[llvm-dev] Source locations missing when using xray-account

Dean Michael Berris via llvm-dev llvm-dev at lists.llvm.org
Mon Dec 3 04:46:24 PST 2018

> 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.


-- Dean

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