From a0197d8d1e2c6c3c5e8a2fcd303f67ef14b39c77 Mon Sep 17 00:00:00 2001
From: Robin Sommer <robin@corelight.com>
Date: Wed, 14 Aug 2024 11:12:41 +0200
Subject: [PATCH] Add peephole optimizer for final AST tuning.

We use this to remove two statement constructs that the main optimizer
may leave behind:

    1. `default<void>()`
    2. `(*self).__error = __error; __error = (*self).__error;`

The second case is a quite specific situation that eventually, once we
have CFG/DFG tracking, the main optimizer should be able to cover more
generically. However, for now, it's just not nice to always have
these blocks in the generated C++ code, so adding this special case
seems useful.

Couples notes on (2):

    - Per #1592, case 2 may also have overhead. Closes #1592.
    - Technically, this optimization isn't always correct: subsequent
      code could assume that `(*self).__error` is set, whereas after
      removal it's not (or not to the expected value). However,
      `__error` is strictly-internal state, and we know that we don't
      use it any different, so this seems ok until we have more
      general optimizer logic.
---
 hilti/toolchain/include/ast/node.h            |  21 ++++
 hilti/toolchain/src/compiler/optimizer.cc     | 102 ++++++++++++++++++
 .../opt.hlt                                   |   8 ++
 .../opt.hlt                                   |   4 +
 .../hilti.optimization.unimplemented_hook/log |   2 +
 .../opt.hlt                                   |   2 -
 .../log                                       |  19 ++++
 .../opt.hlt                                   |  31 +-----
 .../log                                       |  23 ++++
 .../opt.hlt                                   |  33 ------
 .../spicy.optimization.unused-functions/log   |  27 +++++
 .../opt.hlt                                   |  48 +--------
 .../spicy.optimization.unused-types/log       |  47 ++++++++
 .../spicy.optimization.unused-types/opt.hlt   |  83 +-------------
 tests/Baseline/spicy.rt.debug-trace/.stderr   |   5 +-
 .../output                                    |   2 +-
 .../optimization/peephole-default-void.hlt    |  12 +++
 .../optimization/peephole-error-push-pop.hlt  |  19 ++++
 tests/spicy/types/bytes/parse-length.spicy    |   4 +
 19 files changed, 299 insertions(+), 193 deletions(-)
 create mode 100644 tests/Baseline/hilti.optimization.peephole-default-void/opt.hlt
 create mode 100644 tests/Baseline/hilti.optimization.peephole-error-push-pop/opt.hlt
 create mode 100644 tests/hilti/optimization/peephole-default-void.hlt
 create mode 100644 tests/hilti/optimization/peephole-error-push-pop.hlt

diff --git a/hilti/toolchain/include/ast/node.h b/hilti/toolchain/include/ast/node.h
index 0771d01d0f..ad8cdb353d 100644
--- a/hilti/toolchain/include/ast/node.h
+++ b/hilti/toolchain/include/ast/node.h
@@ -456,6 +456,27 @@ class Node {
         return n;
     }
 
+    /**
+     * Returns the subsequent sibling of given child node. This skips over null
+     * children.
+     *
+     * @param n child whose sibling to return
+     * @return sibling of *n*, or null if *n* is the last child or not child at all
+     **/
+    Node* sibling(Node* n) const {
+        auto i = std::find(_children.begin(), _children.end(), n);
+        if ( i == _children.end() )
+            return nullptr;
+
+        while ( true ) {
+            if ( ++i == _children.end() )
+                return nullptr;
+
+            if ( *i )
+                return *i;
+        }
+    }
+
     /**
      * Adds a child node. The node will be appended to the end of the current
      * list of children, and its parent will be set to the current node. If the
diff --git a/hilti/toolchain/src/compiler/optimizer.cc b/hilti/toolchain/src/compiler/optimizer.cc
index 7a8d5b2295..d9c32e6e74 100644
--- a/hilti/toolchain/src/compiler/optimizer.cc
+++ b/hilti/toolchain/src/compiler/optimizer.cc
@@ -907,6 +907,101 @@ struct ConstantFoldingVisitor : OptimizerVisitor {
     }
 };
 
+/**
+ * Visitor running on the final, optimized AST to perform additional peephole
+ * optimizations. Will run repeatedly until it performs no further changes.
+ */
+struct PeepholeOptimizer : visitor::MutatingPostOrder {
+    using visitor::MutatingPostOrder::MutatingPostOrder;
+
+    // Returns true if statement is `(*self).__error = __error`.
+    bool isErrorPush(statement::Expression* n) {
+        auto assign = n->expression()->tryAs<expression::Assign>();
+        if ( ! assign )
+            return false;
+
+        auto lhs = assign->target()->tryAs<operator_::struct_::MemberNonConst>();
+        if ( ! lhs )
+            return false;
+
+        auto op0 = lhs->op0()->tryAs<operator_::value_reference::Deref>();
+        if ( ! op0 )
+            return false;
+
+        auto op1 = lhs->op1()->tryAs<expression::Member>();
+        if ( ! (op1 && op1->id() == "__error") )
+            return false;
+
+        auto self = op0->op0()->tryAs<expression::Name>();
+        if ( ! (self && self->id() == "self") )
+            return false;
+
+        auto rhs = assign->source()->tryAs<expression::Name>();
+        if ( ! (rhs && rhs->id() == "__error") )
+            return false;
+
+        return true;
+    }
+
+    // Returns true if statement is `__error == (*self).__error`.
+    bool isErrorPop(statement::Expression* n) {
+        auto assign = n->expression()->tryAs<expression::Assign>();
+        if ( ! assign )
+            return false;
+
+        auto lhs = assign->target()->tryAs<expression::Name>();
+        if ( ! (lhs && lhs->id() == "__error") )
+            return false;
+
+        auto rhs = assign->source()->tryAs<operator_::struct_::MemberNonConst>();
+        if ( ! rhs )
+            return false;
+
+        auto op0 = rhs->op0()->tryAs<operator_::value_reference::Deref>();
+        if ( ! op0 )
+            return false;
+
+        auto op1 = rhs->op1()->tryAs<expression::Member>();
+        if ( ! (op1 && op1->id() == "__error") )
+            return false;
+
+        auto self = op0->op0()->tryAs<expression::Name>();
+        if ( ! (self && self->id() == "self") )
+            return false;
+
+        return true;
+    }
+
+    void operator()(statement::Expression* n) final {
+        // Remove expression statements of the form `default<void>`.
+        if ( auto ctor = n->expression()->tryAs<expression::Ctor>();
+             ctor && ctor->ctor()->isA<ctor::Default>() && ctor->type()->type()->isA<type::Void>() ) {
+            recordChange(n, "removing default<void> statement");
+            n->parent()->removeChild(n);
+            return;
+        }
+
+        // Remove statement pairs of the form:
+        //
+        //    (*self).__error = __error;
+        //    __error = (*self).__error;
+        //
+        // These will be left behind by the optimizer if a hook call got
+        // optimized out in between them.
+        if ( isErrorPush(n) && n->parent() ) {
+            auto parent = n->parent();
+            if ( auto sibling = parent->sibling(n) ) {
+                if ( auto stmt = sibling->tryAs<statement::Expression>(); stmt && isErrorPop(stmt) ) {
+                    recordChange(n, "removing unneeded error push/pop statements");
+                    parent->removeChild(n);
+                    parent->removeChild(sibling);
+                    return;
+                }
+            }
+        }
+    }
+};
+
 // This visitor collects requirement attributes in the AST and toggles unused features.
 class FeatureRequirementsVisitor : public visitor::MutatingPreOrder {
 public:
@@ -1494,6 +1589,13 @@ void detail::optimizer::optimize(Builder* builder, ASTRoot* root) {
         ++round;
     }
 
+    while ( true ) {
+        auto v = PeepholeOptimizer(builder, hilti::logging::debug::Optimizer);
+        visitor::visit(v, root);
+        if ( ! v.isModified() )
+            break;
+    }
+
     // Clear cached information which might become outdated due to edits.
     auto v = hilti::visitor::PreOrder();
     for ( auto n : hilti::visitor::range(v, root, {}) )
diff --git a/tests/Baseline/hilti.optimization.peephole-default-void/opt.hlt b/tests/Baseline/hilti.optimization.peephole-default-void/opt.hlt
new file mode 100644
index 0000000000..783c4b6086
--- /dev/null
+++ b/tests/Baseline/hilti.optimization.peephole-default-void/opt.hlt
@@ -0,0 +1,8 @@
+### BTest baseline data generated by btest-diff. Do not edit. Use "btest -U/-u" to update. Requires BTest >= 0.63.
+module Test {
+
+
+public function void foo() {
+}
+
+}
diff --git a/tests/Baseline/hilti.optimization.peephole-error-push-pop/opt.hlt b/tests/Baseline/hilti.optimization.peephole-error-push-pop/opt.hlt
new file mode 100644
index 0000000000..e244e143bf
--- /dev/null
+++ b/tests/Baseline/hilti.optimization.peephole-error-push-pop/opt.hlt
@@ -0,0 +1,4 @@
+### BTest baseline data generated by btest-diff. Do not edit. Use "btest -U/-u" to update. Requires BTest >= 0.63.
+module Test {
+
+}
diff --git a/tests/Baseline/hilti.optimization.unimplemented_hook/log b/tests/Baseline/hilti.optimization.unimplemented_hook/log
index 1a89574535..d4cdcfb9c1 100644
--- a/tests/Baseline/hilti.optimization.unimplemented_hook/log
+++ b/tests/Baseline/hilti.optimization.unimplemented_hook/log
@@ -11,3 +11,5 @@
 [debug/optimizer] [unimplemented_hook.hlt:21:5-21:35] declaration::Field "hook void unimplemented_void();" -> null
 [debug/optimizer] removing field for unused method Foo::X::unimplemented_int64
 [debug/optimizer] [unimplemented_hook.hlt:22:5-22:49] declaration::Field "hook optional<int<64>> unimplemented_int64();" -> null
+[debug/optimizer] [unimplemented_hook.hlt:29:1-29:27] statement::Expression "default<void>();" -> removing default<void> statement
+[debug/optimizer] [unimplemented_hook.hlt:35:1-35:22] statement::Expression "default<void>();" -> removing default<void> statement
diff --git a/tests/Baseline/hilti.optimization.unimplemented_hook/opt.hlt b/tests/Baseline/hilti.optimization.unimplemented_hook/opt.hlt
index 90d1083576..8a2af793ea 100644
--- a/tests/Baseline/hilti.optimization.unimplemented_hook/opt.hlt
+++ b/tests/Baseline/hilti.optimization.unimplemented_hook/opt.hlt
@@ -18,8 +18,6 @@ method hook void X::implemented() {
 }
 
