[PATCH] Avoid empty .debug_loc entries and lists

[Duncan P. N. Exon Smith]

> On 2015 Jun 4, at 16:50, David Blaikie <dblaikie at gmail.com> wrote:
> 
> 
> 
> On Thu, Jun 4, 2015 at 4:46 PM, Duncan P. N. Exon Smith <dexonsmith at apple.com> wrote:
> 
> > On 2015 Jun 4, at 16:36, David Blaikie <dblaikie at gmail.com> wrote:
> >
> >
> >
> > On Thu, Jun 4, 2015 at 4:28 PM, Duncan P. N. Exon Smith <dexonsmith at apple.com> wrote:
> >
> > > On 2015 Jun 4, at 16:09, David Blaikie <dblaikie at gmail.com> wrote:
> > >
> > > Judging by the code, does this cause us not to create a variable if it has no locations? That's probably not quite right - even if we have no locations, it's probably a good idea/correct to describe the variable so shadowing, etc, works correctly (if someone tries to print the variable the compiler still finds the right variable but just reports that it doesn't know the location)
> >
> > So the code here already has an early return if there are no ranges:
> >
> >     for (const auto &I : DbgValues) {
> >       InlinedVariable IV = I.first;
> >       if (Processed.count(IV))
> >         continue;
> >
> >       // Instruction ranges, specifying where IV is accessible.
> >       const auto &Ranges = I.second;
> >       if (Ranges.empty())
> >         continue;
> >
> > My patch effectively hits this `continue` more often.  The difference
> > is cases where we *thought* we knew how to emit ranges, but in fact
> > didn't know how.
> >
> > I suppose I assumed the intended behaviour was "don't emit the
> > variable if we don't have any ranges for it" since that's what was
> > going on, but are you saying this `continue` is a bug?  (Or am I
> > totally missing your point?)
> >
> > How do we end up with an empty range list here? That seems strange - I assume we're just lazily creating the rang lists in calculateDbgValueHistory... (I could sort of understand empty entries in a range list - when it turns out the dbg_value intrinsic describes no instructions due to things being optimized away, hoisted here or there, etc).
> 
> No idea, I just assume it happens because there's code.
> 
> Not sure what you mean.
> 
> What I'm trying to understand is whether or not we should be fixing this bug closer to the source (in calculateDbgValueHistory) that's producing empty ranges or empty entries.

One thing that Adrian walked me through is that even if we have
ranges, and we have entries after the call to `buildLocationList()`,
we can still fail to emit any bytes during `DebugLocEntry::finalize()`
since we don't know how to emit every DWARF expression.

Because of that, I changed my approach from "trying to get `Ranges` to
match up with `Entries`" to "popping empty/useless entries and ranges
off the stack", and figured Ranges/Entries inconsistencies were a
compile-time optimization that weren't as urgent.

Maybe we should also be adding the commented out code I had in:
 http://lists.cs.uiuc.edu/pipermail/llvm-commits/Week-of-Mon-20150525/278797.html

It also seems like the `continue` in the current code might be the wrong
thing (and we should create the variable with no DW_AT_location?).

(All of this stuff is a tangent for me.  I wandered in here to remove
the empty .debug_loc entries/lists in the output.)

> What's the right thing to do? 
> 
> I don't have all the context here to know that off-hand. Trying to discuss it to try to wrap my head around enough of it to offer a useful opinion - sorry it takes a while.

Not trying to rush you, sorry if it seems that way.  This isn't really
blocking me (I have other things to work on (although obviously I'd
like to resolve this some time)).

> Should we create a variable the same
> was as the for loop below for function arguments (that doesn't
> attach ranges)?  Should we not create a variable? 
> 
> If we get the same output between creating it here or in the later loop, it doesn't matter in the broad sense.
>  
> Is it actually
> *useful* that we create a .debug_loc list with no entries and point
> to it?
> 
> No, there's no benefit to pointing to an empty loc list compared to just having no DW_AT_location at all.

Thanks for confirming.

I'd be happy to adjust my patch to just address the empty .debug_loc
list/entries and continue to create the same variables that were being
created before (just missing whatever magic makes them create a
`DW_AT_location` -- I'm guessing I just use the constructor logic
that's used for optimized-out function parameters in the loop that
follows?).

(It sounds like it was a poor assumption on my part that we were trying
to avoid emitting variables if they have no ranges.)