Yet another fix to mkAtomicArgs (for floating of casts)

Comment Note [Take care] explains.

mkAtomicArgs is a mess.  A substantial rewrite of Simplify is needed.

diff --git a/compiler/simplCore/Simplify.lhs b/compiler/simplCore/Simplify.lhs
index db08df6..bec02e6 100644 (file)
--- a/compiler/simplCore/Simplify.lhs
+++ b/compiler/simplCore/Simplify.lhs
@@ -1200,7 +1200,7 @@ mkAtomicArgsE env is_strict (Cast rhs co) thing_inside
        -- See also Note [Take care] below
   = do { id <- newId FSLIT("a") (exprType rhs)
        ; completeNonRecX env False id id rhs $ \ env ->
-         thing_inside env (Cast (Var id) co) }
+         thing_inside env (Cast (substExpr env (Var id)) co) }
 
 mkAtomicArgsE env is_strict rhs thing_inside
   | (Var fun, args) <- collectArgs rhs,                                -- It's an application
@@ -1220,11 +1220,18 @@ mkAtomicArgsE env is_strict rhs thing_inside
        | otherwise
        = do { arg_id <- newId FSLIT("a") arg_ty
             ; completeNonRecX env False {- pessimistic -} arg_id arg_id arg $ \env ->
-              go env (App fun (Var arg_id)) args }
+              go env (App fun (substExpr env (Var arg_id))) args }
                -- Note [Take care]:
-               -- This is sightly delicate.  If completeNonRecX was to do a postInlineUnconditionally
+               -- If completeNonRecX was to do a postInlineUnconditionally
                -- (undoing the effect of introducing the let-binding), we'd find arg_id had
-               -- no binding.   The exprIsTrivial is the only time that'll happen, though.
+               -- no binding; hence the substExpr.  This happens if we see
+               --      C (D x `cast` g)
+               -- Then we start by making a variable a1, thus
+               --      let a1 = D x `cast` g in C a1
+               -- But then we deal with the rhs of a1, getting
+               --      let a2 = D x, a1 = a1 `cast` g in C a1
+               -- And now the preInlineUnconditionally kicks in, and we substitute for a1
+       
        where
          arg_ty = exprType arg
          no_float_arg = not is_strict && (isUnLiftedType arg_ty) && not (exprOkForSpeculation arg)