 global_implemented();
-default<void>();
 x.implemented();
-default<void>();
 
 }
diff --git a/tests/Baseline/spicy.optimization.default-parser-functions/log b/tests/Baseline/spicy.optimization.default-parser-functions/log
index b1dcca1e17..0d0e69d683 100644
--- a/tests/Baseline/spicy.optimization.default-parser-functions/log
+++ b/tests/Baseline/spicy.optimization.default-parser-functions/log
@@ -174,6 +174,15 @@
 [debug/optimizer] [<no location>] expression::Name "__feat%foo@@P2%uses_stream" -> expression::Ctor "False" (inlining constant)
 [debug/optimizer] [<no location>] expression::Ternary "False ? (*self).__filters : Null" -> expression::Ctor "Null"
 [debug/optimizer] [<no location>] expression::Ternary "False ? (*self).__filters : Null" -> expression::Ctor "Null"
+[debug/optimizer] [<no location>] statement::Expression "(*self).__error = __error;" -> removing unneeded error push/pop statements
+[debug/optimizer] [<no location>] statement::Expression "(*self).__error = __error;" -> removing unneeded error push/pop statements
+[debug/optimizer] [<no location>] statement::Expression "(*self).__error = __error;" -> removing unneeded error push/pop statements
+[debug/optimizer] [<no location>] statement::Expression "(*self).__error = __error;" -> removing unneeded error push/pop statements
+[debug/optimizer] [<no location>] statement::Expression "(*self).__error = __error;" -> removing unneeded error push/pop statements
+[debug/optimizer] [<no location>] statement::Expression "(*self).__error = __error;" -> removing unneeded error push/pop statements
+[debug/optimizer] [<no location>] statement::Expression "(*self).__error = __error;" -> removing unneeded error push/pop statements
+[debug/optimizer] [<no location>] statement::Expression "(*self).__error = __error;" -> removing unneeded error push/pop statements
+[debug/optimizer] [<no location>] statement::Expression "(*self).__error = __error;" -> removing unneeded error push/pop statements
 [debug/optimizer] [<no location>] statement::If "if ( False ) { (*__unit).__begin = begin(__ncur); }" -> null
 [debug/optimizer] [<no location>] statement::If "if ( False ) { (*__unit).__begin = begin(__ncur); }" -> null
 [debug/optimizer] [<no location>] statement::If "if ( False ) { (*__unit).__begin = begin(__ncur); }" -> null
@@ -330,6 +339,11 @@
 [debug/optimizer] [default-parser-functions.spicy:14:18-14:24] operator_::struct_::MemberCall "(*self).__on_0x25_finally()" -> expression::Ctor "default<void>()" (replacing call to unimplemented method with default value)
 [debug/optimizer] [default-parser-functions.spicy:14:18-14:24] operator_::struct_::MemberCall "(*self).__on_0x25_finally()" -> expression::Ctor "default<void>()" (replacing call to unimplemented method with default value)
 [debug/optimizer] [default-parser-functions.spicy:14:18-14:24] operator_::struct_::MemberCall "(*self).__on_0x25_init()" -> expression::Ctor "default<void>()" (replacing call to unimplemented method with default value)
+[debug/optimizer] [default-parser-functions.spicy:14:18-14:24] statement::Expression "default<void>();" -> removing default<void> statement
+[debug/optimizer] [default-parser-functions.spicy:14:18-14:24] statement::Expression "default<void>();" -> removing default<void> statement
+[debug/optimizer] [default-parser-functions.spicy:14:18-14:24] statement::Expression "default<void>();" -> removing default<void> statement
+[debug/optimizer] [default-parser-functions.spicy:14:18-14:24] statement::Expression "default<void>();" -> removing default<void> statement
+[debug/optimizer] [default-parser-functions.spicy:14:18-14:24] statement::Expression "default<void>();" -> removing default<void> statement
 [debug/optimizer] [default-parser-functions.spicy:16:18-21:1] declaration::Field "hook optional<string> __str__();" -> null
 [debug/optimizer] [default-parser-functions.spicy:16:18-21:1] declaration::Field "hook void __on_0x25_confirmed() &needed-by-feature="synchronization";" -> null
 [debug/optimizer] [default-parser-functions.spicy:16:18-21:1] declaration::Field "hook void __on_0x25_done();" -> null
@@ -349,8 +363,13 @@
 [debug/optimizer] [default-parser-functions.spicy:16:18-21:1] operator_::struct_::MemberCall "(*self).__on_0x25_finally()" -> expression::Ctor "default<void>()" (replacing call to unimplemented method with default value)
 [debug/optimizer] [default-parser-functions.spicy:16:18-21:1] operator_::struct_::MemberCall "(*self).__on_0x25_finally()" -> expression::Ctor "default<void>()" (replacing call to unimplemented method with default value)
 [debug/optimizer] [default-parser-functions.spicy:16:18-21:1] operator_::struct_::MemberCall "(*self).__on_0x25_init()" -> expression::Ctor "default<void>()" (replacing call to unimplemented method with default value)
+[debug/optimizer] [default-parser-functions.spicy:16:18-21:1] statement::Expression "default<void>();" -> removing default<void> statement
+[debug/optimizer] [default-parser-functions.spicy:16:18-21:1] statement::Expression "default<void>();" -> removing default<void> statement
+[debug/optimizer] [default-parser-functions.spicy:16:18-21:1] statement::Expression "default<void>();" -> removing default<void> statement
+[debug/optimizer] [default-parser-functions.spicy:16:18-21:1] statement::Expression "default<void>();" -> removing default<void> statement
 [debug/optimizer] [default-parser-functions.spicy:17:5-17:13] declaration::Field "hook void __on_x(uint<8> __dd);" -> null
 [debug/optimizer] [default-parser-functions.spicy:17:5-17:13] operator_::struct_::MemberCall "(*self).__on_x((*self).x)" -> expression::Ctor "default<void>()" (replacing call to unimplemented method with default value)
+[debug/optimizer] [default-parser-functions.spicy:17:5-17:13] statement::Expression "default<void>();" -> removing default<void> statement
 [debug/optimizer] [spicy_rt.hlt:28:5-28:76] declaration::Field "method void connect_mime_type(string mime_type, string scope) &internal;" -> null (removing unused member)
 [debug/optimizer] [spicy_rt.hlt:29:5-29:75] declaration::Field "method void connect_mime_type(bytes mime_type, string scope) &internal;" -> null (removing unused member)
 [debug/optimizer] removing field for unused method foo::P0::__on_0x25_confirmed
diff --git a/tests/Baseline/spicy.optimization.default-parser-functions/opt.hlt b/tests/Baseline/spicy.optimization.default-parser-functions/opt.hlt
index 4acb68b8f0..797aae34aa 100644
--- a/tests/Baseline/spicy.optimization.default-parser-functions/opt.hlt
+++ b/tests/Baseline/spicy.optimization.default-parser-functions/opt.hlt
@@ -61,35 +61,21 @@ method method tuple<const view<stream>, int<64>, const iterator<stream>, optiona
     try {
         hilti::debugIndent("spicy");
         local iterator<stream> __begin = begin(__cur);
-        (*self).__error = __error;
-        default<void>();
-        __error = (*self).__error;
         local strong_ref<stream> filtered = Null;
 
         if ( ! filtered ) 
             __result = (*self).__parse_foo__P1_stage2(__data, __begin, __cur, __trim, __lah, __lahe, __error);
 
     }
-    catch ( hilti::SystemException __except ) {
-        default<void>();
-        (*self).__error = __error;
-        default<void>();
-        __error = (*self).__error;
+    catch ( hilti::SystemException __except ) 
         throw;
-    }
 
