GCC plugins: mapping AST back to source files, missing locations.

[Ian Lance Taylor]

On Thu, Oct 18, 2012 at 9:34 AM, mefyl <mefyl@gruntech.org> wrote:
>
> * I can retrieve source file location of declarations with
>   DECL_SOURCE_LOCATION without any problem, but I can't get locations for
>   expressions since their EXPR_LOCATION is always null. I used the
>   PLUGIN_PRE_GENERICIZE hook, which AFAIK is the earliest, so I expected
>   locations to still be there. Did I miss something, or is there no way to
>   retrieve such locations from plugins, in which case my example seems
>   impossible ?

Most expressions should have locations these days.  It is certainly
true that there will be some that do not.  Hard to say more without
more details.

> * Even if we suppose I fixed the previous issue, from what I inferred from the
>   AST, a reference to a variable is directly represented by the corresponding
>   VAR_DECL. That is, in "int x; int y; x + y;", "x + y" is a PLUS_EXPR whose
>   operands are the VAR_DECL corresponding to "int x" and "int y". This would
>   make it impossible for me to get the location of "x" and "y", since they are
>   not represented as such by nodes in the AST.

That is correct.

> I fear that due to the compiler nature of GCC, the AST I'm working on is
> altered, with some processing and simplification already applied, preventing
> source-to-source code transformation since one cannot map ast chunks back to
> source files. Could anyone confirm or point me in some direction if I'm wrong
> ? In the case it's indeed impossible, is there any intent to improve this
> aspect, like giving access to earlier stages of the AST (freshly parsed, with
> only variable bindings for instance) ? This would enable to write extremely
> powerfull analysis & refactoring tools.

Unfortunately the GCC AST almost certainly does not retain enough
information for the kind of transformations you want to do.  This is a
deficiency in the AST, but I can't even estimate how difficult it
would be to fix.

Ian