[ShrinkWrapping] Fix an infinite loop while looking for restore point.

This may happen when the input program itself contains an infinite loop with no
exit block. In that case, we would fail to find a block post-dominating the loop
such that this block is outside of the loop.

This fixes PR24823.
Working on reducing the test case.

llvm-svn: 247710

1 files changed, 8 insertions, 0 deletions

diff --git a/llvm/lib/CodeGen/ShrinkWrap.cpp b/llvm/lib/CodeGen/ShrinkWrap.cpp
index 7e2f15163cb..c406c149c00 100644
--- a/llvm/lib/CodeGen/ShrinkWrap.cpp
+++ b/llvm/lib/CodeGen/ShrinkWrap.cpp
@@ -309,6 +309,14 @@ void ShrinkWrap::updateSaveRestorePoints(MachineBasicBlock &MBB) {
         }
       }
       else {
+        // If the loop does not exit, there is no point in looking
+        // for a post-dominator outside the loop.
+        SmallVector<MachineBasicBlock*, 4> ExitBlocks;
+        MLI->getLoopFor(Restore)->getExitingBlocks(ExitBlocks);
+        if (ExitBlocks.empty()) {
+          Restore = nullptr;
+          break;
+        }
         // Push Restore outside of this loop if immediate post-dominator is
         // different from restore block. If immediate post-dominator is not
         // different, bail out.