-    (*self).__error = __error;
-    default<void>();
-    __error = (*self).__error;
     return __result;
 }
 
 method method tuple<const view<stream>, int<64>, const iterator<stream>, optional<hilti::RecoverableFailure>> foo::P1::__parse_foo__P1_stage2(inout value_ref<stream> __data, iterator<stream> __begin, copy view<stream> __cur, copy bool __trim, copy int<64> __lah, copy iterator<stream> __lahe, copy optional<hilti::RecoverableFailure> __error) {
     # "<...>/default-parser-functions.spicy:14:18-14:24"
     local tuple<view<stream>, int<64>, const iterator<stream>, optional<hilti::RecoverableFailure>> __result;
-    (*self).__error = __error;
-    default<void>();
-    __error = (*self).__error;
     hilti::debugDedent("spicy");
     __result = (__cur, __lah, __lahe, __error);
     return __result;
@@ -169,9 +155,6 @@ method method tuple<const view<stream>, int<64>, const iterator<stream>, optiona
     try {
         hilti::debugIndent("spicy");
         local iterator<stream> __begin = begin(__cur);
-        (*self).__error = __error;
-        default<void>();
-        __error = (*self).__error;
         local strong_ref<stream> filtered = Null;
 
         if ( ! filtered ) 
@@ -180,15 +163,9 @@ method method tuple<const view<stream>, int<64>, const iterator<stream>, optiona
     }
     catch ( hilti::SystemException __except ) {
         (*self).__on_0x25_error(hilti::exception_what(__except));
-        (*self).__error = __error;
-        default<void>();
-        __error = (*self).__error;
         throw;
     }
 
-    (*self).__error = __error;
-    default<void>();
-    __error = (*self).__error;
     return __result;
 }
 
@@ -206,9 +183,6 @@ method method tuple<const view<stream>, int<64>, const iterator<stream>, optiona
 
     # End parsing production: Variable: x   -> uint<8>
 
-    (*self).__error = __error;
-    default<void>();
-    __error = (*self).__error;
     # "<...>/default-parser-functions.spicy:18:8-18:12"
 
     # Begin parsing production: Variable: y   -> uint<8>
@@ -223,9 +197,6 @@ method method tuple<const view<stream>, int<64>, const iterator<stream>, optiona
     (*self).__error = __error;
     (*self).__on_y((*self).y);
     __error = (*self).__error;
-    (*self).__error = __error;
-    default<void>();
-    __error = (*self).__error;
     hilti::debugDedent("spicy");
     __result = (__cur, __lah, __lahe, __error);
     return __result;
diff --git a/tests/Baseline/spicy.optimization.feature_requirements/log b/tests/Baseline/spicy.optimization.feature_requirements/log
index 18afe87204..394863e5f0 100644
--- a/tests/Baseline/spicy.optimization.feature_requirements/log
+++ b/tests/Baseline/spicy.optimization.feature_requirements/log
@@ -439,6 +439,16 @@
 [debug/optimizer] [<no location>] expression::Name "__feat%foo@@X7%uses_random_access" -> expression::Ctor "False" (inlining constant)
 [debug/optimizer] [<no location>] expression::Name "__feat%foo@@X7%uses_random_access" -> expression::Ctor "False" (inlining constant)
 [debug/optimizer] [<no location>] expression::Name "__feat%foo@@X7%uses_stream" -> expression::Ctor "False" (inlining constant)
+[debug/optimizer] [<no location>] statement::Expression "(*self).__error = __error;" -> removing unneeded error push/pop statements
+[debug/optimizer] [<no location>] statement::Expression "(*self).__error = __error;" -> removing unneeded error push/pop statements
+[debug/optimizer] [<no location>] statement::Expression "(*self).__error = __error;" -> removing unneeded error push/pop statements
+[debug/optimizer] [<no location>] statement::Expression "(*self).__error = __error;" -> removing unneeded error push/pop statements
+[debug/optimizer] [<no location>] statement::Expression "(*self).__error = __error;" -> removing unneeded error push/pop statements
+[debug/optimizer] [<no location>] statement::Expression "(*self).__error = __error;" -> removing unneeded error push/pop statements
+[debug/optimizer] [<no location>] statement::Expression "(*self).__error = __error;" -> removing unneeded error push/pop statements
+[debug/optimizer] [<no location>] statement::Expression "(*self).__error = __error;" -> removing unneeded error push/pop statements
+[debug/optimizer] [<no location>] statement::Expression "(*self).__error = __error;" -> removing unneeded error push/pop statements
+[debug/optimizer] [<no location>] statement::Expression "(*self).__error = __error;" -> removing unneeded error push/pop statements
 [debug/optimizer] [<no location>] statement::If "if ( False ) { (*(*self).data).close(); }" -> null
 [debug/optimizer] [<no location>] statement::If "if ( False ) { (*(*self).data).close(); }" -> null
 [debug/optimizer] [<no location>] statement::If "if ( False ) { (*__unit).__begin = begin(__ncur); }" -> null
@@ -830,6 +840,11 @@
 [debug/optimizer] [feature_requirements.spicy:32:11-34:1] operator_::struct_::MemberCall "(*self).__on_0x25_finally()" -> expression::Ctor "default<void>()" (replacing call to unimplemented method with default value)
 [debug/optimizer] [feature_requirements.spicy:32:11-34:1] operator_::struct_::MemberCall "(*self).__on_0x25_finally()" -> expression::Ctor "default<void>()" (replacing call to unimplemented method with default value)
 [debug/optimizer] [feature_requirements.spicy:32:11-34:1] operator_::struct_::MemberCall "(*self).__on_0x25_init()" -> expression::Ctor "default<void>()" (replacing call to unimplemented method with default value)
+[debug/optimizer] [feature_requirements.spicy:32:11-34:1] statement::Expression "default<void>();" -> removing default<void> statement
+[debug/optimizer] [feature_requirements.spicy:32:11-34:1] statement::Expression "default<void>();" -> removing default<void> statement
+[debug/optimizer] [feature_requirements.spicy:32:11-34:1] statement::Expression "default<void>();" -> removing default<void> statement
+[debug/optimizer] [feature_requirements.spicy:32:11-34:1] statement::Expression "default<void>();" -> removing default<void> statement
+[debug/optimizer] [feature_requirements.spicy:32:11-34:1] statement::Expression "default<void>();" -> removing default<void> statement
 [debug/optimizer] [feature_requirements.spicy:36:18-40:1] declaration::Field "hook optional<string> __str__();" -> null
 [debug/optimizer] [feature_requirements.spicy:36:18-40:1] declaration::Field "hook void __on_0x25_confirmed() &needed-by-feature="synchronization";" -> null
 [debug/optimizer] [feature_requirements.spicy:36:18-40:1] declaration::Field "hook void __on_0x25_done();" -> null
@@ -849,6 +864,10 @@
 [debug/optimizer] [feature_requirements.spicy:36:18-40:1] operator_::struct_::MemberCall "(*self).__on_0x25_error(hilti::exception_what(__except))" -> expression::Ctor "default<void>()" (replacing call to unimplemented method with default value)
 [debug/optimizer] [feature_requirements.spicy:36:18-40:1] operator_::struct_::MemberCall "(*self).__on_0x25_finally()" -> expression::Ctor "default<void>()" (replacing call to unimplemented method with default value)
 [debug/optimizer] [feature_requirements.spicy:36:18-40:1] operator_::struct_::MemberCall "(*self).__on_0x25_finally()" -> expression::Ctor "default<void>()" (replacing call to unimplemented method with default value)
+[debug/optimizer] [feature_requirements.spicy:36:18-40:1] statement::Expression "default<void>();" -> removing default<void> statement
+[debug/optimizer] [feature_requirements.spicy:36:18-40:1] statement::Expression "default<void>();" -> removing default<void> statement
+[debug/optimizer] [feature_requirements.spicy:36:18-40:1] statement::Expression "default<void>();" -> removing default<void> statement
+[debug/optimizer] [feature_requirements.spicy:36:18-40:1] statement::Expression "default<void>();" -> removing default<void> statement
 [debug/optimizer] [feature_requirements.spicy:43:11-46:1] declaration::Field "hook optional<string> __str__();" -> null
 [debug/optimizer] [feature_requirements.spicy:43:11-46:1] declaration::Field "hook void __on_0x25_confirmed() &needed-by-feature="synchronization";" -> null
 [debug/optimizer] [feature_requirements.spicy:43:11-46:1] declaration::Field "hook void __on_0x25_done();" -> null
@@ -868,6 +887,10 @@
 [debug/optimizer] [feature_requirements.spicy:43:11-46:1] operator_::struct_::MemberCall "(*self).__on_0x25_error(hilti::exception_what(__except))" -> expression::Ctor "default<void>()" (replacing call to unimplemented method with default value)
 [debug/optimizer] [feature_requirements.spicy:43:11-46:1] operator_::struct_::MemberCall "(*self).__on_0x25_finally()" -> expression::Ctor "default<void>()" (replacing call to unimplemented method with default value)
 [debug/optimizer] [feature_requirements.spicy:43:11-46:1] operator_::struct_::MemberCall "(*self).__on_0x25_finally()" -> expression::Ctor "default<void>()" (replacing call to unimplemented method with default value)
+[debug/optimizer] [feature_requirements.spicy:43:11-46:1] statement::Expression "default<void>();" -> removing default<void> statement
+[debug/optimizer] [feature_requirements.spicy:43:11-46:1] statement::Expression "default<void>();" -> removing default<void> statement
+[debug/optimizer] [feature_requirements.spicy:43:11-46:1] statement::Expression "default<void>();" -> removing default<void> statement
+[debug/optimizer] [feature_requirements.spicy:43:11-46:1] statement::Expression "default<void>();" -> removing default<void> statement
 [debug/optimizer] [feature_requirements.spicy:49:1-51:2] declaration::Type "type X7 = struct { optional<hilti::RecoverableFailure> __error &always-emit &internal; method tuple<const view<stream>, int<64>, const iterator<stream>, optional<hilti::RecoverableFailure>> __parse_foo__X7_stage2(inout value_ref<stream> __data, iterator<stream> __begin, copy view<stream> __cur, copy bool __trim, copy int<64> __lah, copy iterator<stream> __lahe, copy optional<hilti::RecoverableFailure> __error); } &on-heap;" -> null (removing unused type)
 [debug/optimizer] [feature_requirements.spicy:49:11-51:1] declaration::Field "hook optional<string> __str__();" -> null
 [debug/optimizer] [feature_requirements.spicy:49:11-51:1] declaration::Field "hook void __on_0x25_confirmed() &needed-by-feature="synchronization";" -> null
diff --git a/tests/Baseline/spicy.optimization.feature_requirements/opt.hlt b/tests/Baseline/spicy.optimization.feature_requirements/opt.hlt
index 7051adbb38..071e6e636f 100644
--- a/tests/Baseline/spicy.optimization.feature_requirements/opt.hlt
+++ b/tests/Baseline/spicy.optimization.feature_requirements/opt.hlt
@@ -150,9 +150,6 @@ method method tuple<const view<stream>, int<64>, const iterator<stream>, optiona
     try {
         hilti::debugIndent("spicy");
         local iterator<stream> __begin = begin(__cur);
-        (*self).__error = __error;
-        default<void>();
-        __error = (*self).__error;
         local strong_ref<stream> filtered = Null;
 
