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 <ma...@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-comm...@lists.llvm.org> >> wrote: >> >> >> >>> -----Original Message----- >>> From: Matthias Braun [mailto:ma...@braunis.de] >>> Sent: Wednesday, October 10, 2018 3:50 PM >>> To: Robinson, Paul >>> Cc: jing...@apple.com; v...@apple.com; llvm-comm...@lists.llvm.org; lldb- >>> d...@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- >>> comm...@lists.llvm.org> wrote: >>>> >>>> >>>> >>>>> -----Original Message----- >>>>> From: jing...@apple.com [mailto:jing...@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 <v...@apple.com> wrote: >>>>>> >>>>>>> On Oct 10, 2018, at 9:16 AM, Matthias Braun <ma...@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 >> >> _______________________________________________ >> llvm-commits mailing list >> llvm-comm...@lists.llvm.org >> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-commits > _______________________________________________ lldb-dev mailing list lldb-dev@lists.llvm.org http://lists.llvm.org/cgi-bin/mailman/listinfo/lldb-dev
