labath marked an inline comment as done.
labath added inline comments.
================
Comment at: lib/DebugInfo/DWARF/DWARFDebugLoc.cpp:291-295
+ EntryIterator Absolute =
+ getAbsoluteLocations(
+ SectionedAddress{BaseAddr, SectionedAddress::UndefSection},
+ LookupPooledAddress)
+ .begin();
----------------
dblaikie wrote:
> labath wrote:
> > dblaikie wrote:
> > > labath wrote:
> > > > dblaikie wrote:
> > > > > labath wrote:
> > > > > > This parallel iteration is not completely nice, but I think it's
> > > > > > worth being able to reuse the absolute range computation code. I'm
> > > > > > open to ideas for improvement though.
> > > > > Ah, I see - this is what you meant about "In particular it makes it
> > > > > possible to reuse this stuff in the dumping code, which would have
> > > > > been pretty hard with callbacks.".
> > > > >
> > > > > I'm wondering if that might be worth revisiting somewhat. A full
> > > > > iterator abstraction for one user here (well, two once you include
> > > > > lldb - but I assume it's likely going to build its own data structure
> > > > > from the iteration anyway, right? (it's not going to keep the
> > > > > iterator around, do anything interesting like partial iterations,
> > > > > re-iterate/etc - such that a callback would suffice))
> > > > >
> > > > > I could imagine two callback APIs for this - one that gets entries
> > > > > and locations and one that only gets locations by filtering on the
> > > > > entry version.
> > > > >
> > > > > eg:
> > > > >
> > > > > // for non-verbose output:
> > > > > LL.forEachEntry([&](const Entry &E, Expected<DWARFLocation> L) {
> > > > > if (Verbose && actually dumping debug_loc)
> > > > > print(E) // print any LLE_*, raw parameters, etc
> > > > > if (L)
> > > > > print(*L) // print the resulting address range, section name
> > > > > (if verbose),
> > > > > else
> > > > > print(error stuff)
> > > > > });
> > > > >
> > > > > One question would be "when/where do we print the DWARF expression" -
> > > > > if there's an error computing the address range, we can still print
> > > > > the expression, so maybe that happens unconditionally at the end of
> > > > > the callback, using the expression in the Entry? (then, arguably, the
> > > > > expression doesn't need to be in the DWARFLocation - and I'd say make
> > > > > the DWARFLocation a sectioned range, exactly the same type as for
> > > > > ranges so that part of the dumping code, etc, can be maximally reused)
> > > > Actually, what lldb currently does is that it does not build any data
> > > > structures at all (except storing the pointer to the right place in the
> > > > debug_loc section. Then, whenever it wants to do something to the
> > > > loclist, it parses it afresh. I don't know why it does this exactly,
> > > > but I assume it has something to do with most locations never being
> > > > used, or being only a couple of times, and the actual parsing being
> > > > fairly fast. What this means is that lldb is not really a single
> > > > "user", but there are like four or five places where it iterates
> > > > through the list, depending on what does it actually want to do with
> > > > it. It also does partial iteration where it stops as soon as it find
> > > > the entry it was interested in.
> > > > Now, all of that is possible with a callback (though I am generally
> > > > trying to avoid them), but it does resurface the issue of what should
> > > > be the value of the second argument for DW_LLE_base_address entries
> > > > (the thing which I originally used a error type for).
> > > > Maybe this should be actually one callback API, taking two callback
> > > > functions, with one of them being invoked for base_address entries, and
> > > > one for others? However, if we stick to the current approaches in both
> > > > LLE and RLE of making the address pool resolution function a parameter
> > > > (which I'd like to keep, as it makes my job in lldb easier), then this
> > > > would actually be three callbacks, which starts to get unwieldy. Though
> > > > one of those callbacks could be removed with the "DWARFUnit
> > > > implementing a AddrOffsetResolver interface" idea, which I really like.
> > > > :)
> > > Ah, thanks for the details on LLDB's location parsing logic. That's
> > > interesting indeed!
> > >
> > > I can appreciate an iterator-based API if that's the sort of usage we've
> > > got, though I expect it doesn't have any interest in the low-level
> > > encoding & just wants the fully processed address ranges/locations - it
> > > doesn't want base_address or end_of_list entries? & I think the
> > > dual-iteration is a fairly awkward API design, trying to iterate them in
> > > lock-step, etc. I'd rather avoid that if reasonably possible.
> > >
> > > Either having an iterator API that gives only the fully processed
> > > data/semantic view & a completely different API if you want to access the
> > > low level primitives (LLE, etc) (this is how ranges works - there's an
> > > API that gives a collection of ranges & abstracts over v4/v5/rnglists/etc
> > > - though that's partly motivated by a strong multi-client need for that
> > > functionality for symbolizing, etc - but I think it's a good
> > > abstraction/model anyway (& one of the reasons the inline range list
> > > printing doesn't include encoding information, the API it uses is too
> > > high level to even have access to it))
> > >
> > > > Now, all of that is possible with a callback (though I am generally
> > > > trying to avoid them), but it does resurface the issue of what should
> > > > be the value of the second argument for DW_LLE_base_address entries
> > > > (the thing which I originally used a error type for).
> > >
> > > Sorry, my intent in the above API was for the second argument to be
> > > Optional's "None" state when... oh, I see, I did use Expected there,
> > > rather than Optional, because there are legit error cases.
> > >
> > > I know it's sort of awkward, but I might be inclined to use
> > > Optional<Expected<AddressRange>> there. I realize two layers of wrapping
> > > is a bit weird, but I think it'd be nicer than having an error state for
> > > what, I think, isn't erroneous.
