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
Test case
Differential revisions
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Blocking
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Operating system
Architecture

changed weight to 5

added Tfeature request Trac import labels

Ideally, express it as a bunch of rewrite rules (prelude/PrelRules.hs). I don't know if that is conveniently possible for assoc/distrib laws.

Don't forget to take care about overflow etc. You will know this better than I but computer arithmetic doesn't obey all the same laws as mathematics!

Trac metadata

Trac field	Value
Differential revisions	- → D2858

#9136 (closed) is about the same problem.

closed

@akio Indeed. Let's continue the discussion on the original ticket.

Trac metadata

Trac field	Value
Resolution	Unresolved → ResolvedDuplicate
Related	- → #9136 (closed)

added Pnormal label