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
Dead code elimination
Does GHC remove code that you're not actually using?
Yes and no. If there is something in a module that isn't exported and isn't used by anything that is exported, it gets ignored. (This makes your compiled program smaller.) So at the module level, yes, GHC does dead code elimination.
On the other hand, if you import a module and use just 1 function from it, all of the code for all of the functions in that module get linked in. So in this sense, no, GHC doesn't do dead code elimination.
(There is a switch to make GHC spit out a separate object file for each individual function in a module. If you use this, only the functions are actually used will get linked into your executable. But this tends to freak out the linker program...)
If you want to be warned about unused code (Why do you have it there if it's unused? Did you forget to type something?) you can use the -fwarn-unused-binds option (or just -Wall).
Common subexpression elimination
First of all, common subexpression elimination (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: