wiki:Commentary/Compiler/Backends/LLVM/GHC_LLVMPorting

Version 2 (modified by dterei, 3 years ago) (diff)

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Porting GHC using LLVM backend

This document is kind of short porting roadmap which serves as a high-level overview for porters of GHC who decided to use LLVM instead of implementing new NCG for their target platform. Please have Design & Implementation at hand since this contains more in-depth information. The list of steps needed for new GHC/LLVM port is:

(1) Make sure GHC unregisterised build is working on your target platform (using the C backend). This guide isn't intended for porting GHC to a completely unsupported platform. If the platform in question doesn't have a GHC unregisterised build then follow the GHC Porting Guide first.

(2) Now try to compile some very simple programs such as 'hello world' or simpler using the GHC you just built. Try with the C backend First to make sure everything is working. Then try with the LLVM backend. If the llvm backend built programs are failing find out why. This is done using a combination of things such as the error message you get when the program fails, tracing the execution with GDB and also just comparing the assembly code produced by the C backend to what LLVM produces. This last method is often the easiest and you can occasionally use techniques like doing doing a 'binary search' for the bug by merging the assembly produced by the C backend and LLVM backend.

(3) When the programs you throw at the LLVM backend are running, try running the GHC testsuite. First run it against the C backend to get a baseline, then run it against the LLVM backend. Fix any failures that are LLVM backend specific.

(4) If the testsuite is passing, now try to build GHC itself using the LLVM backend. This is a very tough test. When working though its a good proof that the LLVM backend is working well on your platform.

(5) Now you have LLVM working in unregistered mode, so the next thing is to implement the GHC calling convention in LLVM that is used by GHC's LLVM backend. This should then allow you to get the LLVM backend working in registered mode but with (TABLES_NEXT_TO_CODE = NO in your build.mk). Majority of this step involves hacking inside the LLVM code. Usually lib/Target/<your target platform name> is the best way to start. Also you might study what David Terei did for x86 support and his patch itself to get an idea what's really needed.

(6) Once (5) is working you have it all running except TABLES_NEXT_TO_CODE. So change that to Yes in your build.mk and get that working. This will probably involve changing the mangler used by LLVM to work on the platform you are targeting.