Teach the DominatorTree fallback to recalculation when applying updates to speedup JT (PR37929)

Summary:
This patch makes the dominatortree recalculate when applying updates with the size of the update vector larger than a threshold. Directly applying updates is usually slower than recalculating the whole domtree in this case. This patch fixes an issue which causes JT running slowly on some inputs.

In bug 37929, the dominator tree is trying to apply 19,000+ updates several times, which takes several minutes.

After this patch, the time used by DT.applyUpdates:

| Input | Before (s) | After (s) | Speedup |
| the 2nd Reproducer in 37929 | 297 | 0.15 | 1980x |
| clang-5.0.0.0.bc | 9.7 | 4.3 | 2.26x |
| clang-5.0.0.4.bc | 11.6 | 2.6 | 4.46x |

Reviewers: kuhar, brzycki, trentxintong, davide, dmgreen, grosser

Reviewed By: kuhar, brzycki

Subscribers: kristina, llvm-commits

Differential Revision: https://reviews.llvm.org/D53245

llvm-svn: 345353

llvm/include/llvm/Support/GenericDomTreeConstruction.h

@@ -1191,6 +1191,20 @@ struct SemiNCAInfo {
     });
     LLVM_DEBUG(dbgs() << "\n");
 
+    // Recalculate the DominatorTree when the number of updates
+    // exceeds a threshold, which usually makes direct updating slower than
+    // recalculation. We select this threshold proportional to the
+    // size of the DominatorTree. The constant is selected
+    // by choosing the one with an acceptable performance on some real-world
+    // inputs.
+
+    // Make unittests of the incremental algorithm work
+    if (DT.DomTreeNodes.size() <= 100) {
+      if (NumLegalized > DT.DomTreeNodes.size())
+        CalculateFromScratch(DT, &BUI);
+    } else if (NumLegalized > DT.DomTreeNodes.size() / 40)
+      CalculateFromScratch(DT, &BUI);
+
     // If the DominatorTree was recalculated at some point, stop the batch
     // updates. Full recalculations ignore batch updates and look at the actual
     // CFG.