From e985a7e59a7a85f61a4eb439068a439cf7adc9c2 Mon Sep 17 00:00:00 2001
From: Roland Conybeare <rconybeare@gmail.com>
Date: Sun, 19 Apr 2026 21:20:38 -0400
Subject: [PATCH] xo-gc: bugfixes + seq3 mlog utest

---
 xo-gc/src/gc/MutationLogStore.cpp     |  5 ++--
 xo-gc/utest/MutationLogStore.test.cpp | 39 ++++++++++++++++++++++++++-
 2 files changed, 41 insertions(+), 3 deletions(-)

diff --git a/xo-gc/src/gc/MutationLogStore.cpp b/xo-gc/src/gc/MutationLogStore.cpp
index 9b141381..e92818c3 100644
--- a/xo-gc/src/gc/MutationLogStore.cpp
+++ b/xo-gc/src/gc/MutationLogStore.cpp
@@ -391,8 +391,9 @@ namespace xo {
                                                             from_entry.parent());
 
                 if (parent_gen_to.is_sentinel()) {
-                    void * parent_fr = *from_entry.p_data();
+                    // parent is not in to-space
 
+                    void * parent_fr = from_entry.parent();
                     AllocInfo parent_info = gc.alloc_info((std::byte *)parent_fr);
 
                     if (parent_info.is_forwarding_tseq()) {
@@ -408,7 +409,7 @@ namespace xo {
                                                          parent_to);
                         parent_info = gc.alloc_info((std::byte *)parent_to);
 
-                        assert(!parent_gen_to.sentinel());
+                        assert(!parent_gen_to.is_sentinel());
 
                         // Since parent already forwarded, we don't have to preserve child
                         // or update parent object.
diff --git a/xo-gc/utest/MutationLogStore.test.cpp b/xo-gc/utest/MutationLogStore.test.cpp
index f5dfd07b..84ce34eb 100644
--- a/xo-gc/utest/MutationLogStore.test.cpp
+++ b/xo-gc/utest/MutationLogStore.test.cpp
@@ -127,6 +127,8 @@ namespace ut {
 
         static TestSequence seq_0 { step_0, phase_0 };
 
+        // ----------------------------------------------------------------
+
         // seq1: side effect on head of cons cell.
         // But no mlog entry b/c all object ages are equal
         // -> no x-age pointers
@@ -150,6 +152,8 @@ namespace ut {
 
         static TestSequence seq_1 { step_1, phase_1 };
 
+        // ----------------------------------------------------------------
+
         static Step step_2[] = {
             // ----- phase 0 -----
             {Cmd::make_bool,   0, 0}, // [0]: #f
@@ -176,6 +180,38 @@ namespace ut {
 
         static TestSequence seq_2 { step_2, phase_2 };
 
+        // ----------------------------------------------------------------
+
+        static Step step_3[] = {
+            // ----- phase 0 -----
+            {Cmd::make_bool,   0, 0}, // [0]: #f
+            {Cmd::make_bool,   1, 0}, // [1]: #t
+            {Cmd::make_nil,    0, 0}, // [2]: #nil
+            {Cmd::make_cons,   0, 2}, // [3]: cons(#f,#nil)
+            // ----- phase 1 -----
+            {Cmd::make_bool,   1, 0}, // [4]: #t
+            {Cmd::assign_head, 3, 4}, //      set-car(cons(#f,#nil),#t)
+            // ----- phase 2 -----
+            // ----- phase 3 -----
+            // ----- end -----
+            {Cmd::sentinel,    0, 0},
+        };
+
+        static Phase phase_3[] = {
+            //
+            // lo   hi    mlog_new_z_[]
+            //  v    v    v
+            {   0,   4,   {0} },  // phase 0
+            {   4,   6,   {1} },  // phase 1. set-car makes 1x xage ptr
+            {   6,   6,   {1} },  // phase 2. now src in g1, dest in g0
+            {   6,   6,   {0} },  // phase 3. now dest in g1
+            {  -1,  -1,   {0} },
+        };
+
+        static TestSequence seq_3 { step_3, phase_3 };
+
+        // ----------------------------------------------------------------
+
 #      define seq_nil TestSequence{}
 #      define nil nullptr
 #      define T true
@@ -202,7 +238,8 @@ namespace ut {
             Testcase(2, 4, 16 * 1024, 8 * 128, T, seq_nil,    0, F, c_selfcycle, 1,  1,  0,  0,  0, F),
             Testcase(2, 4, 16 * 1024, 8 * 128, T,   seq_0,    0, F,     c_fixed, 1,  0,  0,  0,  0, F),
             Testcase(2, 4, 16 * 1024, 8 * 128, T,   seq_1,    0, F,     c_fixed, 1,  0,  0,  0,  0, F),
-            Testcase(2, 1, 16 * 1024, 8 * 128, T,   seq_2,  128, T,     c_fixed, 3,  0,  0,  0,  0, T),
+            Testcase(2, 1, 16 * 1024, 8 * 128, T,   seq_2,  128, T,     c_fixed, 3,  0,  0,  0,  0, F),
+            Testcase(2, 2, 16 * 1024, 8 * 128, T,   seq_3,  128, T,     c_fixed, 4,  0,  0,  0,  0, T),
         };
 
 #      undef T