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

[Villmow, Micah]

Kyriakos,
 We have this same problem with the AMDIL target. The only way to get around it that we have found, and it doesn't work 100% of the time, is to disable all the combine operations. The problem with doing this in a generic way is disabling DAG.combine breaks the x86 backend and generates really weird code in many cases.

Micah

> -----Original Message-----
> From: llvmdev-bounces at cs.uiuc.edu [mailto:llvmdev-bounces at cs.uiuc.edu]
> On Behalf Of Kyriakos Georgiou
> Sent: Tuesday, September 13, 2011 4:01 AM
> To: llvmdev at cs.uiuc.edu
> Subject: [LLVMdev] Handling of DebugLocs during CSE of SelectionDAG
> nodes.
> 
> 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 }
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