         if ( ! filtered ) 
@@ -160,28 +157,18 @@ method method tuple<const view<stream>, int<64>, const iterator<stream>, optiona
 
     }
     catch ( hilti::SystemException __except ) {
-        default<void>();
         
             spicy_rt::filter_forward_eod(self);
 
-        (*self).__error = __error;
-        default<void>();
-        __error = (*self).__error;
         throw;
     }
 
-    (*self).__error = __error;
-    default<void>();
-    __error = (*self).__error;
     return __result;
 }
 
 method method tuple<const view<stream>, int<64>, const iterator<stream>, optional<hilti::RecoverableFailure>> foo::X4::__parse_foo__X4_stage2(inout value_ref<stream> __data, iterator<stream> __begin, copy view<stream> __cur, copy bool __trim, copy int<64> __lah, copy iterator<stream> __lahe, copy optional<hilti::RecoverableFailure> __error) {
     # "<...>/feature_requirements.spicy:32:11-34:1"
     local tuple<view<stream>, int<64>, const iterator<stream>, optional<hilti::RecoverableFailure>> __result;
-    (*self).__error = __error;
-    default<void>();
-    __error = (*self).__error;
     
         spicy_rt::filter_forward_eod(self);
 
@@ -292,28 +279,18 @@ method method tuple<const view<stream>, int<64>, const iterator<stream>, optiona
 
     }
     catch ( hilti::SystemException __except ) {
-        default<void>();
         
             spicy_rt::filter_disconnect(self);
 
-        (*self).__error = __error;
-        default<void>();
-        __error = (*self).__error;
         throw;
     }
 
-    (*self).__error = __error;
-    default<void>();
-    __error = (*self).__error;
     return __result;
 }
 
 method method tuple<const view<stream>, int<64>, const iterator<stream>, optional<hilti::RecoverableFailure>> foo::X5::__parse_foo__X5_stage2(inout value_ref<stream> __data, iterator<stream> __begin, copy view<stream> __cur, copy bool __trim, copy int<64> __lah, copy iterator<stream> __lahe, copy optional<hilti::RecoverableFailure> __error) {
     # "<...>/feature_requirements.spicy:36:18-40:1"
     local tuple<view<stream>, int<64>, const iterator<stream>, optional<hilti::RecoverableFailure>> __result;
-    (*self).__error = __error;
-    default<void>();
-    __error = (*self).__error;
     
         spicy_rt::filter_disconnect(self);
 
@@ -404,28 +381,18 @@ method method tuple<const view<stream>, int<64>, const iterator<stream>, optiona
 
     }
     catch ( hilti::SystemException __except ) {
-        default<void>();
         
             (*(*self).data).close();
 
-        (*self).__error = __error;
-        default<void>();
-        __error = (*self).__error;
         throw;
     }
 
-    (*self).__error = __error;
-    default<void>();
-    __error = (*self).__error;
     return __result;
 }
 
 method method tuple<const view<stream>, int<64>, const iterator<stream>, optional<hilti::RecoverableFailure>> foo::X6::__parse_foo__X6_stage2(inout value_ref<stream> __data, iterator<stream> __begin, copy view<stream> __cur, copy bool __trim, copy int<64> __lah, copy iterator<stream> __lahe, copy optional<hilti::RecoverableFailure> __error) {
     # "<...>/feature_requirements.spicy:43:11-46:1"
     local tuple<view<stream>, int<64>, const iterator<stream>, optional<hilti::RecoverableFailure>> __result;
-    (*self).__error = __error;
-    default<void>();
-    __error = (*self).__error;
     
         (*(*self).data).close();
 
