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

[Matthew Pickering via llvm-dev]

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.

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.

Many thanks for your help,

Matt