[lldb-dev] Heterogeneous debugging and HSA

[Simon Brand via lldb-dev]

Hi everyone,

I am a member of the HSA Foundation tools group, who are attempting to 
standardise debugging and profiling interfaces for heterogeneous systems.

HSA is a low-level heterogeneous system architecture and software 
platform. There are standards describing the architectural requirements, 
an LLVM-IR-like intermediate language for portable HSA applications 
(HSAIL), and a runtime interface for interacting with the system. We are 
currently developing a standard for debugging HSA applications in a 
vendor-independent manner.

By working together with other communities, we are hoping to extend 
existing debuggers and standards to support such systems. To this end, 
I'm hoping to start a conversation with you all about how LLDB could be 
augmented to support heterogeneous debugging, and to ask for input about 
the standards we are writing.

To aid these discussions and provide a testbed for ideas, I have 
modified LLDB to support debugging HSA programs on AMD's hardware. My 
aim for this project is to adapt it to the standard HSA debugging 
interface once this is completed, so that it could debug HSA 
applications on any vendor's implementation.

Currently, the changes I have made to LLDB are in no state for 
upstreaming to trunk LLDB, as I have made some HSA-specific alterations 
to core parts of the code base. My hope is that, aided by discussion 
with the community, I can abstract out these modifications to support 
the necessary features in an elegant and extensible manner. This would 
most likely involve changing the threading model of LLDB to support 
groups of threads which run on different devices and a "some-stop" model 
by which some thread groups can continue to execute whilst others are 
stopped. I plan on starting work on this soon, but I wonder if anyone 
else is already exploring this avenue?

If you would like to examine the code for my LLDB port or try it out, 
you can get the code here: 
https://github.com/HSAFoundation/HSA-Debugger-LLDB-Source

For our debugging standard, we are currently trying to work out the best 
way to express the multiple address spaces of HSA in DWARF and get an 
idea of how this is handled by GDB and LLDB. Our current thinking is to 
use DW_AT_address_class for pointers types and DW_op_xdref for 
variables. I couldn't see anywhere these are handled in LLDB; is there 
any ongoing work to add them? Or would you recommend a different approach?

Feel free to send me any queries or issues regarding the LLDB 
implementation or the HSA standards work.

Thanks,

Simon Brand

-- 
Simon Brand
Staff Software Engineer
Codeplay Software Ltd
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