More heuristics for Combiner-AA. Still catches all important cases, but

compile time penalty on gnugo, the worst case in MultiSource, is down to
about 2.5% from 30%

llvm-svn: 83824

llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp

@@ -6230,6 +6230,7 @@ void DAGCombiner::GatherAllAliases(SDNode *N, SDValue OriginalChain,
 
   // Starting off.
   Chains.push_back(OriginalChain);
+  unsigned Depth = 0;
   
   // Look at each chain and determine if it is an alias.  If so, add it to the
   // aliases list.  If not, then continue up the chain looking for the next
@@ -6238,6 +6239,20 @@ void DAGCombiner::GatherAllAliases(SDNode *N, SDValue OriginalChain,
     SDValue Chain = Chains.back();
     Chains.pop_back();
     
+    // For TokenFactor nodes, look at each operand and only continue up the 
+    // chain until we find two aliases.  If we've seen two aliases, assume we'll 
+    // find more and revert to original chain since the xform is unlikely to be
+    // profitable.
+    // 
+    // FIXME: The depth check could be made to return the last non-aliasing 
+    // chain we found before we hit a tokenfactor rather than the original
+    // chain.
+    if (Depth > 6 || Aliases.size() == 2) {
+      Aliases.clear();
+      Aliases.push_back(OriginalChain);
+      break;
+    }
+
     // Don't bother if we've been before.
     if (!Visited.insert(Chain.getNode()))
       continue;
@@ -6268,8 +6283,7 @@ void DAGCombiner::GatherAllAliases(SDNode *N, SDValue OriginalChain,
       } else {
         // Look further up the chain.
         Chains.push_back(Chain.getOperand(0));
-        // Clean up old chain.
-        AddToWorkList(Chain.getNode());
+        ++Depth;
       }
       break;
     }
@@ -6285,8 +6299,7 @@ void DAGCombiner::GatherAllAliases(SDNode *N, SDValue OriginalChain,
       }
       for (unsigned n = Chain.getNumOperands(); n;)
         Chains.push_back(Chain.getOperand(--n));
-      // Eliminate the token factor if we can.
-      AddToWorkList(Chain.getNode());
+      ++Depth;
       break;
 
     default: