[PATCH] D35391: [Dominators] Teach LoopDeletion to use the new incremental API

[Jakub Kuderski via Phabricator via llvm-commits]

kuhar updated this revision to Diff 106555.
kuhar added a comment.

Make the edited for loop more concise.


https://reviews.llvm.org/D35391

Files:
  lib/Transforms/Scalar/LoopDeletion.cpp


Index: lib/Transforms/Scalar/LoopDeletion.cpp
===================================================================
--- lib/Transforms/Scalar/LoopDeletion.cpp
+++ lib/Transforms/Scalar/LoopDeletion.cpp
@@ -276,25 +276,23 @@
     ++BI;
   }
 
-  // Update the dominator tree and remove the instructions and blocks that will
-  // be deleted from the reference counting scheme.
-  SmallVector<DomTreeNode*, 8> ChildNodes;
-  for (Loop::block_iterator LI = L->block_begin(), LE = L->block_end();
-       LI != LE; ++LI) {
-    // Move all of the block's children to be children of the Preheader, which
-    // allows us to remove the domtree entry for the block.
-    ChildNodes.insert(ChildNodes.begin(), DT[*LI]->begin(), DT[*LI]->end());
-    for (DomTreeNode *ChildNode : ChildNodes) {
-      DT.changeImmediateDominator(ChildNode, DT[Preheader]);
-    }
-
-    ChildNodes.clear();
-    DT.eraseNode(*LI);
-
-    // Remove the block from the reference counting scheme, so that we can
-    // delete it freely later.
-    (*LI)->dropAllReferences();
-  }
+  // Update the dominator tree. Disconnect the loop by bypassing the loop body.
+  //
+  // This is a special use of the incremental API.
+  // We know that all of the loop nodes are dominated by the loop preheader.
+  // Instead of telling the DT of deleting and inserting edges as we go, we
+  // defer it until here. We tell the DT that we delete the edge from the
+  // preaheader to the header and the incremental algorithm will automatically
+  // disconnect the rest of the loop.
+  // When we tell the DT to insert an edge from the preheader to the exit, we
+  // are back in sync with the changes that happened to the CFG.
+  DT.deleteEdge(Preheader, L->getHeader());
+  DT.insertEdge(Preheader, ExitBlock);
+
+  // Remove the block from the reference counting scheme, so that we can
+  // delete it freely later.
+  for (auto *Block : L->blocks())
+    Block->dropAllReferences();
 
   // Erase the instructions and the blocks without having to worry
   // about ordering because we already dropped the references.


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