Re: [PATCH] New pragma handling mechanism.

[Mark Mitchell <mark at codesourcery dot com>]

Richard Henderson wrote:

> Indeed, I don't like it at all.  We've already got one big basket
> data structure in the front end, and it's called "tree".  We don't
> need another big basked data structure called "pragma".

Whereas I would rather have multiple smaller baskets, with more clearly 
defined roles.  To me, optionally attaching a (list of) "struct pragma 
*" to a a statement in the front end, and letting the gimplifier deal 
with it later seems very tidy.  Again, this preserves the high-level 
structure of the user input in the front-end.  Certainly, the fact that 
"everything is a tree" is something we're trying to get away from.

You asked "how is parsing not general?"  It's general, but it's not very 
nicely encapsulated.  In particular, since the C and C++ front ends look 
to be staying separate for the forseeable future, they've got different 
parsers, and their internal representations are already divergent.  I'm 
not sure that a pragma handler can be easily written to apply to these 
different IRs, so if the handler is parsing the subsequent construct, 
it's probably going to have to pass in some representation of the 
#pragma so that the front-end's parsing routine can do the right thign.

It also sounds like, for C++ templates, you're planning on preserving 
the pragma in some form, so that you can substitute into it at 
instantiation-time.  That is just what I would expect.  But why not save 
it as its own data structure, rather than as a tree?

You also said "I don't like trying to design for some uknown future 
requirements".  I just see it as a basic encapsulation step.  I also 
have experience with adding new #pragmas to a front-end that does this, 
and found it convenient.

I certainly wouldn't want to see us fail to support OpenMP because of a 
disagreement about internal representation.  Do any of the other C/C++ 
maintainers have input?

--
Mark Mitchell
CodeSourcery, LLC
mark@codesourcery.com
(916) 791-8304