llvm dev - Feb 2016 - CloneFunction during LTO leads to seg fault?

Hello,

I'm writing an LTO pass and I'd like to be able to duplicate a function 
(with debugging info).  I'm trying to accomplish this with CloneFunction 
but it's leading to a seg fault in ld.

I've whittled down my problem so that it occurs in this small pass [1].

If I run this pass with opt, I get the expected result (i.e. a valid 
program that calls main twice).  If I run the pass during LTO, ld seg 
faults.  Here is a pastebin of when ld seg faults in lldb [2].

If I set the third parameter of CloneFunction (ModuleLevelChanges) to 
false, then then compilation will complete -- but I lose debugging 
information in the cloned function.  I really want to preserve the debug 
info.

Any idea what is wrong?  Is this a bug?  Usually when I screw up the 
module the verifier catches the problem.  This is getting to I think 
CodeGen then crashing.

I've found that the module itself needs to be non-trivial to cause the 
crash.  Here is the module I'm testing with [3].

In case it is relevant: to get my pass to run during LTO I added 
"PM.add(createHelloPass())" to 
PassMangerBuilder::populateLTOPassManager.  I'm using binutils-gold on 
Linux (Ubuntu 14.04 LTS).

Thank you,
Scott A. Carr
PhD Candidate
Purdue University

[1] http://pastebin.com/uL6QMtcT

[2] http://pastebin.com/i1gd0gE6

[3] http://pastebin.com/GEG0F8kU

> On Feb 9, 2016, at 10:24 AM, Scott A. Carr via llvm-dev <llvm-dev at
lists.llvm.org> wrote:
> 
> Hello,
> 
> I'm writing an LTO pass and I'd like to be able to duplicate a
function (with debugging info).  I'm trying to accomplish this with
CloneFunction but it's leading to a seg fault in ld.
> 
> I've whittled down my problem so that it occurs in this small pass [1].
> 
> If I run this pass with opt, I get the expected result (i.e. a valid
program that calls main twice).  If I run the pass during LTO, ld seg faults. 
Here is a pastebin of when ld seg faults in lldb [2].
> 
> If I set the third parameter of CloneFunction (ModuleLevelChanges) to
false, then then compilation will complete -- but I lose debugging information
in the cloned function.  I really want to preserve the debug info.
> 
> Any idea what is wrong?  Is this a bug?  Usually when I screw up the module
the verifier catches the problem.  This is getting to I think CodeGen then
crashing.

What could help is to pass -mllvm -print-after-all to ld and get the IR before
and after your pass ran, and also just before CodeGen.
> 
> I've found that the module itself needs to be non-trivial to cause the
crash.  Here is the module I'm testing with [3].
> 
> In case it is relevant: to get my pass to run during LTO I added
"PM.add(createHelloPass())" to
PassMangerBuilder::populateLTOPassManager.  I'm using binutils-gold on Linux
(Ubuntu 14.04 LTS).
You may want to try to add it at the end of the pipeline, in case something does
not play well with optimizations (just trying to pinpoint where the issue is).

-- 
Mehdi


> 
> Thank you,
> Scott A. Carr
> PhD Candidate
> Purdue University
> 
> [1] http://pastebin.com/uL6QMtcT
> 
> [2] http://pastebin.com/i1gd0gE6
> 
> [3] http://pastebin.com/GEG0F8kU
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev

Hi Medhi,

Thanks for you reply.  Here is the full output of -print-after-all [1] 
and just the module itself after my pass[2].

I've looked over the IR, but I can't see anything obviously wrong.

I'm not sure what you meant by:
> You may want to try to add it at the end of the pipeline
My pass is the last one added inside populateLTOPassManager.  Should I 
add it to the PassManager somewhere else?

Thanks,
Scott

[1] https://gist.github.com/scottcarr/ce0bb6df1cbb44ec46a5
[2] http://pastebin.com/9xc2jpRj

On 02/09/2016 01:36 PM, Mehdi Amini wrote:
>> On Feb 9, 2016, at 10:24 AM, Scott A. Carr via llvm-dev <llvm-dev at
lists.llvm.org> wrote:
>>
>> Hello,
>>
>> I'm writing an LTO pass and I'd like to be able to duplicate a
function (with debugging info).  I'm trying to accomplish this with
CloneFunction but it's leading to a seg fault in ld.
>>
>> I've whittled down my problem so that it occurs in this small pass
[1].
>>
>> If I run this pass with opt, I get the expected result (i.e. a valid
program that calls main twice).  If I run the pass during LTO, ld seg faults. 
Here is a pastebin of when ld seg faults in lldb [2].
>>
>> If I set the third parameter of CloneFunction (ModuleLevelChanges) to
false, then then compilation will complete -- but I lose debugging information
in the cloned function.  I really want to preserve the debug info.
>>
>> Any idea what is wrong?  Is this a bug?  Usually when I screw up the
module the verifier catches the problem.  This is getting to I think CodeGen
then crashing.
>
> What could help is to pass -mllvm -print-after-all to ld and get the IR
before and after your pass ran, and also just before CodeGen.
>
>> I've found that the module itself needs to be non-trivial to cause
the crash.  Here is the module I'm testing with [3].
>>
>> In case it is relevant: to get my pass to run during LTO I added
"PM.add(createHelloPass())" to
PassMangerBuilder::populateLTOPassManager.  I'm using binutils-gold on Linux
(Ubuntu 14.04 LTS).
> You may want to try to add it at the end of the pipeline, in case something
does not play well with optimizations (just trying to pinpoint where the issue
is).
>