X-Git-Url: http://git.megacz.com/?p=ghc-hetmet.git;a=blobdiff_plain;f=compiler%2Fcmm%2FCmmZipUtil.hs;h=a91d76f31d1b4c7033e3a8a9eebf73de707c9110;hp=c7a027e93fb87a4f0e195b9850d04e381577c3c1;hb=edc0bafd3fcd01b85a2e8894e5dfe149eb0e0857;hpb=1241c26f3552a2037263769e5ef7fa68d9f3be36

diff --git a/compiler/cmm/CmmZipUtil.hs b/compiler/cmm/CmmZipUtil.hs
index c7a027e..a91d76f 100644
--- a/compiler/cmm/CmmZipUtil.hs
+++ b/compiler/cmm/CmmZipUtil.hs
@@ -1,13 +1,16 @@
 
 module CmmZipUtil
   ( zipPreds
+  , givesUniquePredecessorTo
   )
 where
+import BlockId
 import Prelude hiding (last, unzip)
 import ZipCfg
+
 import Maybes
 
--- | Compute the predecessors of each *reachable* block
+-- | Compute the predecessors of each /reachable/ block
 zipPreds :: LastNode l => LGraph m l -> BlockEnv BlockSet
 zipPreds g = foldl add emptyBlockEnv (postorder_dfs g)
     where add env block@(Block id _) =
@@ -15,3 +18,22 @@ zipPreds g = foldl add emptyBlockEnv (postorder_dfs g)
                        let preds = lookupBlockEnv env sid `orElse` emptyBlockSet
                        in  extendBlockEnv env sid (extendBlockSet preds id))
             env (succs block)
+
+-- | Tell if a graph gives a block a unique predecessor.  For
+-- efficiency, this function is designed to be partially applied.
+
+givesUniquePredecessorTo :: LastNode l => LGraph m l -> BlockId -> Bool
+givesUniquePredecessorTo g = \id -> elemBlockSet id singlePreds
+    -- accumulates a pair of sets: the set of all blocks containing a single
+    -- predecessor, and the set of all blocks containing at least two predecessors
+    where (singlePreds, _) = fold_blocks add (emptyBlockSet, emptyBlockSet) g
+          add b (single, multi) = foldl add_pred (single, multi) (succs b)
+          add_pred pair@(single, multi) id =
+              if elemBlockSet id multi then pair
+              else if elemBlockSet id single then
+                       (removeBlockSet single id, extendBlockSet multi id)
+                   else
+                       (extendBlockSet single id, multi)
+              
+    
+