Process purity when parsing function declarations

python/tvm/script/parser/relax/parser.py

@@ -21,7 +21,7 @@
 from typing import Any, Dict, Optional
 
 from tvm import relax, tir
-from tvm.ir import GlobalVar, structural_equal
+from tvm.ir import make_node, GlobalVar, structural_equal
 from tvm.relax import Expr, StructInfo
 from tvm.relax.utils import convert_to_expr
 from tvm.script.ir_builder.relax.frame import BlockFrame
@@ -220,7 +220,30 @@ def visit_tvm_declare_function(self: Parser, node: doc.FunctionDef) -> GlobalVar
             param_sinfo = eval_struct_info(self, arg.annotation, eval_str=True)
             params.append(relax.Var(arg.arg, param_sinfo))
 
-    func_signature = relax.Function.create_empty(params, ret_sinfo)
+    # find a call to R.func_attr to see if purity should be indicated
+    # namely, find a call to R.func_attr({..., "IsPure": val, ...})
+    # (we don't need any other attributes at the function declaration stage)
+    attrs = None
+    for item in node.body:
+        if (
+            isinstance(item.value, doc.Call)
+            and isinstance(item.value.func, doc.Attribute)
+            and item.value.func.attr == "func_attr"
+            and len(item.value.args) == 1
+            and isinstance(item.value.args[0], doc.Dict)
+        ):
+            index = None
+            for i, key in enumerate(item.value.args[0].keys):
+                if isinstance(key, doc.Constant) and key.value == "IsPure":
+                    index = i
+                    break
+            if index is not None:
+                val = item.value.args[0].values[index]
+                if isinstance(val, doc.Constant):
+                    purity = bool(val.value)
+                    attrs = make_node("DictAttrs", IsPure=purity)
+
+    func_signature = relax.Function.create_empty(params, ret_sinfo, attrs=attrs)
     return I.decl_function(node.name, func_signature)
 
 

src/relax/ir/expr.cc

@@ -478,7 +478,9 @@ Function Function::CreateEmpty(Array<Var> params, StructInfo ret_struct_info, Di
         << "relax.Function requires params to contain checked_type_.";
     param_sinfo.push_back(GetStructInfo(param));
   }
-  FuncStructInfo finfo(param_sinfo, ret_struct_info);
+  // if unannotated, we assume the function is pure
+  bool purity = attrs.GetAttr<Bool>(relax::attr::kIsPure).value_or(Bool(true))->value;
+  FuncStructInfo finfo(param_sinfo, ret_struct_info, purity);
 
   // set the fields
   ObjectPtr<FunctionNode> n = make_object<FunctionNode>();