> > >
> > > > Maybe this should be actually one callback API, taking two callback
> > > > functions, with one of them being invoked for base_address entries, and
> > > > one for others? However, if we stick to the current approaches in both
> > > > LLE and RLE of making the address pool resolution function a parameter
> > > > (which I'd like to keep, as it makes my job in lldb easier), then this
> > > > would actually be three callbacks, which starts to get unwieldy.
> > >
> > > Don't mind three callbacks too much.
> > >
> > > > Though one of those callbacks could be removed with the "DWARFUnit
> > > > implementing a AddrOffsetResolver interface" idea, which I really like.
> > > > :)
> > >
> > > Sorry, I haven't really looked at where the address resolver callback is
> > > registered and alternative designs being discussed - but yeah, going off
> > > just the one-sentence, it seems reasonable to have the DWARFUnit own an
> > > address resolver/be the thing you consult when you want to resolve an
> > > address (just through a normal function call in DWARFUnit, perhaps -
> > > which might, internally, use a callback registered when it was
> > > constructed).
> > > I know it's sort of awkward, but I might be inclined to use
> > > Optional<Expected<AddressRange>> there. I realize two layers of wrapping
> > > is a bit weird, but I think it'd be nicer than having an error state for
> > > what, I think, isn't erroneous.
> > Actually, my very first attempt at this patch used an
> > `Expected<Optional<Whatever>>`, but then I scrapped it because I didn't
> > think you'd like it. It's not the friendliest of APIs, but I think we can
> > go with that.
> >
> > > Sorry, I haven't really looked at where the address resolver callback is
> > > registered and alternative designs being discussed - but yeah, going off
> > > just the one-sentence, it seems reasonable to have the DWARFUnit own an
> > > address resolver/be the thing you consult when you want to resolve an
> > > address (just through a normal function call in DWARFUnit, perhaps -
> > > which might, internally, use a callback registered when it was
> > > constructed).
> >
> > I think you got that backwards. I don't want the DWARFUnit to be the source
> > of truth for address pool resolutions, as that would make it hard to use
> > from lldb (it's far from ready to start using the llvm version right now).
> > What I wanted was to replace the lambda/function_ref with a single-method
> > interface. Then both DWARFUnits could implement that interface so that
> > passing a DWARFUnit& would "just work" (but you wouldn't be limited to
> > DWARFUnits as anyone could implement that interface, just like anyone can
> > write a lambda).
> As for Expected<Optional<Whatever>> (or Optional<Expected<>>) - yeah, I think
> this is a non-obvious API (both the general problem and this specific
> solution). I think it's probably worth discussing this design a bit more to
> save you time writing/rewriting things a bit. I guess there are a few layers
> of failure here.
>
> There's the possibility that the iteration itself could fail - even for
> debug_loc style lists (if we reached the end of the section before
> encountering a terminating {0,0}). That would suggest a fallible iterator
> idiom:
> http://llvm.org/docs/ProgrammersManual.html#building-fallible-iterators-and-iterator-ranges
>
> But then, yes, when looking at the "processed"/semantic view, that could fail
> too in the case of an invalid address index, etc.
>
> The generic/processed/abstracted-over-ranges-and-rnglists API for ranges
> produces a fully computer vector (& then returns Expected<vector> of that
> range) - is that reasonable? (this does mean manifesting a whole location in
> memory, which may not be needed so I could understand avoiding that even
> without fully implementing & demonstrating the vector solution is inadequate).
>
> But I /think/ maybe the we could/should have two APIs - one generic API that
> abstracts over loc/loclists and only provides the fully processed view, and
> another that is type specific for dumping the underlying representation (only
> used in dumping debug_loclists).
If we were computing the final address ranges from scratch (which would be the
best match for the current lldb usage, but which I am not considering now for
fear of changing too many things), then I agree that we would need the
fallible_iterator iterator thingy. But in this case we are "interpreting" the
already parsed ranges, so we can assume some level of correctness here, and the
thing that can fail is only the computation of a single range, which does not
affect our ability to process the next entry.
This indicates to me that either each entry in the list should be an
Expected<>, or that the invalid entries should be just dropped (possibly
accompanied by some flag which would tell the caller that the result was not
exhaustive).
This is connected to one of the issues I have with the debug ranges API -- it
tries _really_ hard to return *something* -- if resolving the indirect base
address entry fails, it is perfectly happy to use the address _index_ as the
base address. This makes sense for dumping, where you want to show something
(though it would still be good to indicate that you're not showing a real
address), but it definitely does not help consumers which then need to make
decisions based on the returned data.
Anyway, yes, I agree that we need to APIs, and probably callbacks are the
easiest way to achieve that. We could have a "base" callback that is not
particularly nice to use, but provides the full information via a combination
of `UnparsedLL` and `Optional<Expected<ParsedLL>>` arguments. The dumper could
use that to print out everything it needs. And then we could have a second API,
built on top of the first one, which ignores base address entries and the raw
data and returns just a bunch of `Expected<ParsedLL>`. This could be used by
users like lldb, who just want to see the final data. The `ParsedLL` type would
be independent of the location list type, so that the debug_loc parser could
provide the same kind of API (but implemented on top of something else, as the
`UnparsedLL` types would differ). Also, under the hood, the location list
dumper for debug_loclists (but not debug_loc) could reuse some implementation
details with the debug_rnglists dumper via a suitable combination of templates
and callbacks.
How does that sound?
Repository:
rL LLVM
CHANGES SINCE LAST ACTION
https://reviews.llvm.org/D68270/new/
https://reviews.llvm.org/D68270
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