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?
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
>>