[PATCH] D28569: Remove all variants of DWARFDie::getAttributeValueAs...() that had parameters that specified default values.

[David Blaikie via llvm-commits]

On Wed, Jan 11, 2017 at 12:26 PM Adrian Prantl <aprantl at apple.com> wrote:

>
> > On Jan 11, 2017, at 12:07 PM, David Blaikie <dblaikie at gmail.com> wrote:
> >
> >
> >
> > On Wed, Jan 11, 2017 at 12:00 PM Adrian Prantl <aprantl at apple.com>
> wrote:
> >>
> >> > On Jan 11, 2017, at 11:33 AM, David Blaikie <dblaikie at gmail.com>
> wrote:
> >> >
> >> >
> >> >
> >> > On Wed, Jan 11, 2017 at 11:08 AM Greg Clayton <clayborg at gmail.com>
> wrote:
> >> >>
> >> >>> On Jan 11, 2017, at 10:36 AM, David Blaikie <dblaikie at gmail.com>
> wrote:
> >> >>>
> >> >>> Would it make sense to push the value retrieval down into
> DWARFFormValue to separate getting the value from a DIE from getting the
> value into some particular type - they seem pretty orthogonal & it's odd
> that DWARFDie handles the type handling side of that rather than an
> attribute/form abstraction.
> >> >>>
> >> >>> Perhaps it'd be a bit abusive, but we could even overload
> op[](dwarf::Attribute) on DWARFDie for easy map-like attribute lookup.
> Maybe 'find' would be a better name, though.
> >> >>>
> >> >>> I'm OK omitting the 'attribute' part of the name entirely if
> everyone's OK with overloading in this context & if the callsites tend to
> be pretty self documenting. (mostly if you're searching for a particular
> attribute you've probably got the attribute name in a literal at the
> callsite anyway, right?)
> >> >>>
> >> >>> So code like this, perhaps:
> >> >>>
> >> >>>   x = Die.find(DW_AT_name).asString();
> >> >>>
> >> >>> or perhaps:
> >> >>>
> >> >>>   x = asString(Die.find(DW_AT_name));
> >> >>>
> >> >>> (so that find/findAttr/[] whatever we call it can continue to
> return Optional<DWARFFormValue> which asString can pass along as NOne if
> it's passed None (or passed a non-string value), and otherwise decompose
> into a string, etc)
> >> >>>
> >> >>
> >> >> We basically have find already in:
> >> >>
> >> >> Optional<DWARFFormValue>
> DWARFDie::getAttributeValue(dwarf::Attribute);
> >> >>
> >> >> We can easily rename it.
> >> >>
> >> >> If find returns an optional then your first code example would need
> to be:
> >> >>
> >> >> x = Die.find(DW_AT_name).getValueOr(DWARFFormValue()).asString();
> >> >>
> >> >> The second form could be made to work but it doesn’t look as natural
> IMHO.
> >> >>
> >> >> Because of the first is awkward and the second seems less clear than
> just asking the DIE for the value I do think there is value in having the
> calls on DWARFDie. Let me know what you think.
> >> >
> >> > I'd still probably go towards the second - for orthogonality of
> features. Means a DWARFFormValue can be passed around and its value
> retrieved later, etc.
> >> >
> >> > But would be good to hear from Adrian too.
> >>
> >> No matter which version we end up using I would expect any heavy users
> of this to re-wrap it in something more specialized again so
> >>
> >>   x = asString(Die.find(DW_AT_name).getValueOr(DWARFFormValue()));
> >>
> >> becomes
> >>
> >>   x = getAttrAsOr<StringRef>(Die, DW_AT_name, "")
> >
> > Not sure I follow. I would figure:
> >
> >   x = asString(Die.find(DW_AT_name)).getValueOr("");
> >
>
> Ok. That is different from how I understood the proposal, since asString
> here takes and returns an Optional. This also answers my other question,
> since we then can do getValueOr to unwrap the optional result of asString.
>
>
> > to be pretty readable/not be worth having wrappers, but I can see what
> you mean.
>
> It's not awesome :-)
> The order of execution/reading is:
>   2            1                 3
>   asString(Die.find(DW_AT_name)).getValueOr("");
>

It's certainly not uncommon to mix member and non-member function calls in
expressions...


> Sure it's readable, but it takes a second look to correctly parse it
> mentally.
>

Yep. Alternatively, if we had unique names for these functions (perhaps put
them in a 'dwarf' namespace under llvm) we could potentially have:

Optional<const char*> toString(const DWARFFormValue&)

Die.find(DW_AT_name).apply(toString).getValueOr("");

perhaps (adding an Optional<T>::apply(Functor) function - but it'd have to
collapse Optionals on the return value). It'd be nicer because toString
wouldn't have to support an Optional parameter, but would add some
complexity to Optional (& really 'apply' should probably be a free function
since it's not in std::optional - which would defeat some of the point of
the readability improvements).


>
> > I'm not sure if there would be a nice way to keep the orthogonality (so
> that asString/asAddress/etc didn't all need to support/reimplement default
> handling) while making it much more terse. It seems like there are 3
> different things happening there.
>
> Would it make sense to provide both variants?
> I think it would be fine to compose the terse variant out of the generic
> orthogonal building blocks.
>

We certainly could. It wasn't clear to me if it was worth adding the
wrappers for each of these. Whether shaving off a few characters at the
call sites helped that much.


>
> >
> > But, yes, if it comes up often enough to be annoyingly verbose -
> sacrificing the orthogonality (by creating/paying in the form of
> duplication) for convenience by writing asFoo functions that handle the
> defaulting - asString(Die.find(DW_AT_name), "") - same as we had before,
> except now the conversion is orthogonal to the lookup so adding "bool
> recurse"/"find_recursively" doesn't explode the API surface area along
> another axis.
> >
> >> to reclaim some readability again. From that point of view I don't
> think the decision of where asString lives is very important. I am
> wondering how to pass any default value other than an empty value into
> getValueOr(DWARFFormValue Default)? Is it easy to construct an on-the-fly
> DWARFFormValue for e.g., a string?
> >
> > I don't think we'd want people to construct DWARFFormValues to represent
> values that weren't present in the DWARF as a main/common means of handling
> situations like this. That seems to be the wrong layer - I think at the
> point you want an appropriately typed value, then you collapse the
> optionality and values pace into that type if you want to, until then it
> remains Optional.
> >
>
> Yes, I think I was misguided by the
>   asString(Die.find(DW_AT_name).getValueOr(DWARFFormValue()))
> example.
>
> -- adrian
> >>
> >> >
> >> >
> >> > Another option that Adrian might have thoughts on, would be to remove
> the Optional wrapper around the DWARFFormValue from getAttributeValue. It's
> not something I'd be totally happy with - Optional's nice and explicit
> about the fact that there might not be a value, but if DWARFFormValue
> already has a "not present" state in it, we could arguably use that state
> to indicate that.
> >>
> >> I think the answer to this depends on my previous question.
> >> -- adrian
> >
>
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