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

[Devang Patel]

On Sep 14, 2011, at 10:34 AM, Jakob Stoklund Olesen wrote:

> 
> On Sep 14, 2011, at 7:37 AM, Richard Osborne wrote:
> 
>> 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)?
> 
> At -O0, SelectionDAG nodes are scheduled in source order when possible.
> 
> When merging identical nodes, you should make sure that the resulting node has the earlier source order, and the debug location should match.
> 
> That way, CSE simply looks like the last definition of a value has been removed.  Debug location and source order of the first definition should be preserved.

hmm… I agree with Jakob. See SelectionDAG::GetOrdering()

-
Devang