[LLVMdev] Handling of DebugLocs during CSE of SelectionDAG nodes.

[Kyriakos Georgiou]

I've been investigating a case with the XCore target (which doesn't use
FastISel) where the DWARF line number emitted at -O0 results in the xgdb
visiting source lines in an unexpected order. I've tracked down the
problem to the handling of DebugLocs in the selection DAG, in the getNode 
method shown bellow. 

It first tries to find if a similar node already exists in order to use that one
otherwise it creates a new one. The problem of this approach is that it will 
wrongly use the line number debug info from the first node it will find. I 
can not see any reasonable way of dealing with that, but only dropping the
line number info from that node in order to keep the debugger behaving 
correctly. Is that the correct approach? Are there any other suggestions?

02367 /// getNode - Gets or creates the specified node.
02368 ///
02369 SDValue SelectionDAG::getNode(unsigned Opcode, DebugLoc DL, EVT VT) {
02370   FoldingSetNodeID ID;
02371   AddNodeIDNode(ID, Opcode, getVTList(VT), 0, 0);
02372   void *IP = 0;
02373   if (SDNode *E = CSEMap.FindNodeOrInsertPos(ID, IP))
02374     return SDValue(E, 0);
02375 
02376   SDNode *N = new (NodeAllocator) SDNode(Opcode, DL, getVTList(VT));
02377   CSEMap.InsertNode(N, IP);
02378 
02379   AllNodes.push_back(N);
02380 #ifndef NDEBUG
02381   VerifySDNode(N);
02382 #endif
02383   return SDValue(N, 0);
02384 }