[llvm-dev] LLVM on bare-metal

[Brian Clarkson via llvm-dev]

Hi Peter

Thank you for your helpful comments, especially on the RPI.  Since my 
use case is lot simpler than compiling all of Clang, I hopefully can 
take your experience as a good sign.

The RTOS that TI provides for the AM335x actually has pretty complete 
posix layer and other standard libraries.  However, I am working without 
any virtual memory subsystem, so no mmap.  However, I was under the 
impression that LLVM (ORC specifically) should be able to relocate code 
at any memory location so the lack of mmap shouldn't be a problem?

Kind regards
Brian

Orthogonal Devices
Tokyo, Japan
www.orthogonaldevices.com

On 27/06/2019 18:41, Peter Smith wrote:
> On Thu, 27 Jun 2019 at 09:50, Brian Clarkson via llvm-dev
> <llvm-dev at lists.llvm.org> wrote:
>> Hello!
>>
>> Q1
>> Are there any resources or examples on embedding LLVM into an ARM-based
>> bare-metal application?  Searching in this area only turns up
>> information on how to use LLVM to target bare-metal when I want to
>> compile LLVM for linking against a bare-metal application.
>>
> I'm not aware of any examples unfortunately. I suspect that this could
> be quite challenging depending on how rich an environment your RTOS
> offers. It is possible that LLVM depends on Posix or Posix like OS
> calls for things like mmap and other file abstractions. I've not
> looked at this in any detail as it may be possible to strip these out
> with the right configuration options, for example thread support can
> be disabled. One possible approach would be to build LLVM for a linux
> target and look at the dependencies. That might give you an idea of
> what your are up against.
>
>> Q2
>> Are there any memory usage benchmarks for LLVM across the common tasks
>> (especially loading bytecode, doing the optimization passes and finally
>> emitting machine code)?  My target (embedded) system has only 1GB of RAM.
>>
> I don't have anything specific unfortunately. It is, or at least was
> possible a couple of years ago, for Clang to compile Clang on a 1GB
> Raspberry Pi. I'm assuming the plugins will be smaller than the IR
> generated by the largest Clang C++ file, but my Rasberry PI wasn't
> doing anything else but compiling Clang.
>
>> Background:
>> I'm about to embark on an effort to integrate LLVM into my bare-metal
>> application (for AM335x, Cortex-A8, also known as beaglebone black).
>> The application area is sound synthesis and the reason for embedding
>> LLVM is to allow users to develop their own "plugins" on the desktop
>> (using a live coding approach) and then load them (as LLVM bytecode) on
>> the embedded device.  LLVM would be responsible for generating
>> (optimized, and especially vectorized for NEON) machine code directly on
>> the embedded device and it would take care of the relocation and
>> run-time linking duties.  This last task is very important because the
>> RTOS (Texas Instrument's SYS/BIOS) that I'm using  does not have any
>> dynamic linking facilities. Sharing code in the form of LLVM bytecode
>> also seems to sidestep the complex task of setting up a cross-compiling
>> toolchain which is something that I would prefer not to have to force my
>> users to do. In fact, my goal is to have a live coding environment
>> provided as a desktop application (which might also embed Clang as well
>> as LLVM) that allows the user to rapidly and playfully build their sound
>> synthesis idea (in simple C/C++ at first, Faust later maybe) and then
>> save the algorithm as bytecode to be copied over to the AM335x-based
>> device.
>>
>> Thank you in advance for any help or pointers to resources that you can
>> provide!
>>
>> Kind regards
>> Brian
>>
>> --
>> Orthogonal Devices
>> Tokyo, Japan
>> www.orthogonaldevices.com
>>
>>
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