Re: Removing ARCNET stuffs

[Anders Magnusson <ragge%ludd.ltu.se@localhost>]

Andrew Cagney skrev den 2015-06-01 20:41:
> On 1 June 2015 at 12:54, David Holland <dholland-tech%netbsd.org@localhost> wrote:
> > On Mon, Jun 01, 2015 at 11:41:38AM -0400, Andrew Cagney wrote:
> >   > >> systems and generates reasonable code.  Unfortunately, and sorry PCC
> >   > >> (stabs, really?),
> >   > >
> >   > > Feel free to add dwarf, the source is out there, and it wouldn't be
> >   > > especially difficult to do it.  I just haven't had time.
> >   > > Stabs was "for free" :-)
> >   >
> >   > I'm not so sure (a year back I looked at the code with that in mind),
> >   > and wonder if any quick hack would end up being opportunity lost.
> >
> > I have not looked at it, nor have I looked at pcc at all in a long
> > time, so what I'm missing may just be otherwise obvious context, but:
> >
> >   > PCC, as a "classic" C compiler, only generates debug information at
> >   > -O0.  This this is because the stabs code is restricted to the
> >   > un-optimized code generator path.  Having the backend restricted to
> >   > DWARF when '-O0' might just be ok, were it not for SSA (static single
> >   > assignment).
> >   >
> >   > To my mind, and I'm assuming a pure SSA compiler design, having SSA
> >   > forces issues like: [...]
> >
> > I'm missing something; SSA is just a style of program representation.
> Yes.  Lets think of Static Single Assignment as the pure academic theory.
>
> LLVM[Lattner et.al.] and GIMPLE[Novillo et.al.] are real world
> implementations of that theory.
> https://gcc.gnu.org/projects/tree-ssa/#ssa has a good diagram and is a
> relevant read.
>
> PCC, to the best of my knowledge is still in the [very early] planning
> stages.  One of its design choices would be to go pure SSA.  Another
> option, closer to GCC (RTL), would be to retain existing code-gen
> passes.  Tough choices.
I do not understand why either of those choices need to be taken.
Pcc has a reasonable intermediate representation, which in the optimizer
is converted to SSA form, hammered on, and converted back.  This is
done while retaining the intermediate representation, which is no problem.
> I'm being fast and loose.  My reading of the code was that debug info
> was being generated by the back of the front end (very roughly
> "gimplify" in this diagram of GCC
> https://gcc.gnu.org/projects/tree-ssa/#ssa).   It was pretty much hard
> wired printfs, and explained to me why "-g -O" wasn't supported.
printf's are only used for data (which is spit out directly), not code.
All code is dealt with function-wise, otherwise things like the register
allocator would not work.

> > Unless, I guess, what you're talking about is throwing away the
> > existing backend entirely and writing a new SSA-based one, in which
> > case I'd gently suggest that this is a large project :-)
> Exactly :-(
What is wrong with the existing backend?

-- Ragge