Make keepAlive# out-of-line

This is a naive approach to fixing the unsoundness noticed in #21708.
Specifically, we remove the lowering of `keepAlive#` via CorePrep and
instead turn it into an out-of-line primop.
This is simple, inefficient (since the continuation must now be heap
allocated), but good enough for 9.4.1. We will revisit this
(particiularly via #16098) in a future release.

Metric Increase:
    T4978
    T7257
    T9203

3 files changed, 2 insertions, 36 deletions

diff --git a/compiler/GHC/Builtin/primops.txt.pp b/compiler/GHC/Builtin/primops.txt.pp
index d81dd3bc92..ac03c20dbd 100644
--- a/compiler/GHC/Builtin/primops.txt.pp
+++ b/compiler/GHC/Builtin/primops.txt.pp
@@ -3307,6 +3307,7 @@ primop KeepAliveOp "keepAlive#" GenPrimOp
      polymorphic type might suggest; see the section \"RuntimeRep polymorphism
      in continuation-style primops\" for details. }
    with
+   out_of_line = True
    strictness = { \ _arity -> mkClosedDmdSig [topDmd, topDmd, strictOnceApply1Dmd] topDiv }
 
 
diff --git a/compiler/GHC/CoreToStg/Prep.hs b/compiler/GHC/CoreToStg/Prep.hs
index 583badce52..d03df0eedb 100644
--- a/compiler/GHC/CoreToStg/Prep.hs
+++ b/compiler/GHC/CoreToStg/Prep.hs
@@ -28,10 +28,7 @@ import GHC.Tc.Utils.Env
 import GHC.Unit
 
 import GHC.Builtin.Names
-import GHC.Builtin.PrimOps
-import GHC.Builtin.PrimOps.Ids (primOpId)
 import GHC.Builtin.Types
-import GHC.Builtin.Types.Prim ( realWorldStatePrimTy )
 
 import GHC.Core.Utils
 import GHC.Core.Opt.Arity
@@ -1072,36 +1069,6 @@ cpeApp top_env expr
         = let (terminal, args') = collect_args arg
           in cpe_app env terminal (args' ++ args)
 
-    -- See Note [keepAlive# magic].
-    cpe_app env
-            (Var f)
-            args
-        | Just KeepAliveOp <- isPrimOpId_maybe f
-        , CpeApp (Type arg_lev)
-          : CpeApp (Type _result_rep)
-          : CpeApp (Type arg_ty)
-          : CpeApp (Type result_ty)
-          : CpeApp arg
-          : CpeApp s0
-          : CpeApp k
-          : rest <- args
-        = do { y  <- newVar (cpSubstTy env result_ty)
-             ; s2 <- newVar realWorldStatePrimTy
-             ; -- beta reduce if possible
-             ; (floats, k') <- case k of
-                  Lam s body -> cpe_app (extendCorePrepEnvExpr env s s0) body rest
-                  _          -> cpe_app env k (CpeApp s0 : rest)
-             ; let touchId = primOpId TouchOp
-                   expr = Case k' y result_ty [Alt DEFAULT [] rhs]
-                   rhs = let scrut = mkApps (Var touchId) [Type arg_lev, Type arg_ty, arg, Var realWorldPrimId]
-                         in Case scrut s2 result_ty [Alt DEFAULT [] (Var y)]
-             ; (floats', expr') <- cpeBody env expr
-             ; return (floats `appendFloats` floats', expr')
-             }
-        | Just KeepAliveOp <- isPrimOpId_maybe f
-        = pprPanic "invalid keepAlive# application" $
-            vcat [ text "args:" <+> ppr args ]
-
     -- runRW# magic
     cpe_app env (Var f) (CpeApp _runtimeRep@Type{} : CpeApp _type@Type{} : CpeApp arg : rest)
         | f `hasKey` runRWKey
diff --git a/compiler/GHC/StgToCmm/Prim.hs b/compiler/GHC/StgToCmm/Prim.hs
index 22367e48ff..5d459ba7ad 100644
--- a/compiler/GHC/StgToCmm/Prim.hs
+++ b/compiler/GHC/StgToCmm/Prim.hs
@@ -1632,9 +1632,7 @@ emitPrimOp cfg primop =
   TraceEventBinaryOp -> alwaysExternal
   TraceMarkerOp -> alwaysExternal
   SetThreadAllocationCounter -> alwaysExternal
-
-  -- See Note [keepAlive# magic] in GHC.CoreToStg.Prep.
-  KeepAliveOp -> panic "keepAlive# should have been eliminated in CorePrep"
+  KeepAliveOp -> alwaysExternal
 
  where
   profile  = stgToCmmProfile  cfg