Global constant propagation

Simon Peyton-Jones simonpj at microsoft.com
Mon Jan 21 10:48:11 CET 2013

```Your example is complicated by the fact that k is overloaded (will work on any value in class Num), and in fact the numbers end up having type Integer, not Int.

But still, it is indeed like SpecConstr. Maybe we should generate a specialised version of 'k', specialised for k=0.   That might be a worthwhile thing to do, and would be a fairly straightforward modification of the SpecConstr code, to deal with literals as well as constructors.

Simon

|  -----Original Message-----
|  bounces at haskell.org] On Behalf Of C Rodrigues
|  Sent: 20 January 2013 21:18
|  Subject: Global constant propagation
|
|
|  I'm curious about global constant propagation in GHC.  It's a fairly basic
|  optimization in the CFG-based compiler domain, and it's similar to constructor
|  specialization, but it doesn't seem to be in GHC's repertoire.  Perhaps it's usually
|  subsumed by other optimizations or it's more complicated than I am thinking.  Is
|  this optimization worth implementing?
|
|  This optimization can help when a case expression returns a product, some fields
|  of which are the same in all branches.  The following program is a minimal
|  example of an optimizable situation that GHC doesn't exploit.
|
|
|  {-# OPTIONS_GHC -O3 -funbox-strict-fields #-}
|
|
|  data D = D !Int !Int
|
|
|  foo n = if n > 0
|
|          then D 0 0
|
|          else D 0 n
|
|
|  main =
|
|    case foo \$ read "7"
|
|    of D x y -> if x == 0 then return () else print y >> putStrLn "A"
|
|
|  After inlining and case-of-case transformation, GHC produces
|
|
|  main = let n = read "7"
|
|             k x y = case x of {0 -> return (); _ -> print y >> putStrLn "A"}
|
|         in if n > 0
|
|            then k 0 0
|
|            else k 0 n
|
|
|  If the simplifier could discover that x is always 0, it could eliminate one parameter
|  of 'k' and the case expression.
|
|
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