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Re: [epsilon-devel] jitter and libtextstyle

From: Luca Saiu
Subject: Re: [epsilon-devel] jitter and libtextstyle
Date: Thu, 17 Oct 2019 02:02:24 +0200
User-agent: Gnus (Gnus v5.13), GNU Emacs 27.0.50, x86_64-pc-linux-gnu

Hello José and Bruno.

On 2019-10-14 at 16:26 +0200, Jose E. Marchesi wrote:

> I recently "libtextstylized" poke, and to complete the process I would
> need a few changes in the Jitter API.

Very interesting.

I have not looked at libtextstyle beyond what Bruno showed us.  It is
way too late for me to do it now, unfortunately.  I have class at 8.
But I will check it soon.

In the mean time, please tell me if I am misunderstanding anything.

> First, PREFIX_print_program currently is invoked like:
>     pvm_print_program (stdout, prog);
> Where the first argument is a FILE*.

I am seizing the occasion of the upcoming routine API change to change
this a little.  In particular I want to make the APIs for printing a
routine and for disassembling it as similar as possible.  Neither will
become substantially more complex.

> In order to be able to apply style in the printed disassembly

I am assuming you mean printing a VM routine.

> , I would
> need PREFIX_print_program to get a hook instead, that will do the
> printing of the entities.

This is fine, but I would like to make the dependency on libtextstyle
optional.  The hook-enabled printer will be a new function, but
something very close to the current API will remain available as well.

In fact I believe that libtextstyle will be even more useful for the
actual (hardware-instruction) disassembly where a lot of information is
condensed in a small, cluttered space.  In the case of actual
disassembly I suppose that the hook could be completely hidden from the
user, as long as the format customization does not rely on the C API.

I would be quite happy to replace scripts/highlight-disassembly.in ,
which has always been meant as a kludge, with something clean and
directly usable from C.

> The hook should also be made aware of the kind of entity it is
> printing, like instruction mnemonic, or argument type.

Sure, this is easy.  The entities are fixed, and few in number.

Right now Jitter's routine printing performs a trivial kind of
pretty-printing, using the least possible amount of horizontal space for
mnemonics while having the first instruction arguments all aligned on
the same column.

I hope this will not interfere with the library.  If it did in some
troublesome way, I could avoid aligning in the hook-enabled printout.
That would be of course a worst-case scenario.

> Also, the instruction pretty printers currently are implemented as hooks

I am assuming you mean literal.

> that also get a FILE *.  Example:
> [...]
> But then I'm afraid of Jitter relying on the order imposed by the FILE
> stream, or caching it in some other way.

Jitter currently prints routines in two passes:

1) The first pass scans every used VM instruction in order to find the
   longest mnemonic which is actually used as explained above, without
   printing anything;

2) the second pass does the actual output, writing everything to the
   FILE * stream, in order.  No bizarre caching, no ungetc.

I would keep this same idea for the hook version -- possibly replacing
the FILE * argument with something different, as explained below.  Of
course I would re-implement the traditional-style printer as a call to
the hook version, using trivial hooks.  As long as I am not
misunderstanding how libtextstyle works, this should again be easy.

> I am already ignoring the OUT argument in some places, like in:
>     void
>     pvm_literal_printer (FILE *out, jitter_uint val)
>     {
>       /* XXX what about out.  */
>       pvm_print_val ((pvm_val) val, 10, 0);
>     }

I do not understand this.  Where is pvm_print_val printing to?  Always
to the standard output?  I can replace the FILE * object with an
abstract "hook callee argument", in the style of pthread_create 's last
argument.  This would play the role of the FILE * object, and hopefully
interact naturally with libtextstyle.

> I can always flush the libtextstyle output stream in each pretty
> printer, and simply ignore the OUT, but that seems a bit fragile to
> me...  what are your thoughts about an API that would allow
> client-provided printers?  That would be perfect for me, and for other
> users of libtextstyle as well! :)

I have no objections.

This is not directly related to libtextstyle, but the issue will come up
if you want to print structured literal objects in a sophisticated way.

The problem I anticipate is related to printing objects (in your case VM
instruction literals as Poke objects) spanning multiple lines.  If we
want to support that, then Jitter needs to call the data printer (or the
hook?  Are they different things?) by providing information about the
initial column index.

For example (here I am making up VM instructions and Poke data

     push-literal       [struct my-file-field
                          foo = 3:  #2B;
                          bar = 79: #1B]
     branch             $L1

If you want to decorate every syntactic element within the struct in a
sophisticated way and use multiple lines for a single struct, then your
printer hooks need to know how much to indent following each '\n', in
addition to your own indentation showing the data nesting level.  I can
arrange for that.

Are you planning to do this?


Luca Saiu
* My personal web site:  http://ageinghacker.net
* GNU epsilon:           http://www.gnu.org/software/epsilon
* Jitter:                http://ageinghacker.net/projects/jitter

I support everyone's freedom of mocking any opinion or belief, no
matter how deeply held, with open disrespect and the same unrelented
enthusiasm of a toddler who has just learned the word "poo".

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