Buggy optimizations caused function-call return to share the function's entry point

- Block concat and branch-chain elimination were allowing a function call
  to return to the caller's entry point. But that doesn't leave anywhere
  for the infotable on the stack, since the SP on return must be the same
  as the SP on entry to the procedure.

compiler/cmm/CmmContFlowOpt.hs

@@ -83,14 +83,24 @@ replaceLabels env (BasicBlock id stmts)
 ----------------------------------------------------------------
 branchChainElimZ :: Tx CmmGraph
 -- Remove any basic block of the form L: goto L',
+-- and replace L with L' everywhere else,
+-- unless L is the successor of a call instruction and L'
+-- is the entry block. You don't want to set the successor
+-- of a function call to the entry block because there is no good way
+-- to store both the infotables for the call and from the callee,
+-- while putting the stack pointer in a consistent place.
+--
+-- JD isn't quite sure when it's safe to share continuations for different
+-- function calls -- have to think about where the SP will be,
+-- so we'll table that problem for now by leaving all call successors alone.
 branchChainElimZ g@(G.LGraph eid _)
   | null lone_branch_blocks     -- No blocks to remove
   = noTx g
   | otherwise
   = aTx $ replaceLabelsZ env $ G.of_block_list eid (self_branches ++ others)
   where
-  where
+    blocks = G.to_block_list g
+    (lone_branch_blocks, others) = partitionWith isLoneBranchZ blocks
     env = mkClosureBlockEnvZ lone_branch_blocks
     self_branches =
       let loop_to (id, _) =
@@ -101,27 +111,41 @@ branchChainElimZ g@(G.LGraph eid _)
       in  mapMaybe loop_to lone_branch_blocks
     lookup id = lookupBlockEnv env id `orElse` id 
 
-isLoneBranchZ :: CmmBlock -> Either (BlockId, BlockId) CmmBlock
-isLoneBranchZ (G.Block id (G.ZLast (G.LastOther (LastBranch target))))
-    | id /= target  = Left (id,target)
-isLoneBranchZ other = Right other
-   -- An infinite loop is not a link in a branch chain!
+    call_succs = foldl add emptyBlockSet blocks
+      where add succs b =
+              case G.last (G.unzip b) of
+                LastOther (LastCall _ (Just k) _ _ _) -> extendBlockSet succs k
+                _ -> succs
+    isLoneBranchZ :: CmmBlock -> Either (BlockId, BlockId) CmmBlock
+    isLoneBranchZ (G.Block id (G.ZLast (G.LastOther (LastBranch target))))
+        | id /= target && not (elemBlockSet id call_succs) = Left (id,target)
+    isLoneBranchZ other = Right other
+       -- An infinite loop is not a link in a branch chain!
+
+maybeReplaceLabels :: (Last -> Bool) -> BlockEnv BlockId -> CmmGraph -> CmmGraph
+maybeReplaceLabels lpred env =
+  replace_eid . G.map_nodes id middle last
+   where
+     replace_eid (G.LGraph eid blocks) = G.LGraph (lookup eid) blocks
+     middle = mapExpDeepMiddle exp
+     last l = if lpred l then mapExpDeepLast exp (last' l) else l
+     last' (LastBranch bid) = LastBranch (lookup bid)
+     last' (LastCondBranch p t f) = LastCondBranch p (lookup t) (lookup f)
+     last' (LastSwitch e arms) = LastSwitch e (map (liftM lookup) arms)
+     last' (LastCall t k a res r) = LastCall t (liftM lookup k) a res r
+     exp (CmmLit (CmmBlock bid)) = CmmLit (CmmBlock (lookup bid))
+     exp   (CmmStackSlot (CallArea (Young id)) i) =
+       CmmStackSlot (CallArea (Young (lookup id))) i
+     exp e = e
+     lookup id = fmap lookup (lookupBlockEnv env id) `orElse` id 
 
 replaceLabelsZ :: BlockEnv BlockId -> CmmGraph -> CmmGraph
-replaceLabelsZ env = replace_eid . G.map_nodes id middle last
-  where
-    replace_eid (G.LGraph eid blocks) = G.LGraph (lookup eid) blocks
-    middle = mapExpDeepMiddle exp
-    last l = mapExpDeepLast   exp (last' l)
-    last' (LastBranch bid) = LastBranch (lookup bid)
-    last' (LastCondBranch p t f) = LastCondBranch p (lookup t) (lookup f)
-    last' (LastSwitch e arms) = LastSwitch e (map (liftM lookup) arms)
-    last' (LastCall t k a res r) = LastCall t (liftM lookup k) a res r
-    exp (CmmLit (CmmBlock bid)) = CmmLit (CmmBlock (lookup bid))
-    exp   (CmmStackSlot (CallArea (Young id)) i) =
-      CmmStackSlot (CallArea (Young (lookup id))) i
-    exp e = e
-    lookup id = fmap lookup (lookupBlockEnv env id) `orElse` id 
+replaceLabelsZ = maybeReplaceLabels (const True)
+
+-- replaceBranchLabels :: BlockEnv BlockId -> CmmGraph -> CmmGraph
+-- replaceBranchLabels env g@(LGraph _ _) = maybeReplaceLabels lpred env g
+--   where lpred (LastBranch _) = True
+--         lpred _ = False
 
 replaceBranches :: BlockEnv BlockId -> CmmGraph -> CmmGraph
 replaceBranches env g = map_nodes id id last g
@@ -141,9 +165,10 @@ predMap g = G.fold_blocks add_preds emptyBlockEnv g -- find the back edges
           extendBlockEnv env b' $
                 extendBlockSet (lookupBlockEnv env b' `orElse` emptyBlockSet) bid
 ----------------------------------------------------------------
--- If a block B branches to a label L, and L has no other predecessors,
+-- If a block B branches to a label L, L is not the entry block,
+-- and L has no other predecessors,
 -- then we can splice the block starting with L onto the end of B.
--- Because this optmization can be inhibited by unreachable blocks,
+-- Because this optimization can be inhibited by unreachable blocks,
 -- we first take a pass to drops unreachable blocks.
 -- Order matters, so we work bottom up (reverse postorder DFS).
 --
@@ -168,12 +193,12 @@ blockConcatZ' g@(G.LGraph eid blocks) =
                   else unchanged
                _ -> unchanged
         num_preds bid = liftM sizeBlockSet (lookupBlockEnv backEdges bid) `orElse` 0
-        canConcatWith b' = num_preds b' == 1
+        canConcatWith b' = b' /= eid && num_preds b' == 1
         backEdges = predMap g
         splice blocks' h bid' =
           case lookupBlockEnv blocks' bid' of
             Just (G.Block _ t) -> G.zip $ G.ZBlock h t
-            Nothing -> pprPanic "unknown successor block" (ppr bid' <+> ppr blocks' <+> ppr blocks)
+            Nothing -> panic "unknown successor block"
         tx = if changed then aTx else noTx
 ----------------------------------------------------------------
 mkClosureBlockEnv :: [(BlockId, BlockId)] -> BlockEnv BlockId