Strictness analyser missing useful strictness

Roman Beslik (beroal@ukr.net) writes: I provide a sample program which causes a strange behavior of strictness analyzer.

variant 1

module StrictUnusedArg (main) where
f2 :: Int -> Int -> Int
f2 x1 = if x1 == 0 then (\x0 -> x0) else let
     y = x1 - 1
     in f3 y y
f3 :: Int -> Int -> Int -> Int
f3 x2 = if x2 == 0 then f2 else let
     y = x2 - 1
     in f4 y y
f4 :: Int -> Int -> Int -> Int -> Int
f4 x3 = if x3 == 0 then f3 else let
     y = x3 - 1
     in \x2 x1 x0 -> f4 y x2 x1 (y + x0)
main = print (f2 100 0)

I expect that all arguments will be unboxed. "-ddump-simpl" reveals that actually types are

f2 :: Int# -> Int -> Int
f3 :: Int# -> Int -> Int -> Int

So when "f3" calls "f2" it unboxes the argument named "x1" and when "f2" calls "f3" it boxes the argument named "x1". "-ddump-stranal" knows strictness only for the "x2" of "f3" and "x1" of "f2".

f2:
[Arity 1
  Str: DmdType U(L)]
f3:
[Arity 1
  Str: DmdType U(L)]

I also can force the analyzer to think that "x1" and "x0" are strict by eta-expanding "f3": variant 2

f3 x2 x1 x0 = if x2 == 0 then f2 x1 x0 else let
     y = x2 - 1
     in f4 y y x1 x0

"-ddump-stranal" yields:

f3:
[Arity 3
  Str: DmdType U(L)U(L)U(L)m]
f2:
[Arity 2
  Str: DmdType U(L)U(L)m]

I even do not use ($!). So, the questions: Is it possible to change the strictness analyzer so it will treat "variant 1" as "variant 2"? Are these changes big?

Compiled with options:

$ ghc --make -fstrictness -fPIC -O3 -fforce-recomp blah-blah-blah
$ ghc --version
The Glorious Glasgow Haskell Compilation System, version 6.12.1

Trac metadata

Trac field	Value
Version	6.12.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture