[llvm-dev] TableGen trace facility

[Nicolai Hähnle via llvm-dev]

On Tue, Aug 4, 2020 at 11:13 PM Paul C. Anagnostopoulos
<paul at windfall.com> wrote:
> Yes, I understand the problem. To be more useful, TableGen would have to carry the traces along with the classes and records and (re)display the values while the substitutions are being made.
>
> I'm writing a new Programmer's Guide for TableGen and have been digging into the parse-time versus substitution-time issue. I haven't found a document that makes it clear. Can you give a quick summary of the phases?

There aren't really any phases per se, in the sense that the TableGen
frontend doesn't have passes. Maybe that's a mistake, but that's how
it evolved.

So instead, every time a record is "instantiated" -- whether that's by
defining a new class that derives from one or more pre-existing
classes, or by a def in a multi-class, or by a free-standing def, or a
defm (inside or outside a multiclass) -- any free variables that are
"resolved" as template parameters get their substitution applied. The
relevant let-statements take effect just after that.

However, and this is key, reference _between_ fields are only resolved
and substituted once the final defined record at the global scope is
produced. This is what makes:

class A<int x> {
  int a = x;
  int b = a + 1;
}

def B : A<5> { let a = 10; }

... result in b == 11 instead of b == 6.

There's probably subtlety here that I'm forgetting, but that's the
very short version of it ;)

Cheers,
Nicolai


>
> At 8/4/2020 05:01 PM, Nicolai Hähnle wrote:
> >On Mon, Aug 3, 2020 at 6:04 PM Paul C. Anagnostopoulos via llvm-dev
> ><llvm-dev at lists.llvm.org> wrote:
> >> A question for those of you who have developed complex TableGen files: Do you think a trace facility would be useful during debugging? The idea is to add a new statement to TableGen along these lines:
> >>
> >>   trace tag : value1, value2, ... ;
> >>
> >> When encountered, the TableGen parser would display the tag along with the values of the specified value1, value2, etc. The tag is an identifier that makes it easier to distinguish multiple traces.
> >
> >I'm not so sure that that's particularly useful or how it would even
> >work. The issue is that at the point in time where the frontend parses
> >those trace tags, most substitutions haven't taken place yet, so
> >you'll usually get fairly trivial answers that by themselves aren't
> >particularly helpful.
> >
> >Some form of inspection of how values are substituted would indeed be
> >helpful, I just don't think the "trace" is quite it. In a perfect
> >world, we'd have some sort of "record database explorer" that doesn't
> >just let you look at all the final records (TableGen already allows
> >you to do that), but also allows you to interactively explore their
> >"history", i.e. how did the records come to be.
> >
> >Cheers,
> >Nicolai
> >
> >--
> >Lerne, wie die Welt wirklich ist,
> >aber vergiss niemals, wie sie sein sollte.
>


-- 
Lerne, wie die Welt wirklich ist,
aber vergiss niemals, wie sie sein sollte.