[Sol->Yul] Optimizing delete struct.

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.

libsolidity/codegen/YulUtilFunctions.cpp

@@ -1266,19 +1266,31 @@ string YulUtilFunctions::clearStorageStructFunction(StructType const& _type)
 		MemberList::MemberMap structMembers = _type.nativeMembers(nullptr);
 		vector<map<string, string>> memberSetValues;
 
+		set<u256> slotsCleared;
 		for (auto const& member: structMembers)
-		{
-			auto const& [memberSlotDiff, memberStorageOffset] = _type.storageOffsetsOfMember(member.name);
+			if (member.type->storageBytes() < 32)
+			{
+				auto const& slotDiff = _type.storageOffsetsOfMember(member.name).first;
+				if (!slotsCleared.count(slotDiff))
+				{
+					memberSetValues.emplace_back().emplace("clearMember", "sstore(add(slot, " + slotDiff.str() + "), 0)");
+					slotsCleared.emplace(slotDiff);
+				}
+			}
+			else
+			{
+				auto const& [memberSlotDiff, memberStorageOffset] = _type.storageOffsetsOfMember(member.name);
+				solAssert(memberStorageOffset == 0, "");
 
-			memberSetValues.emplace_back().emplace("clearMember", Whiskers(R"(
-					<setZero>(add(slot, <memberSlotDiff>), <memberStorageOffset>)
-				)")
-				("setZero", storageSetToZeroFunction(*member.type))
-				("memberSlotDiff",  memberSlotDiff.str())
-				("memberStorageOffset", to_string(memberStorageOffset))
-				.render()
-			);
-		}
+				memberSetValues.emplace_back().emplace("clearMember", Whiskers(R"(
+						<setZero>(add(slot, <memberSlotDiff>), <memberStorageOffset>)
+					)")
+					("setZero", storageSetToZeroFunction(*member.type))
+					("memberSlotDiff",  memberSlotDiff.str())
+					("memberStorageOffset", to_string(memberStorageOffset))
+					.render()
+				);
+			}
 
 		return Whiskers(R"(
 			function <functionName>(slot) {

test/libsolidity/semanticTests/structs/struct_delete_storage_nested_small.sol

@@ -0,0 +1,35 @@
+contract C {
+    struct S {
+        uint32 a;
+        S[] x;
+    }
+    S s;
+    function f() public returns (uint256 r1, uint256 r2, uint256 r3) {
+        assembly {
+            // 2 ** 150 - 1
+            sstore(s.slot, 1427247692705959881058285969449495136382746623)
+        }
+        s.a = 1;
+        s.x.push(); s.x.push();
+        S storage ptr1 = s.x[0];
+        S storage ptr2 = s.x[1];
+        assembly {
+            // 2 ** 150 - 1
+            sstore(ptr1.slot, 1427247692705959881058285969449495136382746623)
+            sstore(ptr2.slot, 1427247692705959881058285969449495136382746623)
+        }
+        s.x[0].a = 2; s.x[1].a = 3;
+        delete s;
+        assert(s.a == 0);
+        assert(s.x.length == 0);
+        assembly {
+            r1 := sload(s.slot)
+            r2 := sload(ptr1.slot)
+            r3 := sload(ptr2.slot)
+        }
+    }
+}
+// ====
+// compileViaYul: true
+// ----
+// f() -> 0, 0, 0

test/libsolidity/semanticTests/structs/struct_delete_storage_small.sol

@@ -0,0 +1,24 @@
+contract C {
+    struct S {
+        uint64 y;
+        uint64 z;
+    }
+    S s;
+    function f() public returns (uint256 ret) {
+        assembly {
+            // 2 ** 150 - 1
+            sstore(s.slot, 1427247692705959881058285969449495136382746623)
+        }
+        s.y = 1; s.z = 2;
+        delete s;
+        assert(s.y == 0);
+        assert(s.z == 0);
+        assembly {
+            ret := sload(s.slot)
+        }
+    }
+}
+// ====
+// compileViaYul: true
+// ----
+// f() -> 0

test/libsolidity/semanticTests/structs/struct_delete_storage_with_arrays_small.sol

@@ -0,0 +1,29 @@
+contract C {
+    struct S {
+        uint32 a;
+        uint32[3] b;
+        uint32[] x;
+    }
+    S s;
+    function f() public returns (uint256 ret) {
+        assembly {
+            // 2 ** 150 - 1
+            sstore(s.slot, 1427247692705959881058285969449495136382746623)
+        }
+        s.a = 1;
+        s.b[0] = 2; s.b[1] = 3;
+        s.x.push(4); s.x.push(5);
+        delete s;
+        assert(s.a == 0);
+        assert(s.b[0] == 0);
+        assert(s.b[1] == 0);
+        assert(s.x.length == 0);
+        assembly {
+            ret := sload(s.slot)
+        }
+    }
+}
+// ====
+// compileViaYul: true
+// ----
+// f() -> 0