[PATCH] D33769: [SelectionDAG] Get rid of recursion in CalcNodeSethiUllmanNumber

Max Kazantsev via Phabricator via llvm-commits llvm-commits at lists.llvm.org
Fri Jun 9 00:17:18 PDT 2017

mkazantsev added inline comments.

Comment at: lib/CodeGen/SelectionDAG/ScheduleDAGRRList.cpp:1864
+                                          std::vector<unsigned> &SUNumbers) {
+  // Use WorkList to avoid stack overflow on excessively large IRs.
+  struct WorkState {
reames wrote:
> I think you can use a much simpler bit of code here.  The key bit is that we can't compute the current node until all predecessors are done, but that (by assumption from the existing code) the graph is a tree.  (Add an assert that enforces that please!)  Also, Wikipedia helps here.  :)
> If you do something along the lines of:
> while (!Worklist.empty())
>   Cur = Worklist.pop_back();
>   if (not all preds done) {
>      push(Cur)
>      push(each pred)
>     continue
>   }
>   compute answer using preds
> }
> I think you get the same result right?  This will eliminate the need for the intermediate state.
We still need a workstate to keep the current pred in order to avoid pushing an element more than once into the stack, but you are right, it can be done in a much more straightforward way. :)


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