Restrict the definition of loop preheader to avoid EH blocks

Differential Revision: https://reviews.llvm.org/D34487

llvm-svn: 306070

3 files changed, 13 insertions, 2 deletions

diff --git a/llvm/include/llvm/Analysis/LoopInfoImpl.h b/llvm/include/llvm/Analysis/LoopInfoImpl.h
index 6ff4335f1ad..372fc8b2174 100644
--- a/llvm/include/llvm/Analysis/LoopInfoImpl.h
+++ b/llvm/include/llvm/Analysis/LoopInfoImpl.h
@@ -91,8 +91,9 @@ getExitEdges(SmallVectorImpl<Edge> &ExitEdges) const {
 
 /// getLoopPreheader - If there is a preheader for this loop, return it.  A
 /// loop has a preheader if there is only one edge to the header of the loop
-/// from outside of the loop.  If this is the case, the block branching to the
-/// header of the loop is the preheader node.
+/// from outside of the loop and it is legal to hoist instructions into the
+/// predecessor. If this is the case, the block branching to the header of the
+/// loop is the preheader node.
 ///
 /// This method returns null if there is no preheader for the loop.
 ///
@@ -102,6 +103,10 @@ BlockT *LoopBase<BlockT, LoopT>::getLoopPreheader() const {
   BlockT *Out = getLoopPredecessor();
   if (!Out) return nullptr;
 
+  // Make sure we are allowed to hoist instructions into the predecessor.
+  if (!Out->isLegalToHoistInto())
+    return nullptr;
+
   // Make sure there is only one exit out of the preheader.
   typedef GraphTraits<BlockT*> BlockTraits;
   typename BlockTraits::ChildIteratorType SI = BlockTraits::child_begin(Out);
diff --git a/llvm/include/llvm/CodeGen/MachineBasicBlock.h b/llvm/include/llvm/CodeGen/MachineBasicBlock.h
index 051908c40df..97a49ce4dc4 100644
--- a/llvm/include/llvm/CodeGen/MachineBasicBlock.h
+++ b/llvm/include/llvm/CodeGen/MachineBasicBlock.h
@@ -376,6 +376,9 @@ public:
   /// Indicates if this is the entry block of a cleanup funclet.
   void setIsCleanupFuncletEntry(bool V = true) { IsCleanupFuncletEntry = V; }
 
+  /// Returns true if it is legal to hoist instructions into this block.
+  bool isLegalToHoistInto() const;
+
   // Code Layout methods.
 
   /// Move 'this' block before or after the specified block.  This only moves
diff --git a/llvm/include/llvm/IR/BasicBlock.h b/llvm/include/llvm/IR/BasicBlock.h
index 74ad1930909..7a35afcbafc 100644
--- a/llvm/include/llvm/IR/BasicBlock.h
+++ b/llvm/include/llvm/IR/BasicBlock.h
@@ -395,6 +395,9 @@ public:
                     static_cast<const BasicBlock *>(this)->getLandingPadInst());
   }
 
+  /// \brief Return true if it is legal to hoist instructions into this block.
+  bool isLegalToHoistInto() const;
+
 private:
   /// \brief Increment the internal refcount of the number of BlockAddresses
   /// referencing this BasicBlock by \p Amt.