Why does GHC take so long to link?

Question

>cabal update
>cabal install cabal-install
.......
[43 of 44] Compiling Distribution.Client.Install ( Distribution/Client/Install.hs, dist/build/cabal/cabal-tmp/Distribution/Client/Install.o )
[44 of 44] Compiling Main             ( Main.hs, dist/build/cabal/cabal-tmp/Main.o )
Linking dist/build/cabal/cabal ..

then i will wait for a VERY long time for it to finish linking.

Sincerely!

Answer 1

Very likely it's the linker itself. The standard ld from binutils is known to be slow. If you want to speed things up (and live a bit on the edge), try installing the Gold linker. On Ubuntu (and I assume Debian), that would be:

sudo apt-get install binutils-gold

I've been using it on my home system for a while now, no issues yet.

Answer 2

This should be a comment, but I cannot format code like this in comments:

I managed to use gold as the linker used by GHC, working around standard ghc-with-gold errors like /usr/bin/ld.gold: --hash-size=31: unknown option, by installing binutils-gold (as suggested in Michael Snoyman's answer) and then replacing the symlink /usr/bin/ld by the following script (made executable with chmod +x):

#!/usr/bin/env python2

import sys
import os
import subprocess


tofilter = [
    "--hash-size",
    "--reduce-memory-overheads",
]

filtered = [ a for a in sys.argv if not any(a.startswith(p) for p in tofilter) ]
filtered[0] = "/usr/bin/ld.gold"

subprocess.check_call(subprocess.list2cmdline(filtered))

Note that trying to set the linker with ghc -pgml /usr/bin/ld.gold or ghc -pgml /usr/bin/ld.whateverElse is not sufficient because the argument to -pgml needs to be a replacement for GCC, not LD. GHC calls GCC, which calls /usr/bin/ld; this is why the above script works.