Stop ug_boring_info retaining a chain of old CoreExprwip/t20134

It was noticed in #20134 that each simplifier iteration used an
increasing amount of memory and that a certain portion of memory was not
released until the simplfier had completely finished.

I profiled the program using `-hi` profiling and observed that there was
a thunk arising in the computation of `ug_boring_ok`. On each iteration
`ug_boring_ok` would be updated, but not forced, which would leave a
thunk in the shape of

ug_boring_ok = inlineBoringOk expr0 || inlineBoringOk expr2 || inlineBoringOk expr3 || ...

which would retain all previous `expr` until `ug_boring_ok` was forced
or discarded.

Forcing this accumulator eagerly results in a flat profile over multiple
simplifier runs.

This reduces the maximum residency when compiling the test in #20134
from 2GB to 1.3G.

-------------------------
Metric Decrease:
    T11545
-------------------------

1 files changed, 19 insertions, 8 deletions

diff --git a/compiler/GHC/Core/Opt/Simplify.hs b/compiler/GHC/Core/Opt/Simplify.hs
index 1bbb728de6..1cecda63b2 100644
--- a/compiler/GHC/Core/Opt/Simplify.hs
+++ b/compiler/GHC/Core/Opt/Simplify.hs
@@ -4003,8 +4003,7 @@ simplLetUnfolding env top_lvl cont_mb id new_rhs rhs_ty arity unf
 mkLetUnfolding :: UnfoldingOpts -> TopLevelFlag -> UnfoldingSource
                -> InId -> OutExpr -> SimplM Unfolding
 mkLetUnfolding !uf_opts top_lvl src id new_rhs
-  = is_bottoming `seq`  -- See Note [Force bottoming field]
-    return (mkUnfolding uf_opts src is_top_lvl is_bottoming new_rhs)
+  = return (mkUnfolding uf_opts src is_top_lvl is_bottoming new_rhs)
             -- We make an  unfolding *even for loop-breakers*.
             -- Reason: (a) It might be useful to know that they are WHNF
             --         (b) In GHC.Iface.Tidy we currently assume that, if we want to
@@ -4012,8 +4011,11 @@ mkLetUnfolding !uf_opts top_lvl src id new_rhs
             --             to expose.  (We could instead use the RHS, but currently
             --             we don't.)  The simple thing is always to have one.
   where
-    is_top_lvl   = isTopLevel top_lvl
-    is_bottoming = isDeadEndId id
+    -- Might as well force this, profiles indicate up to 0.5MB of thunks
+    -- just from this site.
+    !is_top_lvl   = isTopLevel top_lvl
+    -- See Note [Force bottoming field]
+    !is_bottoming = isDeadEndId id
 
 -------------------
 simplStableUnfolding :: SimplEnv -> TopLevelFlag
@@ -4050,11 +4052,17 @@ simplStableUnfolding env top_lvl mb_cont id rhs_ty id_arity unf
                           , ug_boring_ok = boring_ok
                           }
                           -- Happens for INLINE things
-                     -> let guide' =
+                        -- Really important to force new_boring_ok as otherwise
+                        -- `ug_boring_ok` is a thunk chain of
+                        -- inlineBoringExprOk expr0
+                        --  || inlineBoringExprOk expr1 || ...
+                        --  See #20134
+                     -> let !new_boring_ok = boring_ok || inlineBoringOk expr'
+                            guide' =
                               UnfWhen { ug_arity = arity
                                       , ug_unsat_ok = sat_ok
-                                      , ug_boring_ok =
-                                          boring_ok || inlineBoringOk expr'
+                                      , ug_boring_ok = new_boring_ok
+
                                       }
                         -- Refresh the boring-ok flag, in case expr'
                         -- has got small. This happens, notably in the inlinings
@@ -4075,7 +4083,9 @@ simplStableUnfolding env top_lvl mb_cont id rhs_ty id_arity unf
         | otherwise -> return noUnfolding   -- Discard unstable unfoldings
   where
     uf_opts    = seUnfoldingOpts env
-    is_top_lvl = isTopLevel top_lvl
+    -- Forcing this can save about 0.5MB of max residency and the result
+    -- is small and easy to compute so might as well force it.
+    !is_top_lvl = isTopLevel top_lvl
     act        = idInlineActivation id
     unf_env    = updMode (updModeForStableUnfoldings act) env
          -- See Note [Simplifying inside stable unfoldings] in GHC.Core.Opt.Simplify.Utils
@@ -4216,3 +4226,4 @@ for the RHS as well as the LHS, but that seems more conservative
 than necesary.  Allowing some inlining might, for example, eliminate
 a binding.
 -}
+