[m-dev.] request for opinions about a change to switch detection

[Zoltan Somogyi]

On Fri, 14 Nov 2025 13:17:46 +1100, Peter Wang <novalazy at gmail.com> wrote:
> > > The third "fix" is to simply declare that the warning is an appropriate
> > > response to code like the above.
> > 
> > It is not an appropriate response to such code.
> 
> Why not? There is no description of how smart the compiler is supposed
> to be to implement --warn-unknown-format-calls. I'm surprised the
> compiler didn't warn about that code before. The fact that it didn't
> in that particular case seems like an artefact of the implementation
> more than by design.

Agreed.

> The compiler does produce the warning for this predicate:
> 
>     p(List, Str) :-
> 	(
> 	    List = [],
> 	    Fmt = "empty %d"
> 	;
> 	    List = [_ | _],
> 	    Fmt = "non-empty %d"
> 	),
> 	string.format(Fmt, [i(1)], Str).

A good point.

> I'm not saying the third option is the best, but I wouldn't simply
> reject it.

I agree, which is why I mentioned it, though I added the quotes around
"fix" because I agree with what I take to be Julien's main point, which is that
we should avoid breaking existing user code if possible (and in this case,
avoiding such breakage is possible). 

How about a fourth fix, which goes like this. *If* the format_call
pass generates a warning, *and* the inst of the variable it is warning
about (in this case, Fmt) is a bound inst with two or more alternatives
(as it would be in this case), *then*

- we discard the results of the pass,

- we perform a backwards traversal of the procedure body that looks for
  calls to format predicates that are preceded by a disjunction or a switch
  or if-then-else, and pushes all goals after the disjunction/switch/ite
  up to and including the format call into each arm of the disjunction/switch/ite,

- we fix up quantification, recompute instmap deltas and so on,

- and we then redo the usual format call pass, but this time committing
  to keeping its result, whatever it may be.

This should have the minimal possible performance impact, since code
like this very rare; a bootcheck found exactly one predicate where this
redo of the pass would be needed. *And* it would have the effect that
Peter's example code would be now accepted by the compiler, and the
description of what programs would be accepted in this respect would
be simplified. (You could describe it using text such as "if, for each path
execution can take through the body of the clause, the format string
is fixed, then ...")

Opinions?

Zoltan.