[LoopUnroll] Don't try to unroll non canonical loops.

The current Loop Unroll implementation works with loops having a
single latch that contains a conditional branch to a block outside
the loop (the other successor is, by defition of latch, the header).
If this precondition doesn't hold, avoid unrolling the loop as
the code is not ready to handle such circumstances.

Differential Revision:  https://reviews.llvm.org/D32261

llvm-svn: 301239

1 files changed, 15 insertions, 0 deletions

diff --git a/llvm/lib/Transforms/Utils/LoopUnroll.cpp b/llvm/lib/Transforms/Utils/LoopUnroll.cpp
index 3c669ce..3e3cc5e4 100644
--- a/llvm/lib/Transforms/Utils/LoopUnroll.cpp
+++ b/llvm/lib/Transforms/Utils/LoopUnroll.cpp
@@ -318,6 +318,10 @@ bool llvm::UnrollLoop(Loop *L, unsigned Count, unsigned TripCount, bool Force,
     return false;
   }
 
+  // The current loop unroll pass can only unroll loops with a single latch
+  // that's a conditional branch exiting the loop.
+  // FIXME: The implementation can be extended to work with more complicated
+  // cases, e.g. loops with multiple latches.
   BasicBlock *Header = L->getHeader();
   BranchInst *BI = dyn_cast<BranchInst>(LatchBlock->getTerminator());
 
@@ -328,6 +332,17 @@ bool llvm::UnrollLoop(Loop *L, unsigned Count, unsigned TripCount, bool Force,
     return false;
   }
 
+  auto CheckSuccessors = [&](unsigned S1, unsigned S2) {
+    return BI->getSuccessor(S1) == Header && !L->contains(BI->getSuccessor(S2));
+
+  };
+
+  if (!CheckSuccessors(0, 1) && !CheckSuccessors(1, 0)) {
+    DEBUG(dbgs() << "Can't unroll; only loops with one conditional latch"
+                    " exiting the loop can be unrolled\n");
+    return false;
+  }
+
   if (Header->hasAddressTaken()) {
     // The loop-rotate pass can be helpful to avoid this in many cases.
     DEBUG(dbgs() <<