[LLVMdev] Dead node removal in DAGCombiner

[Duncan Sands]

Hi Akira,

> Is this piece of code in DAGCombiner::visitLOAD removing a dead node?
>
> 06155         if (N->use_empty()) {
> 06156           removeFromWorkList(N);
> 06157           DAG.DeleteNode(N);
> 06158         }

yes.

> If it is, is there a reason it doesn't push its operands to the work
> list as done in line 974-975?
>
> 00970     // If N has no uses, it is dead.  Make sure to revisit all
> N's operands once
> 00971     // N is deleted from the DAG, since they too may now be dead
> or may have a
> 00972     // reduced number of uses, allowing other xforms.
> 00973     if (N->use_empty()&&  N !=&Dummy) {
> 00974       for (unsigned i = 0, e = N->getNumOperands(); i != e; ++i)
> 00975         AddToWorkList(N->getOperand(i).getNode());
> 00976
> 00977       DAG.DeleteNode(N);
> 00978       continue;
> 00979     }

I suspect they could be removed.  Probably a helper function should be added
for this and used all over the place.  Also, the RAUW earlier can cause nodes
to be unified (due to CSE).  Probably the WorkListRemover class should add nodes
to the worklist in the NodeUpdated method.

>
> Or is it one of the corner cases mentioned in SelectionDAGISel.cpp:644?
>
> 00644       // Skip dead nodes. DAGCombiner is expected to eliminate
> all dead nodes,
> 00645       // but there are currently some corner cases that it
> misses. Also, this
> 00646       // makes it theoretically possible to disable the DAGCombiner.
>
> I am seeing an error message "LLVM ERROR: Cannot select:" because
> operands of a dead node do not get removed.

What is it that cannot be selected?

Ciao, Duncan.