[lldb-dev] [llvm] r343874 - DwarfDebug: Pick next location in case of missing location at block begin

Matthias Braun via lldb-dev lldb-dev at lists.llvm.org
Wed Oct 10 16:34:55 PDT 2018

1) So I went and figured out why the lldb testcase at hand fails.

- It seems the debugger stepping logic will follow the program along until it finds a new source location. However if that new source location is part of a new DW_AT_abstract_location it is ignored in the step over mode.
- In the testcase at hand the .loc location of an inlined function was moved to an earlier place without the DW_AT_abstract_location entry getting extended. So the debugger mistook the inlined function as the same scope and stepping incorrectly halted there.

On the LLVM side DW_AT_abstract_location is generated by LexicalScopes which is created by lib/CodeGen/LexicalScopes.cpp / extractLexicalScopes() with completely separate code from the line table generation code in lib/CodeGen/AsmPrinter/DwarfDebug.cpp you have to be aware of that and keep the two algorithms in sync :-/

I fixed LexicalScopes.cpp to be in sync and the lldb test works fine again for me.

2) However talking to Adrian earlier he also expressed having a bad feeling about moving debug location upwards instead of emitting line-0 entries. So I think consensus here is to rather live with some line table bloat instead.

- Unless there are objections I will not go with option 1) but instead revert this commit tomorrow. Note that I will only revert r343874 (i.e. the behavior change for what to emit when the first instructions of a basic block have no location attached), but not r343895 (removing debuglocs from spill/reload instructions) as I still consider this a sensible commit. So in the end we may have bigger line tables than before my changes, but simpler code in the spill/reload generation and occasionally can avoid random source location when spill/reloads get rescheduled.

- Matthias

> On Oct 10, 2018, at 1:17 PM, via llvm-commits <llvm-commits at lists.llvm.org> wrote:
>> -----Original Message-----
>> From: Matthias Braun [mailto:matze at braunis.de]
>> Sent: Wednesday, October 10, 2018 3:50 PM
>> To: Robinson, Paul
>> Cc: jingham at apple.com; vsk at apple.com; llvm-commits at lists.llvm.org; lldb-
>> dev at lists.llvm.org
>> Subject: Re: [lldb-dev] [llvm] r343874 - DwarfDebug: Pick next location in
>> case of missing location at block begin
>>> On Oct 10, 2018, at 12:18 PM, via llvm-commits <llvm-
>> commits at lists.llvm.org> wrote:
>>>> -----Original Message-----
>>>> From: jingham at apple.com [mailto:jingham at apple.com]
>>>> Sent: Wednesday, October 10, 2018 2:20 PM
>>>> To: Vedant Kumar
>>>> Cc: Robinson, Paul; Vedant Kumar via llvm-commits; LLDB; Matthias Braun
>>>> Subject: Re: [lldb-dev] [llvm] r343874 - DwarfDebug: Pick next location
>> in
>>>> case of missing location at block begin
>>>>> On Oct 10, 2018, at 9:54 AM, Vedant Kumar <vsk at apple.com> wrote:
>>>>>> On Oct 10, 2018, at 9:16 AM, Matthias Braun <matze at braunis.de> wrote:
>>>>>> So I haven't worked much on debug info, but here's the explanation
>> for
>>>> my patches:
>>>>>> My original motivation was getting rid of code some code in the llvm
>>>> codegen that for spills and reloads just picked the next debug location
>>>> around. That just seemed wrong to me, as spills and reloads really are
>>>> bookkeeping, we just move values around between registers and memory
>> and
>>>> none of it is related to anything the user wrote in the source program.
>> So
>>>> not assigning any debug information felt right for these instructions.
>>>> Then people noticed line table bloat because of this and I guess we
>>>> assumed that having exact line-0 annotations isn't that useful that it
>>>> warrants bloating the debug information...
>>>>> Right. This doesn't seem any more arbitrary than reusing the previous
>>>> instruction location, which we do all the time. I think it's a
>> reasonable
>>>> tradeoff.
>>> For spills and reloads, the next valid source location might be
>> reasonable.
>>> For top-of-block instructions, I really don't think so; we had this
>> issue
>>> in FastISel, some years back, and ultimately went with line-0 at top of
>>> block because it caused way fewer problems than hoisting the next valid
>>> source location.
>> I assume what happened in the past was that the previous debug location
>> spilled over to the next basic block when the top-of-the-block instruction
>> had line-0 set. I can immediately see why that is a problem. And I assume
>> that was the case that was overlooked in the past. I cannot see however
>> how taking the following location in the same basic block is a problem.
> Because those instructions actually do something, and whether variables are
> visible and expressions evaluate correctly may depend on those instructions
> being executed before the debugger stops.  If you mark them with line 0 the
> debugger doesn't stop. If you hoist the next source location, it does.
> Spills and reloads in the middle of a block *probably* can be associated
> with the next source line without doing any real damage; although it may
> turn out that reloads need to attach to the previous source line.  I'd
> need to see a variety of examples to be sure, one way or the other.
> But long experience has taught me that instructions at the top of a block
> really are better off at line 0 than at the next real source line.  If
> you can prove they are *always* better off with the next source line, for
> all consumers, I'd be very interested to hear about it.
> But, this thread is not the best place for that; bring it up on llvm-dev
> so other interested parties can see what you're looking for.
> --paulr
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