small calldatacopy optimization

tests/unit/compiler/venom/test_memmerging.py

@@ -282,3 +282,22 @@ def test_memzeroing_3():
     assert bb.instructions[1].operands[0].value == 256 + 2 * 32
     assert bb.instructions[1].operands[2].value == 64
     assert len(bb.instructions) == 3
+
+
+def test_memzeroing_small_calldatacopy():
+    ctx = IRContext()
+    fn = ctx.create_function("_global")
+
+    bb = fn.get_basic_block()
+
+    calldatasize = bb.append_instruction("calldatasize")
+    bb.append_instruction("calldatacopy", 32, calldatasize, 64)
+    bb.append_instruction("stop")
+
+    ac = IRAnalysesCache(fn)
+    MemMergePass(ac, fn).run_pass()
+    RemoveUnusedVariablesPass(ac, fn).run_pass()
+
+    assert bb.instructions[0].opcode == "mstore"
+    assert bb.instructions[0].operands[0].value == 0
+    assert bb.instructions[0].operands[1].value == 64

vyper/venom/passes/memmerging.py

@@ -188,24 +188,26 @@ def _barrier():
     # optimize memzeroing operations
     def _optimize_memzero(self, bb: IRBasicBlock, intervals: list[_Interval]):
         for interval in intervals:
-            if interval.length <= 32:
-                # TODO: if interval.length == 32, then we actually want to
-                # turn calldatacopy into (mstore 0)
+            inst = interval.insts[-1]
+            if interval.length == 32 and inst.opcode == "calldatacopy":
+                dst = inst.operands[2]
+                inst.opcode = "mstore"
+                inst.operands = [IRLiteral(0), dst]
+            elif interval.length <= 32:
                 continue
-            inst = interval.insts[0]
-
-            index = bb.instructions.index(inst)
-            calldatasize = bb.parent.get_next_variable()
-            bb.insert_instruction(IRInstruction("calldatasize", [], output=calldatasize), index)
-
-            inst.output = None
-            inst.opcode = "calldatacopy"
-            inst.operands = [
-                IRLiteral(interval.length),
-                calldatasize,
-                IRLiteral(interval.dst_start),
-            ]
-            for inst in interval.insts[1:]:
+            else:
+                index = bb.instructions.index(inst)
+                calldatasize = bb.parent.get_next_variable()
+                bb.insert_instruction(IRInstruction("calldatasize", [], output=calldatasize), index)
+
+                inst.output = None
+                inst.opcode = "calldatacopy"
+                inst.operands = [
+                    IRLiteral(interval.length),
+                    calldatasize,
+                    IRLiteral(interval.dst_start),
+                ]
+            for inst in interval.insts[0:-1]:
                 bb.remove_instruction(inst)
 
         intervals.clear()