Demand: Interleave usage and strictness demands (#18903)

As outlined in #18903, interleaving usage and strictness demands not
only means a more compact demand representation, but also allows us to
express demands that we weren't easily able to express before.

Call demands are *relative* in the sense that a call demand `Cn(cd)`
on `g` says "`g` is called `n` times. *Whenever `g` is called*, the
result is used according to `cd`". Example from #18903:

```hs
h :: Int -> Int
h m =
  let g :: Int -> (Int,Int)
      g 1 = (m, 0)
      g n = (2 * n, 2 `div` n)
      {-# NOINLINE g #-}
  in case m of
    1 -> 0
    2 -> snd (g m)
    _ -> uncurry (+) (g m)
```

Without the interleaved representation, we would just get `L` for the
strictness demand on `g`. Now we are able to express that whenever
`g` is called, its second component is used strictly in denoting `g`
by `1C1(P(1P(U),SP(U)))`. This would allow Nested CPR to unbox the
division, for example.

Fixes #18903.
While fixing regressions, I also discovered and fixed #18957.

Metric Decrease:
    T13253-spj

1 files changed, 6 insertions, 5 deletions

diff --git a/compiler/GHC/Core/Opt/SpecConstr.hs b/compiler/GHC/Core/Opt/SpecConstr.hs
index 4601407723..1abe9f7ab3 100644
--- a/compiler/GHC/Core/Opt/SpecConstr.hs
+++ b/compiler/GHC/Core/Opt/SpecConstr.hs
@@ -1724,11 +1724,12 @@ calcSpecStrictness fn qvars pats
     go env _      _                = env
 
     go_one :: DmdEnv -> Demand -> CoreExpr -> DmdEnv
-    go_one env d   (Var v) = extendVarEnv_C bothDmd env v d
-    go_one env d e
-           | Just ds <- splitProdDmd_maybe d  -- NB: d does not have to be strict
-           , (Var _, args) <- collectArgs e = go env ds args
-    go_one env _         _ = env
+    go_one env d          (Var v) = extendVarEnv_C plusDmd env v d
+    go_one env (_n :* cd) e -- NB: _n does not have to be strict
+      | (Var _, args) <- collectArgs e
+      , Just ds <- viewProd (length args) cd
+      = go env ds args
+    go_one env _               _       = env
 
 {-
 Note [spec_usg includes rhs_usg]