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

Wed Oct 10 17:02:01 PDT 2018

Thanks for looking into this!  

When I was first working on inlined stepping, I found a bunch of cases where the line table info and the ranges for the inlined subroutines disagreed.  I remember seeing cases, for instance, where the address given for foo.h:xxx in the line table was contained in one of the debug_info's inlined subroutine blocks from a different file.  I tried for a little while to put in heuristics to try to work past the disagreement.  But that was too easy to get wrong, and when I got that wrong it often had the result of turning a step into a continue.  It is annoying to stop too early, but it is much worse to stop too late (or never).  So I ended up bagging my attempts at heroics and whenever I get to a place where the line table and the inlined_subroutine bits of info are out of sync, I just stop.

Jim

> On Oct 10, 2018, at 4:34 PM, Matthias Braun <matze at braunis.de> wrote:
> 
> 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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> 