StructurizeCFG: Fix verification failure with some loops.

If the beginning of the loop was also the entry block
of the function, branches were inserted to the entry block
which isn't allowed. If this occurs, create a new dummy
function entry block that branches to the start of the loop.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@195493 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 14 insertions, 0 deletions

diff --git a/lib/Transforms/Scalar/StructurizeCFG.cpp b/lib/Transforms/Scalar/StructurizeCFG.cpp
index 0124dfdbeb..5045ff8fdf 100644
--- a/lib/Transforms/Scalar/StructurizeCFG.cpp
+++ b/lib/Transforms/Scalar/StructurizeCFG.cpp
@@ -779,6 +779,20 @@ void StructurizeCFG::handleLoops(bool ExitUseAllowed,
     handleLoops(false, LoopEnd);
   }
 
+  // If the start of the loop is the entry block, we can't branch to it so
+  // insert a new dummy entry block.
+  Function *LoopFunc = LoopStart->getParent();
+  if (LoopStart == &LoopFunc->getEntryBlock()) {
+    LoopStart->setName("entry.orig");
+
+    BasicBlock *NewEntry =
+      BasicBlock::Create(LoopStart->getContext(),
+                         "entry",
+                         LoopFunc,
+                         LoopStart);
+    BranchInst::Create(LoopStart, NewEntry);
+  }
+
   // Create an extra loop end node
   LoopEnd = needPrefix(false);
   BasicBlock *Next = needPostfix(LoopEnd, ExitUseAllowed);