llvm dev - Sep 2011 - [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 }

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 }
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev

On Sep 13, 2011, at 4:01 AM, Kyriakos Georgiou wrote:
> 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?
Only other alternative is to avoid falling back on DAG,  at -O0, as much as
possible. If … FastISel handles everything then you completely sidestep DAG
nodes at -O0.

-
Devang
> 
> 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 }
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev

On 13/09/11 17:40, Devang Patel wrote:
> On Sep 13, 2011, at 4:01 AM, Kyriakos Georgiou wrote:
>> 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?
> Only other alternative is to avoid falling back on DAG,  at -O0, as much as
possible. If … FastISel handles everything then you completely sidestep DAG
nodes at -O0.
>
> -
> Devang
Compile time at -O0 isn't a high priority for us since, so if we did 
implement FastISel the only reason would be to improve debug information 
at -O0. Since this results in more code in the XCore backend that needs 
to be maintained over time I'd rather explore other options first.

Is there a view on which of the following approaches is better in 
general when two nodes with debug locations are merged because of CSE 
(either in the MachineCSE pass or because of the CSEMap in the 
SelectionDAG):

1) Use the DebugLoc of either of the pair of nodes (chosen arbitrarily).
2) Throw away the line number information on the merged node since it is 
not possible to preserve the information that operation is associated 
with multiple lines.

The current approach seems to be 1). Would you consider patches to 
change the behavior to 2)?

In the cases we have seen at -O0 for each of the pair of nodes to be 
merged there are also other nodes in the DAG with the same debug 
locations that won't be merged. Therefore if the line numbers for merged 
nodes were dropped it would still be possible to set breakpoints on each 
source line. It would also avoid many cases where the debugger jumps 
forward and backward when skipping.

Above -O0 I guess there would be fewer cases where the debugger jumps 
forward or backward, but there may be more cases where there are no 
instructions associated with a line and so lines will be skipped entirely.

-- 
Richard Osborne | XMOS
http://www.xmos.com