wiki:Commentary/Compiler/Backends/LLVM/Installing

Version 16 (modified by dterei, 4 years ago) (diff)

Add in using information

Installing & Using the LLVM Back-end

Installing

The LLVM backend is now included in GHC HEAD. Just grab the darcs HEAD version of GHC and build it. The backend now also supports all modes that GHC can be built in, except perhaps -dynamic which hasn't been tested yet, so you shouldn't need to change your build.mk file either (you used to have disabled an optimisation called tables-next-to-code, but the LLVM backend supports that now).

For instructions on building GHC go here

Using

Once built you can check that you have the LLVM backend GHC will support these extra options:

  • --info - Now will report that it has the llvm backend
  • -fllvm - Compile code using the llvm backend
  • -ddump-llvm' - Dumps the llvm IR while compiling
  • -pgm_la - The program to use as the llvm assembler
  • -pgm_lo - The program to use as the llvm optimiser
  • -pgm_lc - The program to use as the llvm compiler
  • -opt_la - Extra options to pass to the llvm assembler
  • -opt_lo - Extra options to pass to the llvm optimiser
  • -opt_lc - Extra options to pass to the llvm compiler

Supported Platforms & Correctness

  • Linux x86-32/x86-64 are currently well supported. The back-end can pass the test suite and build a working version of GHC (bootstrap test).
  • Mac OS X 10.5 currently has a rather nasty bug with any dynamic lib calls (all libffi stuff) [due to the stack not being 16byte aligned when the calls are made as required by OSX ABI for the curious]. Test suite passes except for most the ffi tests.
  • Windows 32bit: The backend works for most things but no extensive testing or support yet.
  • Other platforms haven't been tested at all. As using the back-end with a registered build of GHC requires a modified version of LLVM, people wanting to try it out on those platforms will need to either make the needed changes to LLVM themselves, or use an unregistered build of GHC which will work with a vanilla install of LLVM. (A patch for LLVM for x86 is linked to below.)

Performance

(All done on linux/x86-32)

A quick summary of the results are that for the 'nofib' benchmark suite, the LLVM code generator was 3.8% slower than the NCG (the C code generator was 6.9% slower than the NCG). The DPH project includes a benchmark suite which I (David Terei) also ran and for this type of code using the LLVM back-end shortened the runtime by an average of 25% compared to the NCG. Also, while not included in my thesis paper as I ran out of time, I did do some benchmarking with the 'nobench' benchmark suite. It gave performance ratios for the back-ends of around:

NCG 1.11
C 1.05
LLVM 1.14

A nice demonstration of the improvements the LLVM back-end can bring to some code though can be see at http://donsbot.wordpress.com/2010/02/21/smoking-fast-haskell-code-using-ghcs-new-llvm-codegen/