Motivation for this change

ld.gold has been known to speed up GHC compilations for some time, and is often recommended whenever linking time becomes unreasonably big. @bgamari and I thought it might be a good idea to use it by default for all GHCs that are built in nixpkgs. Both have bugs but ld.gold tends to be reliable and consistently faster.

I also have concrete numbers to back this up, at the end of this comment.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nix-review --run "nix-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.

First, a tiny script that measures the time that it takes just to link aeson's Setup.hs, producing a Setup executable, with ld.gold and ld.bfd,10 times over, recording the timings (all in seconds).

Here's a table with just the system times reported. The two files with system and user times are available at the end.

Second, a script that measures the time it takes to compile ghc-events-analyze (alone, without its deps) with ld.bfd and ld.gold. Like before, script & raw data available at the end.

Those numbers follow a clear trend that we have been seeing during our work on/around GHC, but I could definitely collect more data points if this is not convincing enough.

aeson Setup.hs data:

#!/run/current-system/sw/bin/env nix-shell
#!nix-shell -i bash -p cabal-install -p haskell.compiler.ghc844 -p binutils

# Using Cabal/Cabal's Setup.hs as an example
cabal get aeson; cd aeson-*
rm -rf Setup Setup.hi Setup.o
ghc -O Setup.lhs

for i in `seq 10`; do
  rm Setup
  command time -o ../gold -a -f '%S %U' ghc -O -optl-fuse-ld=gold Setup.lhs
  rm Setup
  command time -o ../bfd -a -f '%S %U' ghc -O -optl-fuse-ld=bfd Setup.lhs
done

cd ..

bfd.txt
gold.txt

ghc-events-analyze compilation data:

{ nixpkgs ? import <nixpkgs> {} }:
with nixpkgs;

let deps =
  [ cabal-install haskell.compiler.ghc844
    binutils zlib.dev zlib.out
  ];

in

mkShell {
  name = "ghc-gold-bench";
  buildInputs = deps;

  shellHook = ''
    export LD_LIBRARY_PATH=${zlib.out}/lib:$LD_LIBRARY_PATH
  '';
}

#!/usr/bin/env sh

# Building a whole (executable) package
cabal new-update
cabal get ghc-events-analyze
cd ghc-events-analyze*
cabal new-build --ghc-options="-optl-fuse-ld=gold"
cabal new-build --ghc-options="-optl-fuse-ld=bfd"

for i in `seq 10`; do
    rm -r dist-newstyle
    command time -o ../gold-gea -a -f '%S %U' cabal new-build --ghc-options="-optl-fuse-ld=gold"
    rm -r dist-newstyle
    command time -o ../bfd-gea -a -f '%S %U' cabal new-build --ghc-options="-optl-fuse-ld=bfd"
done
cd ..

bfd-gea.txt
gold-gea.txt