llvm dev - Nov 2018 - Source locations missing when using xray-account

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

Is anyone able to help me with this or suggest where I might be able
to get help?

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.
>
> 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

> 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