GHC optimisations

Introduction

This page collects together information about the optimisations that GHC does and does not perform.

• GHC experts: Please check that the info in this page is correct.
• Everybody else: Feel free to add questions!

General optimisations

Common subexpression elimination

First of all, common subexpression elemination (CSE) means that if an expression appears in several places, the code is rearranged so that the value of that expression is computed only once. For example:

  foo x = (bar x) * (bar x)

might be transformed into

  foo x = let x' = bar x in x' * x'

thus, the bar function is only called once. (And if bar is a particularly expensive function, this might save quite a lot of work.)

GHC doesn't actually perform CSE as often as you might expect. The trouble is, performing CSE can affect the strictness/lazyness of the program. So GHC does do CSE, but only in specific circumstances --- see the GHC manual. (Section??)

Long story short: "If you care about CSE, do it by hand."

Inlining

Inlining is where a function call is replaced by that function's definition. For example, the standard map function can be defined as

  map :: (a -> b) -> [a] -> [b]
  map f [] = []
  map f (x:xs) = f x : map f xs

Now if you write something like

  foo = map bar

it's possible that the compiler might inline the definition of map, yielding something like

  foo [] = []
  foo (x:xs) = bar x : foo xs

which is (hopefully!) faster, because it doesn't involve a call to the map function any more, it just does the work directly. (This might also expose new optimisations opportunities; map works for any types, whereas foo probably works for only one type.)

So, that's what inlining is. By default, GHC will inline things if they are 'small enough'. Every time you inline a function, you are in a sense making a (customised) copy of that function. Do too much of this and the compiled program will be enormous. So it's only worth it for 'small' functions.

(How does GHC determine 'small'? Isn't there a switch that adjusts this?)

Execution Model

In order to understand how to write efficient code, and what GHC does with your code to optimise it, it helps to know a bit about what your compiled code looks like and how it works.

Graph reduction

To a first approximation, at any moment your program is a 'graph' of objects in memory. ('Graph' in the graph theory sense --- nodes connected by arcs.) Some of the objects are 'data' --- booleans, integers, strings, lists, etc. Some of those objects are functions (because Haskell lets you pass functions around like data). And some of these are thunks --- unevaluated expressions (because Haskell only evaluates expressions 'as needed').

The program starts off with a single node representing the unevaluated call to main, and proceeds to execute from there. Each time a thunk is executed, the result (whatever it is) overwrites the thunk data. (It's possible that the result of evaluating a thunk is a new thunk of course.)

About STG

GHC compiles to the spineless tagness G-machine (STG). (Finish me!)