[LoopUnrollAnalyzer] Handle out of bounds accesses in visitLoad

While we handed loads past the end of an array, we didn't handle loads
_before_ the array.

This fixes PR28062.

N.B. While the bug in the code is obvious, I am struggling to craft a
test case which is reasonable in size.

llvm-svn: 276510

1 files changed, 10 insertions, 4 deletions

diff --git a/llvm/lib/Analysis/LoopUnrollAnalyzer.cpp b/llvm/lib/Analysis/LoopUnrollAnalyzer.cpp
index f59257ab16b..7bdf3408a58 100644
--- a/llvm/lib/Analysis/LoopUnrollAnalyzer.cpp
+++ b/llvm/lib/Analysis/LoopUnrollAnalyzer.cpp
@@ -115,13 +115,19 @@ bool UnrolledInstAnalyzer::visitLoad(LoadInst &I) {
   // We might have a vector load from an array. FIXME: for now we just bail
   // out in this case, but we should be able to resolve and simplify such
   // loads.
-  if(CDS->getElementType() != I.getType())
+  if (CDS->getElementType() != I.getType())
     return false;
 
-  int ElemSize = CDS->getElementType()->getPrimitiveSizeInBits() / 8U;
-  if (SimplifiedAddrOp->getValue().getActiveBits() >= 64)
+  unsigned ElemSize = CDS->getElementType()->getPrimitiveSizeInBits() / 8U;
+  if (SimplifiedAddrOp->getValue().getActiveBits() > 64)
     return false;
-  int64_t Index = SimplifiedAddrOp->getSExtValue() / ElemSize;
+  int64_t SimplifiedAddrOpV = SimplifiedAddrOp->getSExtValue();
+  if (SimplifiedAddrOpV < 0) {
+    // FIXME: For now we conservatively ignore out of bound accesses, but
+    // we're allowed to perform the optimization in this case.
+    return false;
+  }
+  uint64_t Index = static_cast<uint64_t>(SimplifiedAddrOpV) / ElemSize;
   if (Index >= CDS->getNumElements()) {
     // FIXME: For now we conservatively ignore out of bound accesses, but
     // we're allowed to perform the optimization in this case.