Potentially send a patch to LLVM adding a `call0` triple environment for Xtensa

[alexrp]

LLVM defaults to the Windowed ABI for Xtensa, but seemingly supports the Call0 ABI too. These are fundamentally different enough that they can't be intermixed. As such, we should distinguish the Call0 ABI with a triple environment like call0.

However, it's also quite possible that the Call0 ABI is actually dead; Espressif's documentation explicitly omits details about this ABI. If this is the case, then there's nothing to do. But we need to confirm that.

[alexrp]

However, it's also quite possible that the Call0 ABI is actually dead; Espressif's documentation explicitly omits details about this ABI. If this is the case, then there's nothing to do. But we need to confirm that.

@andreisfr I don't suppose you'd be able to clarify this point?

[andreisfr]

Hi @alexrp, could you please explain which LLVM repository you mention EspressifRepo (https://github.com/espressif/llvm-project/) or MainRepo(https://github.com/llvm/llvm-project/? The Xtensa backend in MainRepo now by default use "generic" processor type for llc and this target implements CALL0 ABI. As for EspressifRepo, we plan to publish release 21 based on MainRepo and we plan also support "generic" target by default in it.
Hope, I understand your questions correctly. Could you share your proposed changes?

[alexrp]

Oh, that's interesting; I was under the impression that the windowed ABI was the prevalent ABI on Xtensa. Does this mean that it's the other way around -- i.e. the windowed ABI is falling out of use -- or rather that both ABIs are still commonly used?

In any case, I'm talking about the main LLVM repo. We have multiple users of Zig who have expressed interest in using Zig for Xtensa, so I've been doing some work on our end to prepare for when upstream LLVM's Xtensa backend is ready for use.

My proposal, assuming both Call0 and windowed ABIs are still relevant, is that it should be possible to select the ABI in the target triple with something like xtensa-*-windowed or xtensa-*-call0 (names TBD). This is similar to how other backends select their default ABI from the triple, e.g. LoongArch (loongarch*-*-(gnu,gnuf32,gnusf)), MIPS (mips*-*-(gnuabi64,gnuabin32)), etc.