Where do I start if I would like help improve GHC compilation times?

David Feuer david at well-typed.com
Sun Apr 9 21:04:41 UTC 2017

Be aware that some of the biggest performance problems with TH simply can't be fixed without changes to the TH language. For details, see Edward Yang's blog post: http://blog.ezyang.com/2016/07/what-template-haskell-gets-wrong-and-racket-gets-right/
There was a Reddit thread discussing that post at https://www.reddit.com/r/haskell/comments/4tfzah/what_template_haskell_gets_wrong_and_racket_gets/

David FeuerWell-Typed, LLP
-------- Original message --------From: Alfredo Di Napoli <alfredo.dinapoli at gmail.com> Date: 4/9/17  5:37 AM  (GMT-05:00) To: Ben Gamari <ben at smart-cactus.org> Cc: ghc-devs at haskell.org Subject: Re: Where do I start if I would like help improve GHC compilation times? 
Hey Ben,
as promised I’m back to you with something more articulated and hopefully meaningful. I do hear you perfectly — probably trying to dive head-first into this without at least a rough understanding of the performance hotspots or the GHC overall architecture is going to do me more harm than good (I get the overall picture and I’m aware of the different stages of the GHC compilation pipeline, but it’s far from saying I’m proficient with the architecture as whole). I have also read a couple of years ago the GHC chapter on the “Architeture of Open Source Applications” book, but I don’t know how much that is still relevant. If it is, I guess I should refresh my memory.
I’m currently trying to move on 2 fronts — please advice if I’m a fool flogging a dead horse or if I have any hope of getting anything done ;)
1. I’m trying to treat indeed the compiler as a black block (as you adviced) trying to build a sufficiently large program where GHC is not “as fast as I would like” (I know that’s a very lame definition of “slow”, hehe). In particular, I have built the stage2 compiler with the “prof” flavour as you suggested, and I have chosen 2 examples as a reference “benchmark” for performance; DynFlags.hs (which seems to have been mentioned multiple times as a GHC perf killer) and the highlighting-kate package as posted here: https://ghc.haskell.org/trac/ghc/ticket/9221 . The idea would be to compile those with -v +RTS -p -hc -RTS enabled, look at the output from the .prof file AND the `-v` flag, find any hotspot, try to change something, recompile, observe diff, rinse and repeat. Do you think I have any hope of making progress this way? In particular, I think compiling DynFlags.hs is a bit of a dead-end; I whipped up this buggy script which escalated into a Behemoth which is compiling pretty much half of the compiler once again :D
```#!/usr/bin/env bash
../ghc/inplace/bin/ghc-stage2 --make -j8 -v +RTS -A256M -qb0 -p -h \-RTS -DSTAGE=2 -I../ghc/includes -I../ghc/compiler -I../ghc/compiler/stage2 \-I../ghc/compiler/stage2/build \-i../ghc/compiler/utils:../ghc/compiler/types:../ghc/compiler/typecheck:../ghc/compiler/basicTypes \-i../ghc/compiler/main:../ghc/compiler/profiling:../ghc/compiler/coreSyn:../ghc/compiler/iface:../ghc/compiler/prelude \-i../ghc/compiler/stage2/build:../ghc/compiler/simplStg:../ghc/compiler/cmm:../ghc/compiler/parser:../ghc/compiler/hsSyn \-i../ghc/compiler/ghci:../ghc/compiler/deSugar:../ghc/compiler/simplCore:../ghc/compile/specialise \-fforce-recomp -c $@```
I’m running it with `./dynflags.sh ../ghc/compiler/main/DynFlags.hs` but it’s taking a lot to compile (20+ mins on my 2014 mac Pro) because it’s pulling in half of the compiler anyway :D I tried to reuse the .hi files from my stage2 compilation but I failed (GHC was complaining about interface file mismatch). Short story short, I don’t think it will be a very agile way to proceed. Am I right? Do you have any recommendation in such sense? Do I have any hope to compile DynFlags.hs in a way which would make this perf investigation feasible?
The second example (the highlighting-kate package) seems much more promising. It takes maybe 1-2 mins on my machine, which is enough to take a look at the perf output. Do you think I should follow this second lead? In principle any 50+ modules package I think would do (better if with a lot of TH ;) ) but this seems like a low-entry barrier start.
2. The second path I’m exploring is simply to take a less holistic approach and try to dive in into a performance ticket like the ones listed here: https://www.reddit.com/r/haskell/comments/45q90s/is_anything_being_done_to_remedy_the_soul/czzq6an/Maybe some are very specific, but it seems like fixing small things and move forward could help giving me understanding of different sub-parts of GHC, which seems less intimidating than the black-box approach.
In conclusion, what do you think is the best approach, 1 or 2, both or none? ;)
Thank you!
On 7 April 2017 at 18:30, Alfredo Di Napoli <alfredo.dinapoli at gmail.com> wrote:
Hey Ben,
thanks for the quite exhaustive reply! I’m on the go right now, but I promise to get back to you with a meaningful reply later this weekend ;)
On 7 April 2017 at 18:22, Ben Gamari <ben at smart-cactus.org> wrote:
Alfredo Di Napoli <alfredo.dinapoli at gmail.com> writes:

> Hey folks,


Hi Alfredo!

First, thanks for writing. More eyes looking at GHC's compiler

performance is badly needed.

