[llvm-dev] Source locations missing when using xray-account
Matthew Pickering via llvm-dev
llvm-dev at lists.llvm.org
Thu Dec 13 08:46:24 PST 2018
David,
Thanks a lot, this fixed the problem with symbol locations. Three more
problems now arise which you might be able to help with.
1. Compiling with `-O1` causes an error in the debug information
verifier: "inlinable function call in a function with debug info must
have a !dbg location"
2. Compiling with `-O2` gives a different verifier error: "!dbg
attachment points at wrong subprogram for function"
Are 1. and 2. bugs in the verifier? It seems that optimisation passes
shouldn't affect the validity of debug information.
3. Compiling with `-O0`, the verifier succeeds and then using
llvm-stack, the complete call hierarchy is completely flat
(an example run for loading into chrome://tracing -
https://gist.githubusercontent.com/mpickering/a6dbaaeaa281b812cf3064a45e73a043/raw/9e5cdfffca1eb1ad269892af6bf7681ff195499e/chrome-tracing.json)
output of llvm-xray stack:
https://gist.github.com/715859a15c088108a8114fcbf68926a5
log.yaml if that helps. https://gist.github.com/86fa7d8252967f6951b63d7a220f2846
Notice that level is 0 for all the entries. Any ideas what could cause this?
Cheers,
Matt
On Sun, Dec 9, 2018 at 9:13 PM David Blaikie <dblaikie at gmail.com> wrote:
>
>
>
> On Thu, Dec 6, 2018 at 4:58 AM Matthew Pickering <matthewtpickering at gmail.com> wrote:
>>
>> Hi David,
>>
>> Sorry for taking a few days to reply. It's not easy for you to compile
>> a Haskell file to see the problem as the debug information is still
>> WIP. Below I prove the IR for a simple hello world program which you
>> can feed into llc.
>>
>> https://gist.github.com/05296933e37e87533a51d493b46aa48d
>>
>> The `out.ir` file can be passed straight to `llc`.
>>
>> Can you see anything obviously wrong?
>
>
> Unless I'm missing something else, I believe none of the instructions have !dbg locations attached (see, for example, a simple empty main.cpp compiled to LLVM IR by Clang, and note the !dbg attached to the ret instruction):
>
> $ clang++-tot -emit-llvm -S -g -c -o - main.cpp
> ; ModuleID = 'main.cpp'
> source_filename = "main.cpp"
> target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
> target triple = "x86_64-unknown-linux-gnu"
>
> ; Function Attrs: noinline norecurse nounwind optnone uwtable
> define dso_local i32 @main() #0 !dbg !7 {
> entry:
> ret i32 0, !dbg !11
> }
>
> attributes #0 = { noinline norecurse nounwind optnone uwtable "correctly-rounded-divide-sqrt-fp-math"="false" "disable-tail-calls"="false" "less-precise-fpmad"="false" "min-legal-vector-width"="0" "no-frame-pointer-elim"="true" "no-frame-pointer-elim-non-leaf" "no-infs-fp-math"="false" "no-jump-tables"="false" "no-nans-fp-math"="false" "no-signed-zeros-fp-math"="false" "no-trapping-math"="false" "stack-protector-buffer-size"="8" "target-cpu"="x86-64" "target-features"="+fxsr,+mmx,+sse,+sse2,+x87" "unsafe-fp-math"="false" "use-soft-float"="false" }
>
> !llvm.dbg.cu = !{!0}
> !llvm.module.flags = !{!3, !4, !5}
> !llvm.ident = !{!6}
>
> !0 = distinct !DICompileUnit(language: DW_LANG_C_plus_plus, file: !1, producer: "clang version 8.0.0 (trunk 348415) (llvm/trunk 348416)", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug, enums: !2, nameTableKind: None)
> !1 = !DIFile(filename: "main.cpp", directory: "/usr/local/google/home/blaikie/dev/scratch/ghc_symbolizer_failure")
> !2 = !{}
> !3 = !{i32 2, !"Dwarf Version", i32 4}
> !4 = !{i32 2, !"Debug Info Version", i32 3}
> !5 = !{i32 1, !"wchar_size", i32 4}
> !6 = !{!"clang version 8.0.0 (trunk 348415) (llvm/trunk 348416)"}
> !7 = distinct !DISubprogram(name: "main", scope: !1, file: !1, line: 1, type: !8, scopeLine: 1, flags: DIFlagPrototyped, spFlags: DISPFlagDefinition, unit: !0, retainedNodes: !2)
> !8 = !DISubroutineType(types: !9)
> !9 = !{!10}
> !10 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
> !11 = !DILocation(line: 2, column: 1, scope: !7)
>
>>
>> Matt
>>
>>
>>
>> On Mon, Dec 3, 2018 at 6:37 PM David Blaikie <dblaikie at gmail.com> wrote:
>> >
>> > Yeah, not sure what GHC might be doing to generate novel DWARF that llvm-symbolizer can't consume - if you could create a small example (preferably source I can compile, but also LLVM IR (since it'll save me having to setup GHC, hopefully) I could feed into clang/llvm and get an object file out that demonstrates the llvm-symbolizer failure) that'd be helpful.
