[llvm-dev] LLVM on bare-metal

[Peter Smith via llvm-dev]

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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