[IR] Add a helper Function::isReturnNonNull

llvm/include/llvm/IR/Function.h

@@ -731,6 +731,11 @@ class LLVM_ABI Function : public GlobalObject, public ilist_node<Function> {
   /// create a Function) from the Function Src to this one.
   void copyAttributesFrom(const Function *Src);
 
+  /// Return true if the return value is known to be not null.
+  /// This may be because it has the nonnull attribute, or because at least
+  /// one byte is dereferenceable and the pointer is in addrspace(0).
+  bool isReturnNonNull() const;
+
   /// deleteBody - This method deletes the body of the function, and converts
   /// the linkage to external.
   ///

llvm/lib/IR/Function.cpp

@@ -873,6 +873,17 @@ void Function::copyAttributesFrom(const Function *Src) {
     setPrologueData(Src->getPrologueData());
 }
 
+bool Function::isReturnNonNull() const {
+  if (hasRetAttribute(Attribute::NonNull))
+    return true;
+
+  if (AttributeSets.getRetDereferenceableBytes() > 0 &&
+      !NullPointerIsDefined(this, getReturnType()->getPointerAddressSpace()))
+    return true;
+
+  return false;
+}
+
 MemoryEffects Function::getMemoryEffects() const {
   return getAttributes().getMemoryEffects();
 }