[ConstantFold] Fix incorrect inbounds inference for [0 x T] GEPs

Previously all indices into [0 x T] arrays were considered in
range, which resulted in us incorrectly inferring inbounds for
all GEPs of that form. We should not consider them in range here,
and instead bail out of the rewriting logic (which would divide
by zero).

Do continue to consider 0 always in range, to avoid changing
behavior for zero-index GEPs.

1 files changed, 9 insertions, 3 deletions

diff --git a/llvm/lib/IR/ConstantFold.cpp b/llvm/lib/IR/ConstantFold.cpp
index 5fa56d3..18f481a 100644
--- a/llvm/lib/IR/ConstantFold.cpp
+++ b/llvm/lib/IR/ConstantFold.cpp
@@ -1951,7 +1951,7 @@ static bool isIndexInRangeOfArrayType(uint64_t NumElements,
   // A negative index or an index past the end of our sequential type is
   // considered out-of-range.
   int64_t IndexVal = CI->getSExtValue();
-  if (IndexVal < 0 || (NumElements > 0 && (uint64_t)IndexVal >= NumElements))
+  if (IndexVal < 0 || (IndexVal != 0 && (uint64_t)IndexVal >= NumElements))
     return false;
 
   // Otherwise, it is in-range.
@@ -2202,11 +2202,17 @@ Constant *llvm::ConstantFoldGetElementPtr(Type *PointeeTy, Constant *C,
       Unknown = true;
       continue;
     }
+
+    // Determine the number of elements in our sequential type.
+    uint64_t NumElements = STy->getArrayNumElements();
+    if (!NumElements) {
+      Unknown = true;
+      continue;
+    }
+
     // It's out of range, but we can factor it into the prior
     // dimension.
     NewIdxs.resize(Idxs.size());
-    // Determine the number of elements in our sequential type.
-    uint64_t NumElements = STy->getArrayNumElements();
 
     // Expand the current index or the previous index to a vector from a scalar
     // if necessary.