DAGCombiner: Don't search up chain through non-memory dependencies

[arsenm]

Summary

This is to try to fix the problem described here :http://lists.cs.uiuc.edu/pipermail/llvm-commits/Week-of-Mon-20150622/284139.html

[hfinkel]

lib/CodeGen/SelectionDAG/DAGCombiner.cpp

14022

Can you explain exactly what situation your trying to prevent and why it is bad? I don't understand the problem from the referenced mailing-list message because everything there looks legal.

[arsenm]

lib/CodeGen/SelectionDAG/DAGCombiner.cpp

14022

My problem/question is is it allowed for a node using the value out of a CopyFromReg to be on the same chain as the CopyFromReg itself?

I'm not seeing it right now with just the isAlias patch, but it looked something like

PtrVal, Chain0 = CopyFromReg EntryToken ...
LoadVal, Chain1 = load EntryToken, PtrVal

It's possible I had something else broken in my tree that I don't remember. I'll see if I can run into this again with some of the other patches I'm working on.

[hfinkel]

lib/CodeGen/SelectionDAG/DAGCombiner.cpp

14022

Yes, I believe this is legal. Data dependencies do not need to be redundantly encoded as chain dependencies also.

lib/CodeGen/SelectionDAG/DAGCombiner.cpp

case ISD::STORE: {
           !cast<LSBaseSDNode>(Chain.getNode())->isVolatile();
 
       // If chain is alias then stop here.
       if (!(IsLoad && IsOpLoad) &&
           isAlias(cast<LSBaseSDNode>(N), cast<LSBaseSDNode>(Chain.getNode()))) {
         DEBUG(dbgs() << "Load alias\n");
         Aliases.push_back(Chain);
       } else {
-        // Look further up the chain.
+        SDValue NextChain = Chain.getOperand(0);
+
+        if (!isa<MemSDNode>(NextChain)) {
+          // Don't continue the search if we see a chain dependency that isn't a
+          // memory access.
+          //
+          // e.g. we could have a dependency on a CopyFromReg into the pointer
+          // value of a load.
+          Chains.push_back(Chain);
+          break;
+        }
+
+        // Look further up the chain at possible memory dependencies.
+        Chains.push_back(NextChain);
         DEBUG(dbgs() << "Look up load / store chain: ");
-        Chain.getOperand(0)->dump(&DAG);
-        Chains.push_back(Chain.getOperand(0));
+        NextChain->dump(&DAG);
+        Chains.push_back(NextChain);
         ++Depth;
       }
       break;
     }
 
     case ISD::TokenFactor:
       DEBUG(dbgs() << "Push token factor\n");
       // We have to check each of the operands of the token factor for "small"