diff --git a/tests/Baseline/spicy.optimization.unused-functions/log b/tests/Baseline/spicy.optimization.unused-functions/log
index cb0fa1a05e..937893397b 100644
--- a/tests/Baseline/spicy.optimization.unused-functions/log
+++ b/tests/Baseline/spicy.optimization.unused-functions/log
@@ -272,6 +272,18 @@
 [debug/optimizer] [<no location>] expression::Name "__feat%foo@@F%uses_random_access" -> expression::Ctor "False" (inlining constant)
 [debug/optimizer] [<no location>] expression::Name "__feat%foo@@F%uses_random_access" -> expression::Ctor "False" (inlining constant)
 [debug/optimizer] [<no location>] expression::Name "__feat%foo@@F%uses_stream" -> expression::Ctor "False" (inlining constant)
+[debug/optimizer] [<no location>] statement::Expression "(*self).__error = __error;" -> removing unneeded error push/pop statements
+[debug/optimizer] [<no location>] statement::Expression "(*self).__error = __error;" -> removing unneeded error push/pop statements
+[debug/optimizer] [<no location>] statement::Expression "(*self).__error = __error;" -> removing unneeded error push/pop statements
+[debug/optimizer] [<no location>] statement::Expression "(*self).__error = __error;" -> removing unneeded error push/pop statements
+[debug/optimizer] [<no location>] statement::Expression "(*self).__error = __error;" -> removing unneeded error push/pop statements
+[debug/optimizer] [<no location>] statement::Expression "(*self).__error = __error;" -> removing unneeded error push/pop statements
+[debug/optimizer] [<no location>] statement::Expression "(*self).__error = __error;" -> removing unneeded error push/pop statements
+[debug/optimizer] [<no location>] statement::Expression "(*self).__error = __error;" -> removing unneeded error push/pop statements
+[debug/optimizer] [<no location>] statement::Expression "(*self).__error = __error;" -> removing unneeded error push/pop statements
+[debug/optimizer] [<no location>] statement::Expression "(*self).__error = __error;" -> removing unneeded error push/pop statements
+[debug/optimizer] [<no location>] statement::Expression "(*self).__error = __error;" -> removing unneeded error push/pop statements
+[debug/optimizer] [<no location>] statement::Expression "(*self).__error = __error;" -> removing unneeded error push/pop statements
 [debug/optimizer] [<no location>] statement::If "if ( False ) { (*__unit).__begin = begin(__ncur); }" -> null
 [debug/optimizer] [<no location>] statement::If "if ( False ) { (*__unit).__begin = begin(__ncur); }" -> null
 [debug/optimizer] [<no location>] statement::If "if ( False ) { (*__unit).__begin = begin(__ncur); }" -> null
@@ -498,6 +510,11 @@
 [debug/optimizer] [unused-functions.spicy:21:17-21:23] operator_::struct_::MemberCall "(*self).__on_0x25_finally()" -> expression::Ctor "default<void>()" (replacing call to unimplemented method with default value)
 [debug/optimizer] [unused-functions.spicy:21:17-21:23] operator_::struct_::MemberCall "(*self).__on_0x25_finally()" -> expression::Ctor "default<void>()" (replacing call to unimplemented method with default value)
 [debug/optimizer] [unused-functions.spicy:21:17-21:23] operator_::struct_::MemberCall "(*self).__on_0x25_init()" -> expression::Ctor "default<void>()" (replacing call to unimplemented method with default value)
+[debug/optimizer] [unused-functions.spicy:21:17-21:23] statement::Expression "default<void>();" -> removing default<void> statement
+[debug/optimizer] [unused-functions.spicy:21:17-21:23] statement::Expression "default<void>();" -> removing default<void> statement
+[debug/optimizer] [unused-functions.spicy:21:17-21:23] statement::Expression "default<void>();" -> removing default<void> statement
+[debug/optimizer] [unused-functions.spicy:21:17-21:23] statement::Expression "default<void>();" -> removing default<void> statement
+[debug/optimizer] [unused-functions.spicy:21:17-21:23] statement::Expression "default<void>();" -> removing default<void> statement
 [debug/optimizer] [unused-functions.spicy:24:10-24:16] declaration::Field "hook optional<string> __str__();" -> null
 [debug/optimizer] [unused-functions.spicy:24:10-24:16] declaration::Field "hook void __on_0x25_confirmed() &needed-by-feature="synchronization";" -> null
 [debug/optimizer] [unused-functions.spicy:24:10-24:16] declaration::Field "hook void __on_0x25_done();" -> null
@@ -519,6 +536,11 @@
 [debug/optimizer] [unused-functions.spicy:24:10-24:16] operator_::struct_::MemberCall "(*self).__on_0x25_finally()" -> expression::Ctor "default<void>()" (replacing call to unimplemented method with default value)
 [debug/optimizer] [unused-functions.spicy:24:10-24:16] operator_::struct_::MemberCall "(*self).__on_0x25_finally()" -> expression::Ctor "default<void>()" (replacing call to unimplemented method with default value)
 [debug/optimizer] [unused-functions.spicy:24:10-24:16] operator_::struct_::MemberCall "(*self).__on_0x25_init()" -> expression::Ctor "default<void>()" (replacing call to unimplemented method with default value)
+[debug/optimizer] [unused-functions.spicy:24:10-24:16] statement::Expression "default<void>();" -> removing default<void> statement
+[debug/optimizer] [unused-functions.spicy:24:10-24:16] statement::Expression "default<void>();" -> removing default<void> statement
+[debug/optimizer] [unused-functions.spicy:24:10-24:16] statement::Expression "default<void>();" -> removing default<void> statement
+[debug/optimizer] [unused-functions.spicy:24:10-24:16] statement::Expression "default<void>();" -> removing default<void> statement
+[debug/optimizer] [unused-functions.spicy:24:10-24:16] statement::Expression "default<void>();" -> removing default<void> statement
 [debug/optimizer] [unused-functions.spicy:25:17-27:1] declaration::Field "hook optional<string> __str__();" -> null
 [debug/optimizer] [unused-functions.spicy:25:17-27:1] declaration::Field "hook void __on_0x25_confirmed() &needed-by-feature="synchronization";" -> null
 [debug/optimizer] [unused-functions.spicy:25:17-27:1] declaration::Field "hook void __on_0x25_done();" -> null
@@ -540,6 +562,11 @@
 [debug/optimizer] [unused-functions.spicy:25:17-27:1] operator_::struct_::MemberCall "(*self).__on_0x25_finally()" -> expression::Ctor "default<void>()" (replacing call to unimplemented method with default value)
 [debug/optimizer] [unused-functions.spicy:25:17-27:1] operator_::struct_::MemberCall "(*self).__on_0x25_finally()" -> expression::Ctor "default<void>()" (replacing call to unimplemented method with default value)
 [debug/optimizer] [unused-functions.spicy:25:17-27:1] operator_::struct_::MemberCall "(*self).__on_0x25_init()" -> expression::Ctor "default<void>()" (replacing call to unimplemented method with default value)
+[debug/optimizer] [unused-functions.spicy:25:17-27:1] statement::Expression "default<void>();" -> removing default<void> statement
+[debug/optimizer] [unused-functions.spicy:25:17-27:1] statement::Expression "default<void>();" -> removing default<void> statement
+[debug/optimizer] [unused-functions.spicy:25:17-27:1] statement::Expression "default<void>();" -> removing default<void> statement
+[debug/optimizer] [unused-functions.spicy:25:17-27:1] statement::Expression "default<void>();" -> removing default<void> statement
+[debug/optimizer] [unused-functions.spicy:25:17-27:1] statement::Expression "default<void>();" -> removing default<void> statement
 [debug/optimizer] [unused-functions.spicy:30:1-32:2] declaration::Type "type F = struct { optional<hilti::RecoverableFailure> __error &always-emit &internal; method tuple<const view<stream>, int<64>, const iterator<stream>, optional<hilti::RecoverableFailure>> __parse_foo__F_stage2(inout value_ref<stream> __data, iterator<stream> __begin, copy view<stream> __cur, copy bool __trim, copy int<64> __lah, copy iterator<stream> __lahe, copy optional<hilti::RecoverableFailure> __error); } &on-heap;" -> null (removing unused type)
 [debug/optimizer] [unused-functions.spicy:30:10-32:1] declaration::Field "hook optional<string> __str__();" -> null
 [debug/optimizer] [unused-functions.spicy:30:10-32:1] declaration::Field "hook void __on_0x25_confirmed() &needed-by-feature="synchronization";" -> null
diff --git a/tests/Baseline/spicy.optimization.unused-functions/opt.hlt b/tests/Baseline/spicy.optimization.unused-functions/opt.hlt
index 5e81d57d41..637066b030 100644
--- a/tests/Baseline/spicy.optimization.unused-functions/opt.hlt
+++ b/tests/Baseline/spicy.optimization.unused-functions/opt.hlt
@@ -82,35 +82,21 @@ method method tuple<const view<stream>, int<64>, const iterator<stream>, optiona
     try {
         hilti::debugIndent("spicy");
         local iterator<stream> __begin = begin(__cur);
-        (*self).__error = __error;
-        default<void>();
-        __error = (*self).__error;
         local strong_ref<stream> filtered = Null;
 
         if ( ! filtered ) 
             __result = (*self).__parse_foo__B_stage2(__data, __begin, __cur, __trim, __lah, __lahe, __error);
 
     }
-    catch ( hilti::SystemException __except ) {
-        default<void>();
-        (*self).__error = __error;
-        default<void>();
-        __error = (*self).__error;
+    catch ( hilti::SystemException __except ) 
         throw;
-    }
 
-    (*self).__error = __error;
-    default<void>();
-    __error = (*self).__error;
     return __result;
 }
 
 method method tuple<const view<stream>, int<64>, const iterator<stream>, optional<hilti::RecoverableFailure>> foo::B::__parse_foo__B_stage2(inout value_ref<stream> __data, iterator<stream> __begin, copy view<stream> __cur, copy bool __trim, copy int<64> __lah, copy iterator<stream> __lahe, copy optional<hilti::RecoverableFailure> __error) {
     # "<...>/unused-functions.spicy:21:17-21:23"
     local tuple<view<stream>, int<64>, const iterator<stream>, optional<hilti::RecoverableFailure>> __result;
-    (*self).__error = __error;
-    default<void>();
-    __error = (*self).__error;
     hilti::debugDedent("spicy");
     __result = (__cur, __lah, __lahe, __error);
     return __result;
@@ -184,35 +170,21 @@ method method tuple<const view<stream>, int<64>, const iterator<stream>, optiona
     try {
         hilti::debugIndent("spicy");
         local iterator<stream> __begin = begin(__cur);
-        (*self).__error = __error;
-        default<void>();
-        __error = (*self).__error;
         local strong_ref<stream> filtered = Null;
 
         if ( ! filtered ) 
             __result = (*self).__parse_foo__C_stage2(__data, __begin, __cur, __trim, __lah, __lahe, __error);
 
     }
-    catch ( hilti::SystemException __except ) {
-        default<void>();
-        (*self).__error = __error;
-        default<void>();
-        __error = (*self).__error;
+    catch ( hilti::SystemException __except ) 
         throw;
-    }
 
-    (*self).__error = __error;
-    default<void>();
-    __error = (*self).__error;
     return __result;
 }
 
 method method tuple<const view<stream>, int<64>, const iterator<stream>, optional<hilti::RecoverableFailure>> foo::C::__parse_foo__C_stage2(inout value_ref<stream> __data, iterator<stream> __begin, copy view<stream> __cur, copy bool __trim, copy int<64> __lah, copy iterator<stream> __lahe, copy optional<hilti::RecoverableFailure> __error) {
     # "<...>/unused-functions.spicy:24:10-24:16"
     local tuple<view<stream>, int<64>, const iterator<stream>, optional<hilti::RecoverableFailure>> __result;
-    (*self).__error = __error;
-    default<void>();
-    __error = (*self).__error;
     hilti::debugDedent("spicy");
     __result = (__cur, __lah, __lahe, __error);
     return __result;
@@ -227,26 +199,15 @@ method method tuple<const view<stream>, int<64>, const iterator<stream>, optiona
     try {
         hilti::debugIndent("spicy");
         local iterator<stream> __begin = begin(__cur);
-        (*self).__error = __error;
-        default<void>();
-        __error = (*self).__error;
         local strong_ref<stream> filtered = Null;
 
         if ( ! filtered ) 
             __result = (*self).__parse_foo__D_stage2(__data, __begin, __cur, __trim, __lah, __lahe, __error);
 
     }
-    catch ( hilti::SystemException __except ) {
-        default<void>();
-        (*self).__error = __error;
-        default<void>();
-        __error = (*self).__error;
+    catch ( hilti::SystemException __except ) 
         throw;
-    }
 
-    (*self).__error = __error;
-    default<void>();
-    __error = (*self).__error;
     return __result;
 }
 
@@ -261,9 +222,6 @@ method method tuple<const view<stream>, int<64>, const iterator<stream>, optiona
     (__cur, __lah, __lahe, __error) = (*__transient__anon).__parse_stage1(__data, __begin, __cur, __trim, __lah, __lahe, __error);
     # End parsing production: Unit: foo__C_2 ->
 
