Avoid infinite loops in branch folding

Differential Revision: https://reviews.llvm.org/D27582

llvm-svn: 289486

1 files changed, 13 insertions, 1 deletions

diff --git a/llvm/lib/CodeGen/BranchFolding.cpp b/llvm/lib/CodeGen/BranchFolding.cpp
index 20dc8805150..913d5d91890 100644
--- a/llvm/lib/CodeGen/BranchFolding.cpp
+++ b/llvm/lib/CodeGen/BranchFolding.cpp
@@ -1624,10 +1624,22 @@ ReoptimizeBlock:
 
       // Okay, there is no really great place to put this block.  If, however,
       // the block before this one would be a fall-through if this block were
-      // removed, move this block to the end of the function.
+      // removed, move this block to the end of the function. There is no real
+      // advantage in "falling through" to an EH block, so we don't want to
+      // perform this transformation for that case.
+      //
+      // Also, Windows EH introduced the possibility of an arbitrary number of
+      // successors to a given block.  The analyzeBranch call does not consider
+      // exception handling and so we can get in a state where a block
+      // containing a call is followed by multiple EH blocks that would be
+      // rotated infinitely at the end of the function if the transformation
+      // below were performed for EH "FallThrough" blocks.  Therefore, even if
+      // that appears not to be happening anymore, we should assume that it is
+      // possible and not remove the "!FallThrough()->isEHPad" condition below.
       MachineBasicBlock *PrevTBB = nullptr, *PrevFBB = nullptr;
       SmallVector<MachineOperand, 4> PrevCond;
       if (FallThrough != MF.end() &&
+          !FallThrough->isEHPad() &&
           !TII->analyzeBranch(PrevBB, PrevTBB, PrevFBB, PrevCond, true) &&
           PrevBB.isSuccessor(&*FallThrough)) {
         MBB->moveAfter(&MF.back());