[llvm-dev] ESP32 Tensilica Xtensa LX6 backend. Interest? Prior attempts?

[Ryan Houdek via llvm-dev]

Hello all,
I'm currently working on a backend for the CPU within ESP32 MCUs. Which is
a Tensilica Xtensa LX6 based CPU.
Some information about the CPU, it's an in-order design with flexible build
options.
These build options include 32bit float instruction, "zero" overhead loops,
MAC16 DSP operations, boolean registers, a 32 or 64 register large register
file with 16 registers exposed in the ISA at any given moment, and SMP
support.
The ISA can be flexible in size with both 24bit and 16bit instruction
encodings and has a few other features.

I'm targeting the Hardkernel ODROID-GO[1] device with this backend, which
is running an ESP32-WROVER chip which supports most of the features with
its LX6 CPU core.

The work on my backend is coming up fast, with object files already being
generated and running on the device with a weekend's worth of work or so.
There is an interest in the community of this device to support more
languages than the original GCC based toolchain supports. Which is where
the LLVM backend comes in.

With that in mind, I do know that people in the past have attempted working
on backends for Xtensa targets. All of these that I could find have died
[2][3][4]. I'm going to want to upstream this work, but I'm curious at what
point it should be merged to cause as little friction as possible.
Currently it is in a state where code is running but it is of course in
heavy development and improving.

I'm planning on getting it to a stable state, where the LLVM backend can
co-exist with the GCC toolchain. Which is going to be a longtime time
investment. Their toolchain has a large amount of GCC-isms stuck in it that
will take a fairly large amount of time to fully weed out in the near-term.
I'm not planning on dropping the code and vanishing for the foreseeable
future due to this.

So the question is, at what point should I start upstreaming code? Right
now, where it is working with a fairly minimal skeleton structure? Some
arbitrary point in the future where it can compile most applications for
these devices? Couple months to a year down the road when it has stabilized
and bloated to the size of a regular target?

In addition, what sort of interest is there from other people in this
mailing list for this target? Are there some Xtensa lovers hiding that
would want this? Is the maintenance burden too much for people to want this
to be upstreamed?

Leaving off, here are my current repos for the target. Loads of activity
happen on them, probably won't be the same commit when you see this mail.
https://github.com/Sonicadvance1/llvm-xtensa
https://github.com/Sonicadvance1/clang-xtensa


[1]
https://www.hardkernel.com/main/products/prdt_info.php?g_code=G152875062626
[2] https://github.com/jdiez17/llvm-xtensa
[3] https://github.com/NyxCode/xtensa-llvm
[4] https://github.com/afonso360/llvm-xtensa
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