-    (*self).__error = __error;
-    default<void>();
-    __error = (*self).__error;
     hilti::debugDedent("spicy");
     __result = (__cur, __lah, __lahe, __error);
     return __result;
diff --git a/tests/Baseline/spicy.optimization.unused-types/log b/tests/Baseline/spicy.optimization.unused-types/log
index eb07bae400..b4e3f2062a 100644
--- a/tests/Baseline/spicy.optimization.unused-types/log
+++ b/tests/Baseline/spicy.optimization.unused-types/log
@@ -506,6 +506,27 @@
 [debug/optimizer] [<no location>] expression::Name "__feat%foo@@Pub3%uses_random_access" -> expression::Ctor "False" (inlining constant)
 [debug/optimizer] [<no location>] expression::Name "__feat%foo@@Pub3%uses_random_access" -> expression::Ctor "False" (inlining constant)
 [debug/optimizer] [<no location>] expression::Name "__feat%foo@@Pub3%uses_stream" -> expression::Ctor "False" (inlining constant)
+[debug/optimizer] [<no location>] statement::Expression "(*self).__error = __error;" -> removing unneeded error push/pop statements
+[debug/optimizer] [<no location>] statement::Expression "(*self).__error = __error;" -> removing unneeded error push/pop statements
+[debug/optimizer] [<no location>] statement::Expression "(*self).__error = __error;" -> removing unneeded error push/pop statements
+[debug/optimizer] [<no location>] statement::Expression "(*self).__error = __error;" -> removing unneeded error push/pop statements
+[debug/optimizer] [<no location>] statement::Expression "(*self).__error = __error;" -> removing unneeded error push/pop statements
+[debug/optimizer] [<no location>] statement::Expression "(*self).__error = __error;" -> removing unneeded error push/pop statements
+[debug/optimizer] [<no location>] statement::Expression "(*self).__error = __error;" -> removing unneeded error push/pop statements
+[debug/optimizer] [<no location>] statement::Expression "(*self).__error = __error;" -> removing unneeded error push/pop statements
+[debug/optimizer] [<no location>] statement::Expression "(*self).__error = __error;" -> removing unneeded error push/pop statements
+[debug/optimizer] [<no location>] statement::Expression "(*self).__error = __error;" -> removing unneeded error push/pop statements
+[debug/optimizer] [<no location>] statement::Expression "(*self).__error = __error;" -> removing unneeded error push/pop statements
+[debug/optimizer] [<no location>] statement::Expression "(*self).__error = __error;" -> removing unneeded error push/pop statements
+[debug/optimizer] [<no location>] statement::Expression "(*self).__error = __error;" -> removing unneeded error push/pop statements
+[debug/optimizer] [<no location>] statement::Expression "(*self).__error = __error;" -> removing unneeded error push/pop statements
+[debug/optimizer] [<no location>] statement::Expression "(*self).__error = __error;" -> removing unneeded error push/pop statements
+[debug/optimizer] [<no location>] statement::Expression "(*self).__error = __error;" -> removing unneeded error push/pop statements
+[debug/optimizer] [<no location>] statement::Expression "(*self).__error = __error;" -> removing unneeded error push/pop statements
+[debug/optimizer] [<no location>] statement::Expression "(*self).__error = __error;" -> removing unneeded error push/pop statements
+[debug/optimizer] [<no location>] statement::Expression "(*self).__error = __error;" -> removing unneeded error push/pop statements
+[debug/optimizer] [<no location>] statement::Expression "(*self).__error = __error;" -> removing unneeded error push/pop statements
+[debug/optimizer] [<no location>] statement::Expression "(*self).__error = __error;" -> removing unneeded error push/pop statements
 [debug/optimizer] [<no location>] statement::If "if ( False ) { (*__unit).__begin = begin(__ncur); }" -> null
 [debug/optimizer] [<no location>] statement::If "if ( False ) { (*__unit).__begin = begin(__ncur); }" -> null
 [debug/optimizer] [<no location>] statement::If "if ( False ) { (*__unit).__begin = begin(__ncur); }" -> null
@@ -877,6 +898,11 @@
 [debug/optimizer] [unused-types.spicy:16:20-16:26] operator_::struct_::MemberCall "(*self).__on_0x25_finally()" -> expression::Ctor "default<void>()" (replacing call to unimplemented method with default value)
 [debug/optimizer] [unused-types.spicy:16:20-16:26] operator_::struct_::MemberCall "(*self).__on_0x25_finally()" -> expression::Ctor "default<void>()" (replacing call to unimplemented method with default value)
 [debug/optimizer] [unused-types.spicy:16:20-16:26] operator_::struct_::MemberCall "(*self).__on_0x25_init()" -> expression::Ctor "default<void>()" (replacing call to unimplemented method with default value)
+[debug/optimizer] [unused-types.spicy:16:20-16:26] statement::Expression "default<void>();" -> removing default<void> statement
+[debug/optimizer] [unused-types.spicy:16:20-16:26] statement::Expression "default<void>();" -> removing default<void> statement
+[debug/optimizer] [unused-types.spicy:16:20-16:26] statement::Expression "default<void>();" -> removing default<void> statement
+[debug/optimizer] [unused-types.spicy:16:20-16:26] statement::Expression "default<void>();" -> removing default<void> statement
+[debug/optimizer] [unused-types.spicy:16:20-16:26] statement::Expression "default<void>();" -> removing default<void> statement
 [debug/optimizer] [unused-types.spicy:19:1-19:21] declaration::Type "type Priv2 = struct { optional<hilti::RecoverableFailure> __error &always-emit &internal; method tuple<const view<stream>, int<64>, const iterator<stream>, optional<hilti::RecoverableFailure>> __parse_foo__Priv2_stage2(inout value_ref<stream> __data, iterator<stream> __begin, copy view<stream> __cur, copy bool __trim, copy int<64> __lah, copy iterator<stream> __lahe, copy optional<hilti::RecoverableFailure> __error); } &on-heap;" -> null (removing unused type)
 [debug/optimizer] [unused-types.spicy:19:14-19:20] declaration::Field "hook optional<string> __str__();" -> null
 [debug/optimizer] [unused-types.spicy:19:14-19:20] declaration::Field "hook void __on_0x25_confirmed() &needed-by-feature="synchronization";" -> null
@@ -972,6 +998,11 @@
 [debug/optimizer] [unused-types.spicy:27:14-27:20] operator_::struct_::MemberCall "(*self).__on_0x25_finally()" -> expression::Ctor "default<void>()" (replacing call to unimplemented method with default value)
 [debug/optimizer] [unused-types.spicy:27:14-27:20] operator_::struct_::MemberCall "(*self).__on_0x25_finally()" -> expression::Ctor "default<void>()" (replacing call to unimplemented method with default value)
 [debug/optimizer] [unused-types.spicy:27:14-27:20] operator_::struct_::MemberCall "(*self).__on_0x25_init()" -> expression::Ctor "default<void>()" (replacing call to unimplemented method with default value)
+[debug/optimizer] [unused-types.spicy:27:14-27:20] statement::Expression "default<void>();" -> removing default<void> statement
+[debug/optimizer] [unused-types.spicy:27:14-27:20] statement::Expression "default<void>();" -> removing default<void> statement
+[debug/optimizer] [unused-types.spicy:27:14-27:20] statement::Expression "default<void>();" -> removing default<void> statement
+[debug/optimizer] [unused-types.spicy:27:14-27:20] statement::Expression "default<void>();" -> removing default<void> statement
+[debug/optimizer] [unused-types.spicy:27:14-27:20] statement::Expression "default<void>();" -> removing default<void> statement
 [debug/optimizer] [unused-types.spicy:28:14-28:20] declaration::Field "hook optional<string> __str__();" -> null
 [debug/optimizer] [unused-types.spicy:28:14-28:20] declaration::Field "hook void __on_0x25_confirmed() &needed-by-feature="synchronization";" -> null
 [debug/optimizer] [unused-types.spicy:28:14-28:20] declaration::Field "hook void __on_0x25_done();" -> null
@@ -993,6 +1024,11 @@
 [debug/optimizer] [unused-types.spicy:28:14-28:20] operator_::struct_::MemberCall "(*self).__on_0x25_finally()" -> expression::Ctor "default<void>()" (replacing call to unimplemented method with default value)
 [debug/optimizer] [unused-types.spicy:28:14-28:20] operator_::struct_::MemberCall "(*self).__on_0x25_finally()" -> expression::Ctor "default<void>()" (replacing call to unimplemented method with default value)
 [debug/optimizer] [unused-types.spicy:28:14-28:20] operator_::struct_::MemberCall "(*self).__on_0x25_init()" -> expression::Ctor "default<void>()" (replacing call to unimplemented method with default value)
+[debug/optimizer] [unused-types.spicy:28:14-28:20] statement::Expression "default<void>();" -> removing default<void> statement
+[debug/optimizer] [unused-types.spicy:28:14-28:20] statement::Expression "default<void>();" -> removing default<void> statement
+[debug/optimizer] [unused-types.spicy:28:14-28:20] statement::Expression "default<void>();" -> removing default<void> statement
+[debug/optimizer] [unused-types.spicy:28:14-28:20] statement::Expression "default<void>();" -> removing default<void> statement
+[debug/optimizer] [unused-types.spicy:28:14-28:20] statement::Expression "default<void>();" -> removing default<void> statement
 [debug/optimizer] [unused-types.spicy:29:20-32:1] declaration::Field "hook optional<string> __str__();" -> null
 [debug/optimizer] [unused-types.spicy:29:20-32:1] declaration::Field "hook void __on_0x25_confirmed() &needed-by-feature="synchronization";" -> null
 [debug/optimizer] [unused-types.spicy:29:20-32:1] declaration::Field "hook void __on_0x25_done();" -> null
@@ -1014,8 +1050,14 @@
 [debug/optimizer] [unused-types.spicy:29:20-32:1] operator_::struct_::MemberCall "(*self).__on_0x25_finally()" -> expression::Ctor "default<void>()" (replacing call to unimplemented method with default value)
 [debug/optimizer] [unused-types.spicy:29:20-32:1] operator_::struct_::MemberCall "(*self).__on_0x25_finally()" -> expression::Ctor "default<void>()" (replacing call to unimplemented method with default value)
 [debug/optimizer] [unused-types.spicy:29:20-32:1] operator_::struct_::MemberCall "(*self).__on_0x25_init()" -> expression::Ctor "default<void>()" (replacing call to unimplemented method with default value)
+[debug/optimizer] [unused-types.spicy:29:20-32:1] statement::Expression "default<void>();" -> removing default<void> statement
+[debug/optimizer] [unused-types.spicy:29:20-32:1] statement::Expression "default<void>();" -> removing default<void> statement
+[debug/optimizer] [unused-types.spicy:29:20-32:1] statement::Expression "default<void>();" -> removing default<void> statement
+[debug/optimizer] [unused-types.spicy:29:20-32:1] statement::Expression "default<void>();" -> removing default<void> statement
+[debug/optimizer] [unused-types.spicy:29:20-32:1] statement::Expression "default<void>();" -> removing default<void> statement
 [debug/optimizer] [unused-types.spicy:31:5-31:13] declaration::Field "hook void __on_x(value_ref<const foo::Priv6> __dd);" -> null
 [debug/optimizer] [unused-types.spicy:31:5-31:13] operator_::struct_::MemberCall "(*self).__on_x((*self).x)" -> expression::Ctor "default<void>()" (replacing call to unimplemented method with default value)
+[debug/optimizer] [unused-types.spicy:31:5-31:13] statement::Expression "default<void>();" -> removing default<void> statement
 [debug/optimizer] [unused-types.spicy:35:1-35:30] declaration::Type "type Priv7 = enum { A = 0, B = 1, C = 2 };" -> null (removing unused type)
 [debug/optimizer] [unused-types.spicy:43:22-46:1] declaration::Field "hook optional<string> __str__();" -> null
 [debug/optimizer] [unused-types.spicy:43:22-46:1] declaration::Field "hook void __on_0x25_confirmed() &needed-by-feature="synchronization";" -> null
@@ -1038,6 +1080,11 @@
 [debug/optimizer] [unused-types.spicy:43:22-46:1] operator_::struct_::MemberCall "(*self).__on_0x25_finally()" -> expression::Ctor "default<void>()" (replacing call to unimplemented method with default value)
 [debug/optimizer] [unused-types.spicy:43:22-46:1] operator_::struct_::MemberCall "(*self).__on_0x25_finally()" -> expression::Ctor "default<void>()" (replacing call to unimplemented method with default value)
 [debug/optimizer] [unused-types.spicy:43:22-46:1] operator_::struct_::MemberCall "(*self).__on_0x25_init()" -> expression::Ctor "default<void>()" (replacing call to unimplemented method with default value)
+[debug/optimizer] [unused-types.spicy:43:22-46:1] statement::Expression "default<void>();" -> removing default<void> statement
+[debug/optimizer] [unused-types.spicy:43:22-46:1] statement::Expression "default<void>();" -> removing default<void> statement
+[debug/optimizer] [unused-types.spicy:43:22-46:1] statement::Expression "default<void>();" -> removing default<void> statement
+[debug/optimizer] [unused-types.spicy:43:22-46:1] statement::Expression "default<void>();" -> removing default<void> statement
+[debug/optimizer] [unused-types.spicy:43:22-46:1] statement::Expression "default<void>();" -> removing default<void> statement
 [debug/optimizer] removing field for unused method foo::Priv10::__on_0x25_confirmed
 [debug/optimizer] removing field for unused method foo::Priv10::__on_0x25_done
 [debug/optimizer] removing field for unused method foo::Priv10::__on_0x25_error
diff --git a/tests/Baseline/spicy.optimization.unused-types/opt.hlt b/tests/Baseline/spicy.optimization.unused-types/opt.hlt
index a9430daf50..a95b0314ac 100644
--- a/tests/Baseline/spicy.optimization.unused-types/opt.hlt
+++ b/tests/Baseline/spicy.optimization.unused-types/opt.hlt
@@ -136,35 +136,21 @@ method method tuple<const view<stream>, int<64>, const iterator<stream>, optiona
     try {
         hilti::debugIndent("spicy");
         local iterator<stream> __begin = begin(__cur);
-        (*self).__error = __error;
-        default<void>();
-        __error = (*self).__error;
         local strong_ref<stream> filtered = Null;
 
