[PASS] StorageRewrite Fold Inplace op storage when possible (#759)

* [PASS] StorageRewrite Fold Inplace op storage when possible

* update comment to fix typos

Author: tqchen

include/tvm/ir.h

@@ -152,6 +152,12 @@ constexpr const char* coproc_scope = "coproc_scope";
 constexpr const char* coproc_uop_scope = "coproc_uop_scope";
 /*! \brief Mark the scope as volatile access for certain handle. */
 constexpr const char* volatile_scope = "volatile_scope";
+/*!
+ * \brief Mark the scope as generated by extern primitive.
+ *  such scope can contain arbitrary ir program and we need to be careful
+ *  when make certain assumptions about the structure of the program.
+ */
+constexpr const char* extern_scope = "extern_scope";
 /*!
  * \brief Mark the scope as when computation start to happen
  *  This can hint some code generator to create a new function for compute.

src/op/extern_op.cc

@@ -130,7 +130,7 @@ Stmt ExternOpNode::BuildProvide(
     const Stage& stage,
     const std::unordered_map<IterVar, Range>& dom_map) const {
   CHECK_EQ(stage->op.operator->(), this);
-  Stmt ret = this->body;
+  Stmt ret = AttrStmt::make(make_zero(Int(32)), attr::extern_scope, 0, this->body);
   auto f_push_bind = [&ret](Buffer buffer, Tensor tensor) {
     Array<NodeRef> bind_spec;
     Array<Expr> tuple;