[m-rev.] Compiler bug found

[Volker Wysk]

Hi

Are you done with it? Is the fix in the latest ROTD?

Regards,
Volker

Am Sonntag, dem 06.08.2023 um 00:47 +1000 schrieb Zoltan Somogyi:
> On 2023-08-04 20:29 +02:00 CEST, "Volker Wysk" <post at volker-wysk.de> wrote:
> > I've found a bug in MMC rotd-2023-03-03. I've attached the compiler output
> > and made a small example program.
> > 
> > Compile it with the mmc-make script. The bug doesn't occur when the option
> > "-O 6" is left out.
> 
> I have started looking at this.
> 
> The problem occurs during a simplification pass on the amend predicate
> When the pass tests whether the goal that is the whole body of amend can be
> replaced by "true", part of the test is to see whether it changes the state
> of the instantiation of any of the nonlocals. (This is the call to to instmap_delta_
> no_output_vars in line 191 in simplify_goal.) I haven't zeroed in on the problem yet,
> but I don't think the problem is in the mode checks so much as in the updating of the
> merge inst table, because the immediate cause of the abort is a map.lookup
> in that table failing. I suspect that somewhere, we are adding an entry to that table,
> attaching the resulting merge-inst in an instmap delta somewhere, but throwing away
> the updated merge inst table. That scenario would perfectly explain the symptom.
> 
> Attached are ARGSY, which is an argument list; invoking
> "mmc `cat ARGSY`" generates the crash described in Volker's email. The output
> of that invocation, from a somewhat-modified local debug-grade compiler
> is in the other attached file, BAD_OUTPUT.
> 
> The optimization level is a red herring. The predicate the problem occurs in,
> amend, is never called, so if dead proc elimination is enabled, it gets deleted
> from the HLDS, and the crash never happens. If the optimization level is high
> enough, then some compiler pass (e.g. deforestation) will invoke simplify
> *before* dead proc elimination runs, but that compiler pass is NOT the cause
> of the problem. I know that at -O0, with dead proc elim enabled (which is by default)
> you don't get crash, but with dead proc elim explicitly disabled, you do.
> In that last case, it is the pre-code-generation simplification pass that crashes.
> 
> The end-of-the-front-end simplify pass does not crash in any of the scenarios
> I tested. It is possible that it is the culprit, setting up the conditions for a crash
> on its next invocation.
> 
> If you want to try out ARGSY, you will need to update the paths in it.
> 
> Zoltan.

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