         if ( ! filtered ) 
             __result = (*self).__parse_foo__Pub2_stage2(__data, __begin, __cur, __trim, __lah, __lahe, __error);
 
     }
-    catch ( hilti::SystemException __except ) {
-        default<void>();
-        (*self).__error = __error;
-        default<void>();
-        __error = (*self).__error;
+    catch ( hilti::SystemException __except ) 
         throw;
-    }
 
-    (*self).__error = __error;
-    default<void>();
-    __error = (*self).__error;
     return __result;
 }
 
 method method tuple<const view<stream>, int<64>, const iterator<stream>, optional<hilti::RecoverableFailure>> foo::Pub2::__parse_foo__Pub2_stage2(inout value_ref<stream> __data, iterator<stream> __begin, copy view<stream> __cur, copy bool __trim, copy int<64> __lah, copy iterator<stream> __lahe, copy optional<hilti::RecoverableFailure> __error) {
     # "<...>/unused-types.spicy:16:20-16:26"
     local tuple<view<stream>, int<64>, const iterator<stream>, optional<hilti::RecoverableFailure>> __result;
-    (*self).__error = __error;
-    default<void>();
-    __error = (*self).__error;
     hilti::debugDedent("spicy");
     __result = (__cur, __lah, __lahe, __error);
     return __result;
@@ -247,35 +233,21 @@ method method tuple<const view<stream>, int<64>, const iterator<stream>, optiona
     try {
         hilti::debugIndent("spicy");
         local iterator<stream> __begin = begin(__cur);
-        (*self).__error = __error;
-        default<void>();
-        __error = (*self).__error;
         local strong_ref<stream> filtered = Null;
 
         if ( ! filtered ) 
             __result = (*self).__parse_foo__Priv5_stage2(__data, __begin, __cur, __trim, __lah, __lahe, __error);
 
     }
-    catch ( hilti::SystemException __except ) {
-        default<void>();
-        (*self).__error = __error;
-        default<void>();
-        __error = (*self).__error;
+    catch ( hilti::SystemException __except ) 
         throw;
-    }
 
-    (*self).__error = __error;
-    default<void>();
-    __error = (*self).__error;
     return __result;
 }
 
 method method tuple<const view<stream>, int<64>, const iterator<stream>, optional<hilti::RecoverableFailure>> foo::Priv5::__parse_foo__Priv5_stage2(inout value_ref<stream> __data, iterator<stream> __begin, copy view<stream> __cur, copy bool __trim, copy int<64> __lah, copy iterator<stream> __lahe, copy optional<hilti::RecoverableFailure> __error) {
     # "<...>/unused-types.spicy:27:14-27:20"
     local tuple<view<stream>, int<64>, const iterator<stream>, optional<hilti::RecoverableFailure>> __result;
-    (*self).__error = __error;
-    default<void>();
-    __error = (*self).__error;
     hilti::debugDedent("spicy");
     __result = (__cur, __lah, __lahe, __error);
     return __result;
@@ -290,35 +262,21 @@ method method tuple<const view<stream>, int<64>, const iterator<stream>, optiona
     try {
         hilti::debugIndent("spicy");
         local iterator<stream> __begin = begin(__cur);
-        (*self).__error = __error;
-        default<void>();
-        __error = (*self).__error;
         local strong_ref<stream> filtered = Null;
 
         if ( ! filtered ) 
             __result = (*self).__parse_foo__Priv6_stage2(__data, __begin, __cur, __trim, __lah, __lahe, __error);
 
     }
-    catch ( hilti::SystemException __except ) {
-        default<void>();
-        (*self).__error = __error;
-        default<void>();
-        __error = (*self).__error;
+    catch ( hilti::SystemException __except ) 
         throw;
-    }
 
-    (*self).__error = __error;
-    default<void>();
-    __error = (*self).__error;
     return __result;
 }
 
 method method tuple<const view<stream>, int<64>, const iterator<stream>, optional<hilti::RecoverableFailure>> foo::Priv6::__parse_foo__Priv6_stage2(inout value_ref<stream> __data, iterator<stream> __begin, copy view<stream> __cur, copy bool __trim, copy int<64> __lah, copy iterator<stream> __lahe, copy optional<hilti::RecoverableFailure> __error) {
     # "<...>/unused-types.spicy:28:14-28:20"
     local tuple<view<stream>, int<64>, const iterator<stream>, optional<hilti::RecoverableFailure>> __result;
-    (*self).__error = __error;
-    default<void>();
-    __error = (*self).__error;
     hilti::debugDedent("spicy");
     __result = (__cur, __lah, __lahe, __error);
     return __result;
@@ -333,26 +291,15 @@ method method tuple<const view<stream>, int<64>, const iterator<stream>, optiona
     try {
         hilti::debugIndent("spicy");
         local iterator<stream> __begin = begin(__cur);
-        (*self).__error = __error;
-        default<void>();
-        __error = (*self).__error;
         local strong_ref<stream> filtered = Null;
 
         if ( ! filtered ) 
             __result = (*self).__parse_foo__Pub3_stage2(__data, __begin, __cur, __trim, __lah, __lahe, __error);
 
