Add RubbishLit for absent bindings of UnliftedRep

Summary:
Trac #9279 reminded us that the worker wrapper transformation copes
really badly with absent unlifted boxed bindings.

As `Note [Absent errors]` in WwLib.hs points out, we can't just use
`absentError` for unlifted bindings because there is no bottom to hide
the error in.
So instead, we synthesise a new `RubbishLit` of type
`forall (a :: TYPE 'UnliftedRep). a`, which code-gen may subsitute for
any boxed value. We choose `()`, so that there is a good chance that
the program crashes instead instead of leading to corrupt data, should
absence analysis have been too optimistic (#11126).

Reviewers: simonpj, hvr, goldfire, bgamari, simonmar

Reviewed By: simonpj

Subscribers: osa1, rwbarton, carter

GHC Trac Issues: #15627, #9279, #4306, #11126

Differential Revision: https://phabricator.haskell.org/D5153

1 files changed, 4 insertions, 0 deletions

diff --git a/compiler/ghci/ByteCodeAsm.hs b/compiler/ghci/ByteCodeAsm.hs
index 476a9b2efd..4473a9e9b2 100644
--- a/compiler/ghci/ByteCodeAsm.hs
+++ b/compiler/ghci/ByteCodeAsm.hs
@@ -460,6 +460,10 @@ assembleI dflags i = case i of
       LitNumWord64  -> int64 (fromIntegral i)
       LitNumInteger -> panic "ByteCodeAsm.literal: LitNumInteger"
       LitNumNatural -> panic "ByteCodeAsm.literal: LitNumNatural"
+    -- We can lower 'RubbishLit' to an arbitrary constant, but @NULL@ is most
+    -- likely to elicit a crash (rather than corrupt memory) in case absence
+    -- analysis messed up.
+    literal RubbishLit         = int 0
 
     litlabel fs = lit [BCONPtrLbl fs]
     addr (RemotePtr a) = words [fromIntegral a]