Missed constant folding oportunities (associativity)

Consider this simple example:

import System.Environment

main :: IO ()
main = do
   args <- getArgs
   print ((length args + 10) - 10)

Compiled with -O2, we get the following Core:

case GHC.Show.$wshowSignedInt
              0#
              (GHC.Prim.-# (GHC.Prim.+# ww2_a6Pd 10#) 10#)
              (GHC.Types.[] @ Char)
       of

I would expect GHC to perform constant folding there to remove the unnecessary operation.

Basically, for any expression composed of Int#/Word# +/-/* primops, I think we should use associativity and distributivity laws to push the constants in the outer operation. It would allow the "scrutinee constant folding" optimization to be applied more often too.

I have a patch that does this for case scrutinees, but where should I put it in GHC so that the optimization gets applied more generally?

Trac metadata

Trac field	Value
Version	8.0.1
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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