[llvm-dev] [RFC] Tensilica Xtensa (ESP32) backend

Fri Mar 8 15:12:52 PST 2019

On Thu, Mar 7, 2019 at 12:47 PM Andrei Safronov <safronov at espressif.com>
wrote:

> Hello, James,
>
> Thank you very much for your advices! The next step in compiler
> development on Espressif is object file generation. There are no essential
> problems with this step, it will be implemented in nearest future.
> Currently Xtensa backend is able to print and parse assembly, I used about
> 1300 tests from gcc torture testsuite and GNU binutils to debug assembly
> output and now all tests could be compiled and executed successfully. So,
> with object file generation Xtensa backend will be significally closer to
> be used in real projects, but I'm agree that it is not ready yet for
> upstreaming. There is no llvm tests and also it is hard to review such big
> amount of code. So I need to do something with these issues.
>
> I reviewed integration process of  the RISC-V backend, which you
> mentioned. Did I understand correctly that you suggest to split the
> proposed Xtensa backend into ordered patches, each of which defines some
> backend functionality (starting from Asm/MC layer)that is easy to review?
> And also include in each patch some set of tests to verify such
> functionality?
>
> Yes, exactly.

It's not important to follow exactly the same breakup as the RISCV target,
but it makes sense to follow roughly the same order of work.

That is, the overall stages you'll want to tackle are going to be something
like:
1. Initial addition of the target, triple parsing, etc. (riscv patches #2
through #4)
2. Working MC for the baseline ISA, (#5 through #11).
3. Codegen for the baseline ISA (#12-#32)
4. MC layer for ISA extensions (#33-#41)
5. Codegen for ISA extensions, and further fixes throughout (#42-...)

Having patches that are small enough to be usefully-reviewable is very
helpful, and each patch should have a full set of test-cases included
testing its functionality (as much as is feasible).

But it's also important to be able to see enough of the implementation to
have the necessary context to sanely review. So, that's why I suggest that
your initial goal should be to create a set of patches fully-implementing
just the first two items on that list, and post them for review at the same
time. After that's committed, move on to extracting patches for further
functionality, adding tests, and posting for review as you go.

Best regards,
> Andrei Safronov
> 07.03.2019 1:31, James Y Knight пишет:
>
> Sounds like a good idea to me! It seems there's plenty of interest in this
> architecture.
>
> From a quick glance at your existing code, it looks like you haven't added
> any llvm tests -- that'll certainly be a requirement.  But barring some
> particular reason why it's not feasible, I think that having object-file
> generation working is basically considered a requirement for new
> architectures. (It's also not clear to me why you cannot with your current
> code, it looks like instruction definitions already have their encodings
> specified and such).
>
> If you haven't yet, you ought to check out at the great patch series
> that Alex Bradbury created to demonstrate the addition of RISCV support to
> LLVM, as a guide to what order it probably makes sense to make the changes,
> and how to split the changes into reviewable pieces for upstreaming:
> https://github.com/lowRISC/riscv-llvm
>
> I'd suggest as your first step, you should work on getting just the Asm/MC
> layer rebased onto trunk and working for the core ISA for your target --
> able to parse and print assembly, both textual and object files -- with a
> full suite of test-cases at each step. Just like the first ~10 patches in
> the above patchset.
>
>
>
> On Wed, Mar 6, 2019 at 6:29 AM Andrei Safronov via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
>> Hello,
>>
>> I'm from Espressif Systems company, software department. Our company
>> develops processors based on Xtensa architecture like ESP32 and ESP8266. We
>> propose the integration of a backend targeting Xtensa architecture.
>>
>> We started to develop LLVM Xtensa backend almost a year ago. The reason
>> was that we saw a demand from our large developers community. Currently
>> only GNU compiler supports Xtensa architecture. The company has approved me
>> to develop and maintain Xtensa backend.
>>
>> We already have the initial version of the Xtensa backend, based on LLVM
>> Compiler Infrastructure, release 6.0.0. It was successfully tested using
>> GCC torture testsuite and multiple applications.
>>
>> These are the links to LLVM and Clang repositories.
>>
>> https://github.com/espressif/llvm-xtensa
>> https://github.com/espressif/clang-xtensa
>>
>> Current version can generate Xtensa assembly code as output, not object
>> files yet, and has to be used together with GNU Binutils and GCC-built
>> libraries to create object and binary files.
>>
>> Xtensa backend features implemented:
>>
>> - Xtensa target description(Xtnesa.td, XtensaTargetMachine.cpp,
>> XtensaSubTarget.cpp)
>> - ISA desciption (XtensaInstrInfo.td, XtensaInstrFormats.td,
>> XtensaREgisterInfo.td)
>> - Xtensa Call ABI (XtensaCallingConv.td, XtensaFrameLowering.cpp)
>> - ASM printer/parser(XtesaAsmPrinter.cpp, XtensaInstrPrinter.cpp,
>> XtensaAsmParser.cpp)
>>
>> Xtensa architecture features implemented in compiler:
>>
>> - Xtensa Core Architecture instructions
>> - Code Density option
>> - Windowed Register option
>> - Floating-Point Coprocessor option
>> - Boolean option (only a subset of instructions)
>> - Thread Pointer option
>> - atomic operations
>>
>> Current Xtensa target list:
>>
>> - support Xtensa LX6 target (ESP32) by default
>>
>> Compiler optimization levels include O0/O1/O2/O3/Os options.
>>
>> With LLVM community approval, my next plans will be
>>
>> - rebasing on the upstream version of LLVM.
>> - object code generation (XtensaMC package)
>> - implement test cases
>> - support for LX106 target (ESP8266)
>> - improvements of generated code performance
>> - support for zero-overhead loop option
>> - MAC16 option
>>
>> There were some discussions about implementation of the Xtensa backend
>> and attempt to implement it:
>> http://lists.llvm.org/pipermail/llvm-dev/2018-July/124789.html
>> http://lists.llvm.org/pipermail/llvm-dev/2018-April/122676.html
>>
>> Also there were attempts to implement a LLVM Xtensa backend, but recently
>> I found only one actual link:
>> https://github.com/jdiez17/llvm-xtensa
>>
>> All comments and suggesions are welcome!
>>
>> Andrei Safronov
>>
>> _______________________________________________
>> LLVM Developers mailing list
>> llvm-dev at lists.llvm.org
>> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20190308/6014dcf9/attachment-0001.html>