Don't keep exit join points so much

We were religiously keeping exit join points throughout, which
had some bad effects (#21148, #22084).

This MR does two things:

* Arranges that exit join points are inhibited from inlining
  only in /one/ Simplifier pass (right after Exitification).

  See Note [Be selective about not-inlining exit join points]
  in GHC.Core.Opt.Exitify

  It's not a big deal, but it shaves 0.1% off compile times.

* Inline used-once non-recursive join points very aggressively
  Given join j x = rhs in
        joinrec k y = ....j x....

  where this is the only occurrence of `j`, we want to inline `j`.
  (Unless sm_keep_exits is on.)

  See Note [Inline used-once non-recursive join points] in
  GHC.Core.Opt.Simplify.Utils

  This is just a tidy-up really.  It doesn't change allocation, but
  getting rid of a binding is always good.

Very effect on nofib -- some up and down.

1 files changed, 4 insertions, 2 deletions

diff --git a/compiler/GHC/Core/Opt/SpecConstr.hs b/compiler/GHC/Core/Opt/SpecConstr.hs
index 9119671f95..b8a77875a6 100644
--- a/compiler/GHC/Core/Opt/SpecConstr.hs
+++ b/compiler/GHC/Core/Opt/SpecConstr.hs
@@ -1512,8 +1512,10 @@ scExpr' env (Case scrut b ty alts)
                 scrut_occ = case con of
                                DataAlt dc -- See Note [Do not specialise evals]
                                   | not (single_alt && all deadArgOcc arg_occs)
-                                  -> ScrutOcc (unitUFM dc arg_occs)
-                               _  -> UnkOcc
+                                  -> -- pprTrace "sc_alt1" (ppr b' $$ ppr con $$ ppr bs $$ ppr arg_occs) $
+                                     ScrutOcc (unitUFM dc arg_occs)
+                               _  -> -- pprTrace "sc_alt1" (ppr b' $$ ppr con $$ ppr bs $$ ppr arg_occs) $
+                                     UnkOcc
           ; return (usg', b_occ `combineOcc` scrut_occ, Alt con bs2 rhs') }