diff --git a/mlir/lib/Transforms/DialectConversion.cpp b/mlir/lib/Transforms/DialectConversion.cpp
--- a/mlir/lib/Transforms/DialectConversion.cpp
+++ b/mlir/lib/Transforms/DialectConversion.cpp
@@ -611,12 +611,11 @@
   TypeConversion
 };
 
-/// Original position of the given block in its parent region.  We cannot use
-/// a region iterator because it could have been invalidated by other region
-/// operations since the position was stored.
+/// Original position of the given block in its parent region. During undo
+/// actions, the block needs to be placed after `insertAfterBlock`.
 struct BlockPosition {
   Region *region;
-  Region::iterator::difference_type position;
+  Block *insertAfterBlock;
 };
 
 /// Information needed to undo the merge actions.
@@ -634,8 +633,8 @@
   static BlockAction getCreate(Block *block) {
     return {BlockActionKind::Create, block, {}};
   }
-  static BlockAction getErase(Block *block, BlockPosition originalPos) {
-    return {BlockActionKind::Erase, block, {originalPos}};
+  static BlockAction getErase(Block *block, BlockPosition originalPosition) {
+    return {BlockActionKind::Erase, block, {originalPosition}};
   }
   static BlockAction getMerge(Block *block, Block *sourceBlock) {
     BlockAction action{BlockActionKind::Merge, block, {}};
@@ -644,8 +643,8 @@
                                          : &(block->back())};
     return action;
   }
-  static BlockAction getMove(Block *block, BlockPosition originalPos) {
-    return {BlockActionKind::Move, block, {originalPos}};
+  static BlockAction getMove(Block *block, BlockPosition originalPosition) {
+    return {BlockActionKind::Move, block, {originalPosition}};
   }
   static BlockAction getSplit(Block *block, Block *originalBlock) {
     BlockAction action{BlockActionKind::Split, block, {}};
@@ -990,9 +989,11 @@
     // Put the block (owned by action) back into its original position.
     case BlockActionKind::Erase: {
       auto &blockList = action.originalPosition.region->getBlocks();
-      blockList.insert(
-          std::next(blockList.begin(), action.originalPosition.position),
-          action.block);
+      blockList.insert((action.originalPosition.insertAfterBlock
+                            ? std::next(Region::iterator(
+                                  action.originalPosition.insertAfterBlock))
+                            : blockList.end()),
+                       action.block);
       break;
     }
     // Split the block at the position which was originally the end of the
@@ -1013,7 +1014,10 @@
     case BlockActionKind::Move: {
       Region *originalRegion = action.originalPosition.region;
       originalRegion->getBlocks().splice(
-          std::next(originalRegion->begin(), action.originalPosition.position),
+          (action.originalPosition.insertAfterBlock
+               ? std::next(
+                     Region::iterator(action.originalPosition.insertAfterBlock))
+               : originalRegion->end()),
           action.block->getParent()->getBlocks(), action.block);
       break;
     }
@@ -1191,8 +1195,9 @@
 
 void ConversionPatternRewriterImpl::notifyBlockIsBeingErased(Block *block) {
   Region *region = block->getParent();
-  auto position = std::distance(region->begin(), Region::iterator(block));
-  blockActions.push_back(BlockAction::getErase(block, {region, position}));
+  Block *origPrevBlock =
+      (&*region->begin() == block ? nullptr : &(*(--Region::iterator(block))));
+  blockActions.push_back(BlockAction::getErase(block, {region, origPrevBlock}));
 }
 
 void ConversionPatternRewriterImpl::notifyCreatedBlock(Block *block) {
@@ -1211,10 +1216,12 @@
 
 void ConversionPatternRewriterImpl::notifyRegionIsBeingInlinedBefore(
     Region &region, Region &parent, Region::iterator before) {
+  Block *origPrevBlock = nullptr;
   for (auto &pair : llvm::enumerate(region)) {
     Block &block = pair.value();
-    Region::iterator::difference_type position = pair.index();
-    blockActions.push_back(BlockAction::getMove(&block, {&region, position}));
+    blockActions.push_back(
+        BlockAction::getMove(&block, {&region, origPrevBlock}));
+    origPrevBlock = █
   }
 }