[Hexagon] Weaken over-aggressive assertion in RDFGraph

[niravd]

Summary

Improvements in consecutive merge stores in SelectionDAG (D14834)
trigger assertion failures in getNextRef during clang
bootstrapping. Rework to make cases that assertion failure return
default "nullptr" instead.

[kparzysz]

Hitting this assertion is very bad. The list of nodes is a circular list with a single "code" and a bunch of "refs". Once you visit the last ref, the next node should be the code, there is no other possibility.

Something is seriously wrong somewhere.

[kparzysz]

How are you getting this assertion? I applied D14384, built clang, then built another clang from scratch with the previous one, ran check-llvm-codegen-hexagon and I didn't get any errors.

[niravd]

In D25791#577097, @kparzysz wrote:

How are you getting this assertion? I applied D14384, built clang, then built another clang from scratch with the previous one, ran check-llvm-codegen-hexagon and I didn't get any errors.

If you checkout 284154 you should see the problem replicated during bootstrapping. I've verified that the tests generate the same SelectionDAG as before and that the problem still holds with -O0 for the stage 1 compiler where D14834 should do nothing. This should mean that the problem is in the release compilation of Hexagon backend with D14834.

[kparzysz]

What is stage 1 compiler? One built using the default C++ compiler? If the problem happens when clang is built with -O0, why do you conclude that the problem is in the release build?

lib/Target/Hexagon/RDFGraph.h

NodeAddr<RefNode*> RefNode::getNextRef(RegisterRef RR, Predicate P,
     while (NA.Addr != this) {
       if (NA.Addr->getType() == NodeAttrs::Ref) {
         NodeAddr<RefNode*> RA = NA;
         if (RA.Addr->getRegRef(G) == RR && P(NA))
           return NA;
         if (NextOnly)
           break;
         NA = G.addr<NodeBase*>(NA.Addr->getNext());
-      } else {
+      } else if (NA.Addr->getType() == NodeAttrs::Code) {
         // We've hit the beginning of the chain.
-        assert(NA.Addr->getType() == NodeAttrs::Code);
         NodeAddr<CodeNode*> CA = NA;
         NA = CA.Addr->getFirstMember(G);
+      } else { // No node found.
+        break;
       }
     }
     // Return the equivalent of "nullptr" if such a node was not found.
     return NodeAddr<RefNode*>();
   }
 
   template <typename Predicate>
   NodeList CodeNode::members_if(Predicate P, const DataFlowGraph &G) const {