> maybe I’m setting up for something too ambitious for me, but I would like

> to take an active stance to the overlasting “GHC compilation times are

> terrible” matter, instead of simply stare at the screen with despair

> whenever GHC compiles a sufficiently large Haskell program ;)


> To make this even more interesting, I have never contributed to GHC either!

> The max I have pushed myself into was 2 years ago when I successfully built

> GHC head from source and tried to fix an Haddock “easy” ticket I don’t even

> recall (full disclosure, eventually I didn’t :D ).


> Specifically, I would love community recommendations & guidance about:


> 1. Is this simply too daunting for somebody like me? Maybe is better to

> first start contributing more regularly, take confidence with the code base

> AND then move forward?


As with any software project, it is possible to treat the compiler as a

black box, throw a profiler at it and see what hotspots show up. This

gives you a place to focus your effort, allowing you to learn a small

area and broaden your knowledge as necessary.

However, I think it's fair to say that you will be significantly more

productive if you first develop a basic understanding of the compilation

pipeline. I'd recommend having a look at the GHC Commentary [1] for a


I think it also helps to have a rough idea of what "slow" means to you.

I find it is quite helpful if you have a particular program which you

feel compiles more slowly than you would like (especially if it even

compiles slowly with -O0, since then much less of the compiler is

involved in compilation). Another approach is to look for programs whose

compilation time has regressed over the course of GHC releases. It is

not hard to find these examples and it is often possible to bisect your

way back to the regressing commit.

Also, note that I have collected some notes pertaining to compiler

performance on the Wiki [2]. Here you will find a number of tickets of

interest (as well a some rough themes which I've noticed), some nofib

results which might guide your efforts, as well as a list of some

fixes which have been committed in the past.

[1] https://ghc.haskell.org/trac/ghc/wiki/Commentary/Compiler

[2] https://ghc.haskell.org/trac/ghc/wiki/Performance/Compiler

> 2. Are compilation times largely dependant from the target platform (I’m on

> Darwin) or there is something which can be done “globally” so that the

> benefits can be experienced by everybody?


There are some external considerations (e.g. the platform's compiler and

linking toolchain) which contribute to GHC's runtime. For instance, it

is known that the BFD ld linker implementation that many Linux

distributions use by default is a great deal slower than it could be.

This particular issue has come up recently and I'm currently working on

allowing us to use the more performant gold linker when available.

However, I think it's fair to say that for most programs GHC's runtime

is largely independent of platform. I would invite you to try compiling

a package which you consider GHC to compile "slowly" with GHC's -v flag

(and GHC 8.0.1 or newer). This will give you a rough breakdown of where

time is spent. For many packages you will find that the simplifier

and/or typechecker dominate, followed (often distantly) by native code

generation. Of these steps native code generation is the only one with a

strong platform dependence.

> 3. Is there any recommended workflow to profile GHC compilation times? Is

> there any build flavour one should prefer when doing so? (Maybe the full,

> slowest one?)


There are a few options here:

 * As of GHC 8.0 the compiler will output timing and allocation

   information for its various stages if run with -v. This can be

   extremely helpful to get a high-level picture of where the compiler

   is spending its time while compiling your program. This is almost

   always the right place to start.

 * As with any Haskell program, the cost centre profiler can be used to

   characterize the memory and CPU behavior of various parts of the


   GHC's source tree includes a "prof" build flavour which builds the

   compiler with profiling enabled. However it only includes a handful

   of cost-centres and is best used when you already have a rough idea

   where you are looking and can add further cost-centres to drill down

   to your hotspot.

   Simply enabling -fprof-exported across the entire tree just doesn't

   work in my experience: not only is the resulting compiler quite slow,

   but the profile you get is far too unwieldy to learn from.

 * Occassionally the ticky-ticky profiler can be helpful in identifying

   allocation hotspots without the full overhead of the cost-centre


 * In principle our newly-stable DWARF debug information can be used for

   profiling, although this is still a work in progress and requires a

   patched GHC for best results. It's probably best to stick to the more

   traditional profiling mechanisms for now.

Anyways, I hope this helps. Always feel free to get in touch with me

personally (IRC and email are both great) if you would like to discuss

particular issues. Thanks again for your interest!


- Ben

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