Optimization for multiple memory-zeroing instructions

vyper/optimizer.py

@@ -61,6 +61,60 @@ def optimize(lll_node: LLLnode) -> LLLnode:
 def apply_general_optimizations(node: LLLnode) -> LLLnode:
     # TODO refactor this into several functions
     argz = [apply_general_optimizations(arg) for arg in node.args]
+
+    if node.value == "seq":
+        # look for sequential mzero / calldatacopy operations that are zero'ing memory
+        # and merge them into a single calldatacopy
+        mzero_nodes: List = []
+        initial_offset = 0
+        total_length = 0
+        for lll_node in [i for i in argz if i.value != "pass"]:
+            if (
+                lll_node.value == "mstore"
+                and isinstance(lll_node.args[0].value, int)
+                and lll_node.args[1].value == 0
+            ):
+                # mstore of a zero value
+                offset = lll_node.args[0].value
+                if not mzero_nodes:
+                    initial_offset = offset
+                if initial_offset + total_length == offset:
+                    mzero_nodes.append(lll_node)
+                    total_length += 32
+                    continue
+
+            if (
+                lll_node.value == "calldatacopy"
+                and isinstance(lll_node.args[0].value, int)
+                and lll_node.args[1].value == "calldatasize"
+                and isinstance(lll_node.args[2].value, int)
+            ):
+                # calldatacopy from the end of calldata - efficient zero'ing via `empty()`
+                offset, length = lll_node.args[0].value, lll_node.args[2].value
+                if not mzero_nodes:
+                    initial_offset = offset
+                if initial_offset + total_length == offset:
+                    mzero_nodes.append(lll_node)
+                    total_length += length
+                    continue
+
+            # if we get this far, the current node is not a zero'ing operation
+            # it's time to apply the optimization if possible
+            if len(mzero_nodes) > 1:
+                new_lll = LLLnode.from_list(
+                    ["calldatacopy", initial_offset, "calldatasize", total_length],
+                    pos=mzero_nodes[0].pos,
+                )
+                # replace first zero'ing operation with optimized node and remove the rest
+                idx = argz.index(mzero_nodes[0])
+                argz[idx] = new_lll
+                for i in mzero_nodes[1:]:
+                    argz.remove(i)
+
+            initial_offset = 0
+            total_length = 0
+            mzero_nodes.clear()
+
     if node.value in arith and int_at(argz, 0) and int_at(argz, 1):
         left, right = get_int_at(argz, 0), get_int_at(argz, 1)
         # `node.value in arith` implies that `node.value` is a `str`