[llvm-dev] [RFC] Nios II backend

[Grischenko, Andrei L via llvm-dev]

Hi,

I'm from Intel compiler department. 
I am proposing the integration of a backend targeting Nios II processor architecture.
Nios II is a 32-bit general-purpose RISC processor core designed specifically for the Altera family of FPGAs.

All information at about Nios II can be found at Altera website https://www.altera.com/products/processors/support.html, including the current ISA specification https://www.altera.com/en_US/pdfs/literature/hb/nios2/n2cpu-nii5v1gen2.pdf

Nios II has two different binary incompatible ISA, that is called R1 and R2. I'm going to support both of them.

Nios II code generation for both ISA already is supported by GNU compiler. GNU compiler also supports big-endian mode of code generation that is not a high priority for me. I'm going to have the same level of support (as GNU compiler has )in LLVM including major number of Nios II specific options and built-ins, but without big-endian mode at the moment. The link below illustrates current GNU compiler specific related to Nios II.
https://gcc.gnu.org/onlinedocs/gcc/Nios-II-Options.html#Nios-II-Options
https://gcc.gnu.org/onlinedocs/gcc/Altera-Nios-II-Built-in-Functions.html#Altera-Nios-II-Built-in-Functions

LLVM is a key part of our development plan, and with community approval I would like to act as maintainer for the backend. 
The current status is that I just started the backend implementation based on Cpu0 backend described here http://jonathan2251.github.io/lbd/TutorialLLVMBackendCpu0.pdf as it based on backend for MIPS 32-bit architecture that is quite similar to Nios II.
I've added Nios II register and instruction set. I was managed to compile an assembly for "Hello World" program for Nios II target.

My development roadmap corresponds to the book for Cpu0 backend implementation. Even I already have the workable backend I plan to review it part by part and prepare appropriate patches for community review.

All Nios II specific names will have prefix "Nios2". For example, for the files "Nios2CallingConv.td", for classes "Nios2AsmPrinter".

Plan for initial implementation corresponding to patches (obviously the plan can be changed after comments)
- Target registration
	a)  Nios2.td, Nios2.h, Nios2TargetMachine.h, Nios2TargetMachine.cpp, Nios2Subtarget.h, Nios2Subtarget.cpp, TargetInfo/Nios2TargetInfo.cpp
	b) MCTargetDesc/Nios2MCTargetDesc.h, MCTargetDesc/Nios2MCTargetDesc.cpp...
- Register information (Nios2RegisterInfo.cpp, Nios2RegisterInfo.h, Nios2RegisterInfo.td)
- ISA specification (Nios2InstrFormats.td, Nios2InstrInfo.cpp, Nios2InstrInfo.h, Nios2InstrInfo.td)
- GlobalISel (At the moment I use DAG selector, but work on porting to GlobalISel is already started)
- Assembly printer (InstPrinter/Nios2InstPrinter.h, InstPrinter/Nios2InstPrinter.cpp, Nios2AsmPrinter.cpp, Nios2AsmPrinter.h, MCTargetDesc/Nios2MCAsmInfo.cpp, MCTargetDesc/Nios2MCAsmInfo.h, .)
- Target triple registration in clang
- Built-in functions (clang and codegen part)
- New options support (clang driver and codegen part)
- Direct ELF streaming (binary emission)

All comments and suggestions are very welcome,
Andrei Grischenko.

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