     }
-    catch ( hilti::SystemException __except ) {
-        default<void>();
-        (*self).__error = __error;
-        default<void>();
-        __error = (*self).__error;
+    catch ( hilti::SystemException __except ) 
         throw;
-    }
 
-    (*self).__error = __error;
-    default<void>();
-    __error = (*self).__error;
     return __result;
 }
 
@@ -374,12 +321,6 @@ method method tuple<const view<stream>, int<64>, const iterator<stream>, optiona
     (__cur, __lah, __lahe, __error) = (*(*self).x).__parse_stage1(__data, __begin, __cur, __trim, __lah, __lahe, __error);
     # End parsing production: Unit: foo__Priv6_2 ->
 
-    (*self).__error = __error;
-    default<void>();
-    __error = (*self).__error;
-    (*self).__error = __error;
-    default<void>();
-    __error = (*self).__error;
     hilti::debugDedent("spicy");
     __result = (__cur, __lah, __lahe, __error);
     return __result;
@@ -453,35 +394,21 @@ method method tuple<const view<stream>, int<64>, const iterator<stream>, optiona
     try {
         hilti::debugIndent("spicy");
         local iterator<stream> __begin = begin(__cur);
-        (*self).__error = __error;
-        default<void>();
-        __error = (*self).__error;
         local strong_ref<stream> filtered = Null;
 
         if ( ! filtered ) 
             __result = (*self).__parse_foo__Priv10_stage2(__data, __begin, __cur, __trim, __lah, __lahe, __error);
 
     }
-    catch ( hilti::SystemException __except ) {
-        default<void>();
-        (*self).__error = __error;
-        default<void>();
-        __error = (*self).__error;
+    catch ( hilti::SystemException __except ) 
         throw;
-    }
 
-    (*self).__error = __error;
-    default<void>();
-    __error = (*self).__error;
     return __result;
 }
 
 method method tuple<const view<stream>, int<64>, const iterator<stream>, optional<hilti::RecoverableFailure>> foo::Priv10::__parse_foo__Priv10_stage2(inout value_ref<stream> __data, iterator<stream> __begin, copy view<stream> __cur, copy bool __trim, copy int<64> __lah, copy iterator<stream> __lahe, copy optional<hilti::RecoverableFailure> __error) {
     # "<...>/unused-types.spicy:43:22-46:1"
     local tuple<view<stream>, int<64>, const iterator<stream>, optional<hilti::RecoverableFailure>> __result;
-    (*self).__error = __error;
-    default<void>();
-    __error = (*self).__error;
     hilti::debugDedent("spicy");
     __result = (__cur, __lah, __lahe, __error);
     return __result;
diff --git a/tests/Baseline/spicy.rt.debug-trace/.stderr b/tests/Baseline/spicy.rt.debug-trace/.stderr
index f2ae95977d..931d64939c 100644
--- a/tests/Baseline/spicy.rt.debug-trace/.stderr
+++ b/tests/Baseline/spicy.rt.debug-trace/.stderr
@@ -16,7 +16,7 @@
 [hilti-trace] : (__ncur, __lahead, __lahead_end, __error) = (*__unit).__parse_stage1(__data, begin(__ncur), __ncur, True, __lahead, __lahead_end, __error);
 [hilti-trace] : # "<...>/debug-trace.spicy:8:20-13:1"
 [hilti-trace] : local tuple<view<stream>, int<64>, const iterator<stream>, optional<hilti::RecoverableFailure>> __result;
-[hilti-trace] : try { hilti::debug("spicy", "Mini::Test"); hilti::debugIndent("spicy"); local iterator<stream> __begin = begin(__cur); (*self).__error = __error; (*self).__on_0x25_init(); __error = (*self).__error; local strong_ref<stream> filtered = Null; if ( ! filtered ) __result = (*self).__parse_Mini__Test_stage2(__data, __begin, __cur, __trim, __lah, __lahe, __error); } catch ( hilti::SystemException __except ) { default<void>(); (*self).__error = __error; default<void>(); __error = (*self).__error; throw; }
+[hilti-trace] : try { hilti::debug("spicy", "Mini::Test"); hilti::debugIndent("spicy"); local iterator<stream> __begin = begin(__cur); (*self).__error = __error; (*self).__on_0x25_init(); __error = (*self).__error; local strong_ref<stream> filtered = Null; if ( ! filtered ) __result = (*self).__parse_Mini__Test_stage2(__data, __begin, __cur, __trim, __lah, __lahe, __error); } catch ( hilti::SystemException __except ) { throw; }
 [hilti-trace] : hilti::debug("spicy", "Mini::Test");
 [hilti-trace] : hilti::debugIndent("spicy");
 [hilti-trace] : local iterator<stream> __begin = begin(__cur);
@@ -72,9 +72,6 @@
 [hilti-trace] : hilti::debugDedent("spicy");
 [hilti-trace] : __result = (__cur, __lah, __lahe, __error);
 [hilti-trace] : return __result;
-[hilti-trace] : (*self).__error = __error;
-[hilti-trace] debug-trace.spicy:8:20-13:1: : default<void>();
-[hilti-trace] : __error = (*self).__error;
 [hilti-trace] : return __result;
 [hilti-trace] : hilti::debugDedent("spicy-verbose");
 [hilti-trace] : # End parsing production: Unit: Mini__Test -> f1 f2
diff --git a/tests/Baseline/spicy.types.sink.reassembler.wrong-init-seq/output b/tests/Baseline/spicy.types.sink.reassembler.wrong-init-seq/output
index 0259ea0fc5..3e80a1dd70 100644
--- a/tests/Baseline/spicy.types.sink.reassembler.wrong-init-seq/output
+++ b/tests/Baseline/spicy.types.sink.reassembler.wrong-init-seq/output
@@ -1,2 +1,2 @@
 ### BTest baseline data generated by btest-diff. Do not edit. Use "btest -U/-u" to update. Requires BTest >= 0.63.
-[error] terminating with uncaught exception of type spicy::rt::SinkError: sink cannot update initial sequence number after activity has already been seen (<...>/wrong-init-seq.spicy:20:19-38:1)
+[error] terminating with uncaught exception of type spicy::rt::SinkError: sink cannot update initial sequence number after activity has already been seen (<...>/wrong-init-seq.spicy:14:9-14:50)
diff --git a/tests/hilti/optimization/peephole-default-void.hlt b/tests/hilti/optimization/peephole-default-void.hlt
new file mode 100644
index 0000000000..b7e189a268
--- /dev/null
+++ b/tests/hilti/optimization/peephole-default-void.hlt
@@ -0,0 +1,12 @@
+# @TEST-EXEC: hiltic -p %INPUT >opt.hlt
+# @TEST-EXEC: btest-diff opt.hlt
+#
+# @TEST-DOC: Test removal of no-op `default<void>()` statements
+
+module Test {
+
+public function void foo() {
+    default<void>();
+}
+
+}
diff --git a/tests/hilti/optimization/peephole-error-push-pop.hlt b/tests/hilti/optimization/peephole-error-push-pop.hlt
new file mode 100644
index 0000000000..23a4828c88
--- /dev/null
+++ b/tests/hilti/optimization/peephole-error-push-pop.hlt
@@ -0,0 +1,19 @@
+# @TEST-EXEC: hiltic -V -p %INPUT >opt.hlt
+# @TEST-EXEC: btest-diff opt.hlt
+#
+# @TEST-DOC: Test removal of redundant __error assignments.
+
+module Test {
+
+type Foo = struct {
+    method void test();
+    int<64> __error; # name is internal, hence compiling with -V; type does not matter
+};
+
+function void Foo::test() {
+    local int<64> __error;
+    (*self).__error = __error;
+    __error = (*self).__error;
+}
+
+}
diff --git a/tests/spicy/types/bytes/parse-length.spicy b/tests/spicy/types/bytes/parse-length.spicy
index f2ff438959..3e67259933 100644
--- a/tests/spicy/types/bytes/parse-length.spicy
+++ b/tests/spicy/types/bytes/parse-length.spicy
@@ -9,6 +9,10 @@
 #
 # Ensure we don't get any look-ahead checks when parsing the literals, we don't need them here.
 # @TEST-EXEC: spicyc -p %INPUT | grep -vq 'if.*lah'
+#
+# Ensure our peephole optimizer removes `(*self).__error = __error; default<void>(); __error = (*self).__error;` blocks
+# @TEST-EXEC-FAIL: spicyc -p %INPUT | grep -q 'default<void>()'
+# @TEST-EXEC-FAIL: spicyc -p %INPUT | grep -v 'default<void>()' | grep -A1 '(.self).__error = __error' | grep -q -B1 "__error = (.self).__error"
 
 module Test;