[Sol->Yul] Optimizing delete struct.

Co-authored-by: Daniel Kirchner <daniel@ekpyron.org>

docs/ir/ir-breaking-changes.rst

@@ -4,3 +4,32 @@ Solidity IR-based Codegen Changes
 
 This section highlights the main differences between the old and the IR-based codegen,
 along with the reasoning behind the changes and how to update affected code.
+
+Semantic Only Changes
+=====================
+
+This section lists the changes that are semantic-only, thus potentially
+hiding new and different behavior in existing code.
+
+ * When storage structs are deleted, every storage slot that contains a member of the struct is set to zero entirely. Formally, padding space was left untouched.
+Consequently, if the padding space within a struct is used to store data (e.g. in the context of a contract upgrade), you have to be aware that ``delete`` will now also clear the added member (while it wouldn't have been cleared in the past).
+
+::
+    // SPDX-License-Identifier: GPL-3.0
+    pragma solidity >0.7.0;
+
+    contract C {
+        struct S {
+            uint64 y;
+            uint64 z;
+        }
+        S s;
+        function f() public {
+            // ...
+            delete s;
+            // s occupies only first 16 bytes of the 32 bytes slot
+            // delete will write zero to the full slot
+        }
+    }
+
+We have the same behavior for implicit delete, for example when array of structs is shortened.