[cfe-dev] Parser Stmt/Expr Owning Pointer

[Sebastian Redl]

Hi,

In my quest to have parser and sema use smart pointers to ensure that 
all nodes are always freed, I have now replaced ASTGuard with ASTOwner. 
ASTGuard was a class that simply sat by and watched expressions, 
deleting them if necessary, but it was an add-on, which led to very ugly 
code:

ExprResult Res = Parse...();
ExprGuard Guard(Actions);

The new ASTOwner is a proper move-emulated smart pointer, and also holds 
the validity information of ActionResult. Thus, the code becomes

ExprOwner Res(Actions, Parse...());

Passing Actions to every pointer is an unfortunate necessity.

The move emulation of this pointer is specialized to support backwards 
compatibility. The ASTMove class, core of the move emulation, cannot 
only be implicitly converted to a new ASTOwner (which will take 
ownership), but also to an ActionResult or a void pointer, which also 
have ownership. This allows the use of move() in many contexts, such as 
returning an ASTOwner where an ActionResult is expected, or passing a 
void* to the Action by calling move(). This is a significant advantage 
in making the use of the smart pointer intuitive: move() is used 
whenever you give up ownership, no matter what receives it.


The change is simple but very far-reaching, because I've excised every 
single ExprResult/StmtResult that isn't part of a function signature. 
Thus, I'll await comments before committing it. It also influences the 
way everyone should write code in the parser.

I'd also like to see if the use of the smart pointer, which is a word 
larger than ActionResult and two words larger than a raw pointer, 
negatively affects performance. Do you people have a hint, or perhaps 
even a complete framework, for tracking parser performance?


In the next step, I'll make the Parser's internal signatures use the 
smart pointer, which should reduce the number of times Actions is passed 
greatly, but increases the number of times move() is called greatly, 
too. After that, we'll want to use smart pointers in the Action 
interface, and finally use smart pointers inside Sema.

Sebastian
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