[Dominators] Make slow walks shorter

[kuhar]

Summary

When DFS numbers are not yet calculated for a dominator tree, we have to walk it up to say whether one node dominates some other.

This patch makes the slow walks shorter by only walking until the level of the node we check against is reached. This is because a node cannot possibly dominate something higher in its tree.

When running opt with -O3, the patch results in:

• 25% fewer loop iterations for opt (fullLTO)
• 30% fewer loop iterations for sqlite

[NutshellySima]

LGTM.

llvm/trunk/include/llvm/Support/GenericDomTree.h

 
  private:
   bool dominatedBySlowTreeWalk(const DomTreeNodeBase<NodeT> *A,
                                const DomTreeNodeBase<NodeT> *B) const {
     assert(A != B);
     assert(isReachableFromEntry(B));
     assert(isReachableFromEntry(A));
 
+    const unsigned ALevel = A->getLevel();
     const DomTreeNodeBase<NodeT> *IDom;
-    while ((IDom = B->getIDom()) != nullptr && IDom != A && IDom != B)
+
+    // Don't walk nodes above A's subtree. When we reach A's level, we must
+    // either find A or be in some other subtree not dominated by A.
+    while ((IDom = B->getIDom()) != nullptr && IDom->getLevel() >= ALevel)
       B = IDom;  // Walk up the tree
-    return IDom != nullptr;
+
+    return B == A;
   }
 
   /// Wipe this tree's state without releasing any resources.
   ///
   /// This is essentially a post-move helper only. It leaves the object in an
   /// assignable and destroyable state, but otherwise invalid.
   void wipe() {
     DomTreeNodes.clear();