[cfe-dev] Future of AST transformation

Olaf Krzikalla Olaf.Krzikalla at tu-dresden.de
Wed Jul 18 04:56:45 PDT 2012

Hi @clang,

I'm working at my source-to-source transformator based on clang for 
quite some time now. However, my last clang update constantly causes me 
some serious headaches. Mainly it is the new expr::EvaluateAsRValue 
framework, which asserts like hell. Of course rightly since I cannot 
preserve all - often hidden - AST invariants during transformations.
Some assertions were fixable, but for a lot of others I have no clue.
An example: transforming a+=b to a=a+b works like this in my framework:

CompoundAssignOperator* Node;
// compute binOpc
Expr* firstRhs = Clone_(Node->getLHS());
BinaryOperator* TC = Assign_(
   BinaryOp_(firstRhs, Node->getRHS(), binOpc));

I had to figure out that I had to set the value kind. But in the end 
that was still an easy fix. But recently a new problem popped up:

struct A { float x; };
A a;
a.x += y;  // assertion: APValue.h:306: isStruct() && "Invalid accessor"

This comes from the fact that "a" in "a.x" itself is treated as an 
lvalue. And this is only a small example of all the assertion hell I've 
got with the expr::EvaluateAsRValue framework.
So my question is whether there is or there will be a AST transformation 
framework capable of transforming expressions like above? Or whether 
clang/AST transformation itself is a dead end in the long run?

Best regards
Olaf Krzikalla

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