Fix #17724 by running the simplifier before late CSEwip/andreask/T17724

CSE seems to depend on bindings being in dependency order.
The simplifier put's bindings into dependency order.

So do a simplifier run first.

We used to do in order

* ...
* optional: late spec + simplifier "post-late-spec" run
* optional: late cse
* simplifier "final" run
* ...

We now do:

* ...
* optional: late spec
* optional: simplifier "pre-late-cse" run + cse
* simplifier "final" run.
* ...

The result is that:

* At -O we now potentially do fewer runs of the simplifier.
  "post-late-spec" is gone if late-spec is on
* At -O2 we do an additional simplifier run : "pre-late-cse"
* At -O2 with -fspec-late we do what we already did.
  Only that the intermediate simplifier run is called "pre-late-cse"
  instead of "post-late-spec".

We also removed a case where occAnal dropped code it assumed to be dead.

1 files changed, 1 insertions, 0 deletions

diff --git a/compiler/simplCore/CSE.hs b/compiler/simplCore/CSE.hs
index 9a0945e290..0835f87bb0 100644
--- a/compiler/simplCore/CSE.hs
+++ b/compiler/simplCore/CSE.hs
@@ -327,6 +327,7 @@ the program; it's a kind of synthetic key for recursive bindings.
 ************************************************************************
 -}
 
+-- | Requires bindings to be in dependency order.
 cseProgram :: CoreProgram -> CoreProgram
 cseProgram binds = snd (mapAccumL (cseBind TopLevel) emptyCSEnv binds)