## A Tip for Profiling GHC

Posted on March 26, 2020

GHC developers are often in a situation where we want to profile a new change to GHC to see how it affects memory usage or runtime performance. In this post I will describe quite an ergonomic way of profiling any merge request without having to build the branch yourself from source or in any special mode. We’ll download the bindist from GitLab CI and then compile a simple GHC API application which models the compilation pipeline which we can profile.

## Step 1: Enter an environment with the bindist

I recently wanted to profile one of Sebastian Graf’s great patches, seeing as he put up a merge request on GitLab, CI built his patch and produced a bindist for a large number of platforms. This included a Fedora bindist which can be passed to my tool ghc-head-from in order to enter an environment with his patched version of GHC available.

I found the URL for the bindist for his patch by navigating through the GitLab interface.

ghc-head-from https://gitlab.haskell.org/ghc/ghc/-/jobs/289084/artifacts/raw/ghc-x86_64-fedora27-linux.tar.xz

Once it has finished downloading and installed, which takes a surprising amount of time, the patched version of GHC will be available.

> ghc --version
The Glorious Glasgow Haskell Compilation System, version 8.11.0.20200324

## Step 2: The simple GHC API program

Now we need the program we are going to profile. This is a simple GHC API program which will read arguments from a file called args and then just compile the modules as specified by the arguments.

module Main where

import Lib
import GHC as G
import GHC.Driver.Session as G
import GHC.Driver.Session
import SrcLoc as G

import System.Environment
import System.Mem
import Control.Concurrent
import Outputable

initGhcM :: [String] -> Ghc ()
initGhcM xs = do
df1 <- getSessionDynFlags
let cmdOpts = ["-fforce-recomp"] ++ xs
(df2, leftovers, warns) <- G.parseDynamicFlags df1 (map G.noLoc cmdOpts)
setSessionDynFlags df2
ts <- mapM (flip G.guessTarget Nothing) $map unLoc leftovers setTargets ts pprTraceM "Starting" (ppr ts) void$ G.load LoadAllTargets

main :: IO ()
main = do
xs <- words <$> readFile "args" let libdir = "/nix/store/c7113gcm42jjjzpgygfmmrivdhrxgvvk-ghc-8.11.0.20200324/lib/ghc-8.11.0.20200324" runGhc (Just libdir)$ initGhcM xs

In the program you need to set the libdir to the libdir for the version of ghc we just downloaded.

> ghc --print-libdir
/nix/store/c7113gcm42jjjzpgygfmmrivdhrxgvvk-ghc-8.11.0.20200324/lib/ghc-8.11.0.20200324

The args file contains a list of arguments that you would normally pass to GHC. The wrapper is then compiled as normal, passing both the -package and -prof flags.

ghc Profile.hs -package ghc -prof

## Step 3: The program you want to profile

Say for this example we want to profile a single compilation of Cabal, how do we know what options we should pass to the wrapper program in order to perform the compilation? The easiest way to work this out is to ask cabal to compile the project and then copy the arguments it uses to invoke GHC. So in the locally cloned Cabal repository, we can compile it like normal and pass the -v2 flag to get cabal to print the options it will use to call GHC.

cabal v2-build -v2 Cabal | tee args

Then open the args file and delete everything apart from the arguments for the final call to ghc. The final file should contain a single line with just the options you want to pass to GHC to compile the project.

By using cabal to get the arguments it will also build any necessary dependencies for us.

Note: You might need to fix some of the include paths if you are running the executable in a different directory.

## Step 4: Running the profile

So now we have the program to profile and something to compile, we can profile using any of the normal profiling modes.

-- Run a time profile
./Profile +RTS -p -l-au
-- Run a heap profile
./Profile +RTS -hy -l-au

Then you can use hs-speedscope to view the time profile or eventlog2html to view the heap profile. You will observe that the simplifier is very slow.

# Conclusion

The main disadvantage of this approach is that you can’t add any cost centres into the build. GHC comes with a limited number of hand written cost centres but not covering a lot of functions.

It would be nice in future to automate some of these steps to make it even more seamless to profile a specific MR.