1 && "Extra clauses in allocate directive");
  return OMPAllocateDirective::Create(Context, StartLoc, EndLoc, Clauses);

Now it is time to call our function from ActOnOpenMPExecutableDirective. Let us go to the switch case in the function ActOnOpenMPExecutableDirective (line 3420). We add our case OMPD_allocate where we call the ActOnOpenMPAllocateDirective function.

  case OMPD_allocate:
    assert(AStmt == nullptr && 
	"No associated statement allowed for 'omp allocate' directive");
    Res = ActOnOpenMPAllocateDirective(ClausesWithImplicit, StartLoc, EndLoc);

Since allocate directive should not have any captured region, we need to make sure that the function ActOnOpenMPRegionStart handles the case accordingly. For that we go to the end of the switch statement (line 2815) and add our case before llvm_unreachble statement is called. This way we make sure if the allocate directive has any associated statemnt we throw an error.

  case OMPD_allocate:

Step 3 - Building LLVM and testing code

To build LLVM go to the LLVM_BUILD directory and run make. We are redirecting the 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

You might get a couple of warnings about enumeration value 'OMPD_allocate' not handled in switch. 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 get a new test file


Now you have a new test file alloc.c which uses the allocate directive. The content of the file should be as follows:

int main()
#pragma omp allocate
    return 0;

Build this file using your Clang compiler.

clang -cc1 -ast-dump -fopenmp alloc.c

You should get an AST tree with OMPAllocateDirective Node in it.

Congratulations you were successfully able to add an AST Node for a new directive to openmp in Clang compiler.