LoopUnrollAndJam] Only allow loops with single exit(ing) blocks

Summary:
This resolves an issue posted on Bugzilla. https://bugs.llvm.org/show_bug.cgi?id=48764
In this issue, the loop had  multiple exit blocks, which resulted in the
function getExitBlock to return a nullptr, which resulted in hitting the assert.
This patch ensures that loops which only have one exit block as allowed to be
unrolled and jammed.

Reviewed By: Whitney, Meinersbur, dmgreen

Differential Revision: https://reviews.llvm.org/D95806

1 files changed, 17 insertions, 0 deletions

diff --git a/llvm/lib/Transforms/Utils/LoopUnrollAndJam.cpp b/llvm/lib/Transforms/Utils/LoopUnrollAndJam.cpp
index 6e32a2b..c62049c 100644
--- a/llvm/lib/Transforms/Utils/LoopUnrollAndJam.cpp
+++ b/llvm/lib/Transforms/Utils/LoopUnrollAndJam.cpp
@@ -831,6 +831,23 @@ static bool isEligibleLoopForm(const Loop &Root) {
     if (SubLoopsSize != 1)
       return false;
 
+    // Only loops with a single exit block can be unrolled and jammed.
+    // The function getExitBlock() is used for this check, rather than
+    // getUniqueExitBlock() to ensure loops with mulitple exit edges are
+    // disallowed.
+    if (!L->getExitBlock()) {
+      LLVM_DEBUG(dbgs() << "Won't unroll-and-jam; only loops with single exit "
+                           "blocks can be unrolled and jammed.\n");
+      return false;
+    }
+
+    // Only loops with a single exiting block can be unrolled and jammed.
+    if (!L->getExitingBlock()) {
+      LLVM_DEBUG(dbgs() << "Won't unroll-and-jam; only loops with single "
+                           "exiting blocks can be unrolled and jammed.\n");
+      return false;
+    }
+
     L = L->getSubLoops()[0];
   } while (L);