Fix a bug in the liveness analysis

We were being overly conservative, registers that were live only at a
branch target were not recorded as being dead in the branch-not-taken
code.

diff --git a/compiler/nativeGen/RegisterAlloc.hs b/compiler/nativeGen/RegisterAlloc.hs
index 2031fa7..0a732fb 100644 (file)
--- a/compiler/nativeGen/RegisterAlloc.hs
+++ b/compiler/nativeGen/RegisterAlloc.hs
@@ -353,7 +353,10 @@ computeLiveness sccs
 
   liveness liveregs blockmap done []  = (liveregs, done)
   liveness liveregs blockmap done (instr:instrs) 
-       = liveness liveregs2 blockmap ((instr,r_dying,w_dying):done) instrs
+        | not_a_branch = liveness liveregs1 blockmap 
+                                ((instr,r_dying,w_dying):done) instrs
+        | otherwise = liveness liveregs_br blockmap
+                                ((instr,r_dying_br,w_dying):done) instrs
        where 
              RU read written = regUsage instr
 
@@ -362,24 +365,32 @@ computeLiveness sccs
              liveregs1 = (liveregs `delListFromUniqSet` written)
                                    `addListToUniqSet` read
 
+             -- registers that are not live beyond this point, are recorded
+             --  as dying here.
+             r_dying  = [ reg | reg <- read, reg `notElem` written,
+                                not (elementOfUniqSet reg liveregs) ]
+
+             w_dying = [ reg | reg <- written,
+                               not (elementOfUniqSet reg liveregs) ]
+
              -- union in the live regs from all the jump destinations of this
              -- instruction.
              targets = jumpDests instr [] -- where we go from here
-             liveregs2 = unionManyUniqSets
-                           (liveregs1 : map targetLiveRegs targets)
+              not_a_branch = null targets
 
               targetLiveRegs target = case lookupUFM blockmap target of
                                         Just ra -> ra
                                         Nothing -> emptyBlockMap
 
-             -- registers that are not live beyond this point, are recorded
-             --  as dying here.
-             r_dying  = [ reg | reg <- read, reg `notElem` written,
-                                not (elementOfUniqSet reg liveregs) ]
+              live_from_branch = unionManyUniqSets (map targetLiveRegs targets)
 
-             w_dying = [ reg | reg <- written,
-                               not (elementOfUniqSet reg liveregs) ]
+             liveregs_br = liveregs1 `unionUniqSets` live_from_branch
 
+              -- registers that are live only in the branch targets should
+              -- be listed as dying here.
+              live_branch_only = live_from_branch `minusUniqSet` liveregs
+              r_dying_br = uniqSetToList (mkUniqSet r_dying `unionUniqSets`
+                                          live_branch_only)
 
 -- -----------------------------------------------------------------------------
 -- Linear sweep to allocate registers