[cfe-dev] Custom Clang Code Transformation Addon

[Eli Friedman]

On Mon, Mar 26, 2012 at 1:27 PM, Daniel Powell
<divot.powell at googlemail.com> wrote:
> Hi,
>
> I am attempting to write a code transformation tool for clang and was hoping
> for some help and advice.
>
> The general idea is to take c code marked up with custom pragma statements
> similar to openmp markup, and extract the basic block immediately following.
> A new c++ class that extends another already existing class will then be
> generated inside the compiler with the basic block added as a method inside
> that class, along with some administration code etc.
>
> My current plan for doing this is as follows:
>
> In the preprocessor create a new PragmaHandler that inserts an annotation
> token marking the start location of the block to be extracted (and a few
> other tokens for parameters/config etc.)
> During parsing and ast construction, extract the block following any marker
> encountered and generate the new class directly into the AST.
> Insert instances of this class globally and then replace the original block
> with calls to use the new objects.
> Emit everything as llvm bytecode so that I can do further modifications and
> optimisations in an llvm pass.
>
> My first question is, does this sound sane?
>
> I have no problems with implementing the first stage, however I am hitting
> humps during the second. Using the already existing "unused pragma" as a
> base, I have the parser appropriately recognising my new pragma tokens,
> which calls my HandlePragma method inside the Parser object. This then
> consumes my custom tokens and calls my ActOnPragma method inside the Sema
> object. At this stage I am unsure of how to proceed. I could attempt to
> generate my new class inside this action (Using CXXRecordDecls I believe),
> or I could insert another marker such as a custom statement here, and then
> attempt to generate the new class inside of an ASTConsumer or elsewhere. Or
> should I be using a BuildXXX method to generate my new class as suggested by
> the Clang Internals document (struggling with finding examples of doing so)?
> Do you have any suggestions or advice on this part?
>
> Finally, the actual generation of the new class I am having difficulties
> with and struggling with documentation for doing so (unsurprising given the
> whole c++ side of clang appears to still be in flux). I am able to generate
> a new CXXRecordDecl object inside my ActOnPragma method, and print it out
> for analysis fine, (so my class exists but hasn't been inserted into the AST
> just yet). However when I attempt to extend my class using an already
> existing base class, or add in methods and variables etc into my class
> nothing appears to happen. Do you have any advice, documentation, examples
> of generating additional AST nodes on the fly like this? I have been rifling
> through the doxygen and source code and whatever else I can find up to this
> point.
>
> Many thanks in advance for your help.

It sounds like the closest existing thing in clang to what you are
trying to do is LambdaExpr (which represents C++11 lambda
expressions); reading the relevant code for that would be a decent
place to start.

-Eli