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A work colleague of mine found that some expressions were not being constant folded as expected.

For example:

module Test where
import Data.Int
number :: Int64
number = 2 ^ 10 - 3

compiled using:

ghc -O2 -keep-s-file -c test.hs

should result in an assembler file containing the constant 1021 but doesn't. However, if I change the above expression to:

number = 1024 -3

then the output assembler files does contain the value 1021.

I did a bit of a search and found some constant folding code in the hoopl library, but that doesn't seem to be what GHC actually uses.