Delay dominator updation while cloning loop.

[tvvikram]

Summary

Dominator updation fails for a loop inserted with a new basicblock.

A block required by DT to set the IDom might not have been cloned yet. This is because there is no predefined ordering of loop blocks (except for the header block which should be the first block in the list).

The patch first creates DT nodes for the cloned blocks and then separately updates the DT in a follow-on loop.

[tvvikram]

Ping!

[• dberlin]

This looks correct to me

[tvvikram]

In D20899#453116, @dberlin wrote:

This looks correct to me

Can I commit the change? Or should I wait till the patch is formally accepted?

[llvm-commits]

Sorry, though i clicked accept, i'll do so.
please commit.

• msg-31516-14.txt162 BDownload

[tvvikram]

Thanks!

lib/Transforms/Utils/CloneFunction.cpp

Loop *llvm::cloneLoopWithPreheader(BasicBlock *Before, BasicBlock *LoopDomBB,
 
   for (BasicBlock *BB : OrigLoop->getBlocks()) {
     BasicBlock *NewBB = CloneBasicBlock(BB, VMap, NameSuffix, F);
     VMap[BB] = NewBB;
 
     // Update LoopInfo.
     NewLoop->addBasicBlockToLoop(NewBB, *LI);
 
-    // Update DominatorTree.
-    BasicBlock *IDomBB = DT->getNode(BB)->getIDom()->getBlock();
-    DT->addNewBlock(NewBB, cast<BasicBlock>(VMap[IDomBB]));
+    // Add DominatorTree node. After seeing all blocks, update to correct IDom.
+    DT->addNewBlock(NewBB, NewPH);
 
     Blocks.push_back(NewBB);
   }
 
+  for (BasicBlock *BB : OrigLoop->getBlocks()) {
+    // Update DominatorTree.
+    BasicBlock *IDomBB = DT->getNode(BB)->getIDom()->getBlock();
+    DT->changeImmediateDominator(cast<BasicBlock>(VMap[BB]),
+                                 cast<BasicBlock>(VMap[IDomBB]));
+  }
+
   // Move them physically from the end of the block list.
   F->getBasicBlockList().splice(Before->getIterator(), F->getBasicBlockList(),
                                 NewPH);
   F->getBasicBlockList().splice(Before->getIterator(), F->getBasicBlockList(),
                                 NewLoop->getHeader()->getIterator(), F->end());
 
   return NewLoop;
 }