I reverted things in r344318 now. > On Oct 10, 2018, at 5:02 PM, Jim Ingham <jing...@apple.com> wrote: > > 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. Makes total sense from lldbs point of view. If anything this is something a verifier could catch during testing, but it's a small issue so I'm not sure it's worth starting this now.
- Matthias > > 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