>> >
>> > It may also be useful to compare Clang's behavior on similar C or C++ source to see if there are any critical differences in the DWARF that might explain the difference in behavior.
>> >
>> > - Dave
>> >
>> > On Mon, Dec 3, 2018 at 4:46 AM Dean Michael Berris <dean.berris at gmail.com> wrote:
>> >>
>> >>
>> >>
>> >> > On 3 Dec 2018, at 23:01, Matthew Pickering via llvm-dev <llvm-dev at lists.llvm.org> wrote:
>> >> >
>> >> > Is anyone able to help me with this or suggest where I might be able
>> >> > to get help?
>> >>
>> >> You’ve asked the right place, apologies for the delay I’m supposed to be answering these questions.
>> >>
>> >> >
>> >> > Matt
>> >> > On Mon, Nov 26, 2018 at 3:02 PM Matthew Pickering
>> >> > <matthewtpickering at gmail.com> wrote:
>> >> >>
>> >> >> Hi all,
>> >> >>
>> >> >> I am trying to add XRay support to the IR produced by GHC. Getting the
>> >> >> basics working is not too complicated after adding the right function
>> >> >> attribute but any analysis shows the function names but not the
>> >> >> locations of the functions.
>> >> >>
>> >> >> In particular, I run a program which has been instrumented as follows:
>> >> >>
>> >> >> XRAY_OPTIONS="patch_premain=true xray_mode=xray-basic verbosity=1" ./llvm
>> >> >>
>> >> >> which then writes out an xray-log file.
>> >> >>
>> >> >> ==4193==XRay: Log file in 'xray-log.llvm.hgD9oi'
>> >> >> ==4193==Cleaned up log for TID: 4193
>> >> >>
>> >> >> I then attempt to use the xray-account utility to analyse the log but
>> >> >> whilst the functions are named, the locations of the functions are
>> >> >> displayed as <invalid>:0:0:.
>> >> >>
>> >> >>> llvm-xray account -sort=sum -sortorder=dsc -instr_map ./llvm xray-log.llvm.hgD9oi
>> >> >>
>> >> >> Functions with latencies: 5
>> >> >> funcid count [ min, med, 90p, 99p,
>> >> >> max] sum function
>> >> >> 6 9 [ 0.000006, 0.000006, 0.000040, 0.000040,
>> >> >> 0.000040] 0.000089 <invalid>:0:0: fqux
>> >> >> ... 4 lines omitted
>> >> >>
>> >> >> If I then inspect the executable with llvm-dwarfdump, I can see that
>> >> >> the debug information for fqux clearly lists the location as well.
>> >> >>
>> >> >> 0x00000918: DW_TAG_subprogram
>> >> >> DW_AT_low_pc (0x00000000004207c8)
>> >> >> DW_AT_high_pc (0x0000000000420817)
>> >> >> DW_AT_frame_base (DW_OP_reg7 RSP)
>> >> >> DW_AT_linkage_name ("fqux")
>> >> >> DW_AT_name ("fqux")
>> >> >> DW_AT_decl_file ("TODO/llvm.hs")
>> >> >> DW_AT_decl_line (8)
>> >> >> DW_AT_external (true)
>> >> >>
>> >> >> I suspect this is a problem with the DWARF information as when I try
>> >> >> to use `llvm-symboliser` with address 0x00000000004207c8 as retrieved
>> >> >> from the above paste, the source location is also not reported.
>> >> >>
>> >>
>> >> Yes, this is the issue. Getting llvm-symboliser to understand the DWARF information generated will get you better debugging information for XRay.
>> >>
>> >> >> So, can anyone give me some practical advice about how to troubleshoot
>> >> >> this problem/validate the DWARF information that is produced?
>> >> >>
>> >> >> It might also be helpful is someone could point me to some
>> >> >> documentation which explains at a high-level how llvm-symboliser
>> >> >> works. Reading the source code was too difficult for me as I am not
>> >> >> very experienced with C++ programming.
>> >> >>
>> >>
>> >> I suspect all the usual resources apply — resources on DWARF4, and potentially looking at the various encodings. I’m not a DWARF expert, but
>> >> David Blaikie might be able to help more specifically on emitting useful DWARF information via LLVM.
>> >>
>> >> >> Many thanks for your help,
>> >> >>
>> >>
>> >> Apologies for the delay again.
>> >>
>> >> Cheers
>> >>
>> >> -- Dean
>> >>
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