3 files changed, 8 insertions, 10 deletions
diff --git a/compiler/nativeGen/RegAlloc/Linear/JoinToTargets.hs b/compiler/nativeGen/RegAlloc/Linear/JoinToTargets.hs
index 6294743c48..768ddab788 100644
--- a/compiler/nativeGen/RegAlloc/Linear/JoinToTargets.hs
+++ b/compiler/nativeGen/RegAlloc/Linear/JoinToTargets.hs
@@ -17,7 +17,6 @@ import Instruction
 import Reg
 
 import BlockId
-import OldCmm  hiding (RegSet)
 import Digraph
 import DynFlags
 import Outputable
diff --git a/compiler/nativeGen/RegAlloc/Linear/Main.hs b/compiler/nativeGen/RegAlloc/Linear/Main.hs
index fc5b992603..fa71457808 100644
--- a/compiler/nativeGen/RegAlloc/Linear/Main.hs
+++ b/compiler/nativeGen/RegAlloc/Linear/Main.hs
@@ -116,7 +116,7 @@ import Instruction
 import Reg
 
 import BlockId
-import OldCmm hiding (RegSet)
+import Cmm hiding (RegSet)
 
 import Digraph
 import DynFlags
@@ -743,12 +743,13 @@ allocateRegsAndSpill reading keep spills alloc (r:rs)
                 Just (InMem slot) | reading   -> doSpill (ReadMem slot)
                                   | otherwise -> doSpill WriteMem
                 Nothing | reading   ->
-                   -- pprPanic "allocateRegsAndSpill: Cannot read from uninitialized register" (ppr r)
-                   -- ToDo: This case should be a panic, but we
-                   -- sometimes see an unreachable basic block which
-                   -- triggers this because the register allocator
-                   -- will start with an empty assignment.
-                   doSpill WriteNew
+                   pprPanic "allocateRegsAndSpill: Cannot read from uninitialized register" (ppr r)
+                   -- NOTE: if the input to the NCG contains some
+                   -- unreachable blocks with junk code, this panic
+                   -- might be triggered.  Make sure you only feed
+                   -- sensible code into the NCG.  In CmmPipeline we
+                   -- call removeUnreachableBlocks at the end for this
+                   -- reason.
 
                         | otherwise -> doSpill WriteNew
 
diff --git a/compiler/nativeGen/RegAlloc/Linear/Stats.hs b/compiler/nativeGen/RegAlloc/Linear/Stats.hs
index bfd196ac05..d8ca77537d 100644
--- a/compiler/nativeGen/RegAlloc/Linear/Stats.hs
+++ b/compiler/nativeGen/RegAlloc/Linear/Stats.hs
@@ -17,8 +17,6 @@ import RegAlloc.Linear.Base
 import RegAlloc.Liveness
 import Instruction
 
-import OldCmm  (GenBasicBlock(..))
-
 import UniqFM
 import Outputable