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Wednesday
Sep172008

Parsing the Linux kernel with Haskell: experience with Language.C

At Galois, Aaron Tomb has been experimenting with the new Haskell Language.C libraries recently (a Summer of Code project by Benedikt Huber, mentored by a Galois engineer, Iavor Diatchki), and he's been impressed by what it can do. Here are his thoughts on parsing the Linux kernel with Haskell, with an eye to future static analysis work:My interest in the library is for use in static analysis of very large bodies of legacy C code, which means two issues matter a lot to me: 1) rock-solid support for all of GCC's numerous extensions, and 2) speed. I have used CIL, and tools based on CIL in the past, but have been disappointed with its speed.As a simple scalability and robustness experiment, I decided to see how well Language.C would do on the Linux source tree. It doesn't yet have an integrated preprocessor (depending on GCC's for now), but I happened to have an already-preprocessed set of sources for Linux 2.6.24.3 sitting around (configured with defconfig).Could Language.C handle the Linux kernel?I wrote a little wrapper around the C parser to essentially just syntax-check all of the code.


import Language.Cimport Language.C.System.GCCimport System.Environmentprocess :: String -> IO ()process file = do putStr file stream <- readInputStream file putStr (take (20 - length file) $ repeat ' ') either print (const $ putStrLn "Pass") (parseC stream nopos)main :: IO ()main = do files <- getArgs mapM_ process files

It prints the filename followed by "Pass" if the parse succeeds, or details about a syntax error if the parse fails. When I ran this on the Linux code mentioned above, I was amazed to find that it processed it all successfully! All 18 million lines of pre-processed source without a hitch.Since I also care about speed, I wanted to compare it with GCC. GCC has a handy flag, -fsyntax-only, which tells it to just check the syntax of the input file and quit. I ran both the Language.C wrapper(compiled with GHC 6.8.3 and the -O2 option) and GCC on all that code, on a 2.2GHz/4GB MacBook Pro. The result: Language.C parsed all of the code in about 6 minutes, while GCC managed it in a little over 2. GCC is still faster, but I'm happy to take a 3x speed hit for the benefit of being able to write all the subsequent analysis in Haskell.The following table shows the precise time and memory statistics for Langugage.C and GCC, both on the entire source collection and on the single largest file in the tree, bnx2.i, the driver for the Broadcom NetXtreme II network adapter. For the Language.C tests, I compared the performance when the garbage collector used 2 generations (the default) to 4 generations (specified with the +RTS -G4 option). Increasing the number of generations helped slightly.












































 User TimeSystem TimeMemory Use
L.C, all5:590:09144MB
L.C, all, -G45:270:08131MB
L.C, bnx2.i0:02.150:01.89133MB
L.C, bnx2.i, -G40:01.960:01.7685MB
gcc, all2:020:17????
gcc, bnx2.i0:00.560:00.0733MB

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