Design Considerations of GIMPLE Front End

[Basile Starynkevitch]

On Tue, 2010-05-18 at 15:59 +0200, Michael Matz wrote:
> Hi,
> 
> On Tue, 18 May 2010, Diego Novillo wrote:
> 
> > On Mon, May 17, 2010 at 16:15, Sandeep Soni <soni.sandeepb@gmail.com> wrote:
> > 
> > > 1. What should be the format of representation of the GIMPLE tuples in 
> > >    text?
> > 
> > I liked Andrew's suggestion about S-expressions.
> 
> I can see that for describing types, maybe.  But isn't that artificially 
> awkward for representing tuple instructions?  I mean most instructions 
> will look like
> 
>   (= i_1 (+ k_1 m_1))
> or
>   (= j_1 (call func arg1 arg2))
> 
> I don't see how that is much easier to parse compared to
>   i_1 = k_1 + m_1
>   j_1 = func (arg1, arg2)

My intuition might be that once a Gimple parser exists, most of its use
would be writing various translators (e.g. external front-ends) to this
syntax, so people might probably code more Gimple-syntax printers than
Gimple-syntax parsers.

Still, I prefer the Lispy S-expression syntax everywhere -including
Gimple- because it is so simple to define and to implement, and because
GCC MELT already have [almost] the infrastructure for it.

However, I tend to think that the Gimple syntax would also [at least
optionally] contain the location information, so it would be instead
   (= (@ "foo.cobol" 23 2) i_1 (+ k_1 m_1)))
or even
   (= (@ "foo.cobol" 23 3) i_1 (+ (@ "foo.cobol" 23 5) k_1 m_1)))
I am using, perhaps wrongly, @ as an "operator" giving the location
information as a file name, line number, column number. I am not sure to
have the syntax right (because I am not sure to remember what exactly
has a location information).

I believe a Gimple-syntax should provide the features (or hooks, or
syntax) to convey all the Gimple information, and this includes the
source file location. This is needed both for external (GPLv3+ or
compatibly licensed) programs producing Gimple (such as an hypothetical
Cobol frontend) and for external (GPLv3+ or compatibly licensed)
programs consuming Gimple (like sophisticated static analyzers) or for
external programs both consuming & producing Gimple (e.g. an
"optimization" implemented by an external program).

Otherwise, what is the purpose of Gimple-syntax? Why making it if it
does not contain all the information inside GCC?


BTW, is it possible today to have a GCC plugin providing a front-end to
GCC? [last time I looked, I believe the answer is no]

Cheers.