From 32abec651d9775ca338da1d8a43c6363fd7a1190 Mon Sep 17 00:00:00 2001
From: "Enzo Persillet (Tutez)" <enzopersillet@gmail.com>
Date: Sat, 3 Oct 2026 17:08:17 +0200
Subject: [PATCH] Align the objects that jitted cppia code allocates in the
 nursery.

With HXCPP_GC_GENERATIONAL, ImmixAllocator::alloc skips 4 bytes when
needed so that each nursery object is 8-byte aligned, and the nursery
scan that finds conservatively referenced objects (GetEnclosingNurseryType)
walks the holes with that alignment. The inline allocation that the JIT
emits for a new did not align. After an object it placed off alignment,
the scan read a wrong header and missed the objects that follow, so an
object referenced only from the stack was freed by the next collection.

The cppia host now builds with HXCPP_GC_GENERATIONAL. The new test has
jitted code allocate two objects, then keeps an array only on the stack
across a generational collection. Without the fix, the -jit run crashes.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
---
 src/hx/cppia/Cppia.cpp         |  6 ++++++
 test/cppia/Client.hx           | 13 +++++++++++++
 test/cppia/cases/TestCommon.hx | 14 ++++++++++++++
 test/cppia/compile-host.hxml   |  1 +
 4 files changed, 34 insertions(+)

diff --git a/src/hx/cppia/Cppia.cpp b/src/hx/cppia/Cppia.cpp
index ba68a29fa..ea5d1f43c 100644
--- a/src/hx/cppia/Cppia.cpp
+++ b/src/hx/cppia/Cppia.cpp
@@ -1619,6 +1619,12 @@ struct NewExpr : public CppiaDynamicExpr
             // sJitReturnReg = alloc->spaceFirst - will be the result if all goes well
             compiler->move(sJitReturnReg, sJitCtx.star(jtPointer, offsetof(hx::StackContext,spaceFirst) ) );
 
+            #ifdef HXCPP_ALIGN_ALLOC
+            // As ImmixAllocator::alloc: skip 4 bytes to align the object, which the nursery scan expects
+            compiler->bitOp(bitOpAnd, sJitTemp1.as(jtInt), sJitReturnReg.as(jtInt), (int)4 );
+            compiler->add(sJitReturnReg.as(jtPointer), sJitReturnReg.as(jtPointer), sJitTemp1.as(jtPointer) );
+            #endif
+
             // sJitTemp1 = end = spaceFirst + size + sizeof(int)
             compiler->add(sJitTemp1.as(jtPointer), sJitReturnReg.as(jtPointer), (int)(size + sizeof(int) ) );
 
diff --git a/test/cppia/Client.hx b/test/cppia/Client.hx
index 710373f4b..36e73f776 100644
--- a/test/cppia/Client.hx
+++ b/test/cppia/Client.hx
@@ -14,6 +14,19 @@ class ClientFoo implements IFoo {
    }
 }
 
+class ClientNursery {
+
+   public var value:Dynamic;
+
+   public function new() {}
+
+   // With -jit, the jitted code allocates these in the nursery itself.
+   public static function allocate() {
+      new ClientNursery();
+      new ClientNursery();
+   }
+}
+
 class Client
 {
    public static var clientBool0 = true;
diff --git a/test/cppia/cases/TestCommon.hx b/test/cppia/cases/TestCommon.hx
index eaacb5a76..b317566aa 100644
--- a/test/cppia/cases/TestCommon.hx
+++ b/test/cppia/cases/TestCommon.hx
@@ -1,6 +1,7 @@
 package cases;
 
 import cpp.cppia.Host;
+import cpp.vm.Gc;
 import utest.Test;
 import utest.Assert;
 
@@ -59,6 +60,19 @@ class TestCommon extends Test {
         Assert.equals(2, Common.callbackSet, 'Bad cppia closure');
     }
 
+    @:depends(testStatus)
+    function testStackObjectAfterScriptAllocations() {
+        final nursery = Type.resolveClass('ClientNursery');
+        Reflect.callMethod(nursery, Reflect.field(nursery, 'allocate'), []);
+        // Only the stack references it, so only the nursery scan finds it.
+        final kept = [42];
+
+        Gc.run(false);
+        final junk = [for (i in 0...100000) [i]];
+
+        Assert.equals(42, kept[0]);
+    }
+
     @:depends(testStatus)
     function testInterfaceCalling() {
         final obj : IFoo = Type.createInstance(Type.resolveClass('ClientFoo'), []);
diff --git a/test/cppia/compile-host.hxml b/test/cppia/compile-host.hxml
index 76bf2d790..051e26186 100644
--- a/test/cppia/compile-host.hxml
+++ b/test/cppia/compile-host.hxml
@@ -1,6 +1,7 @@
 -m CppiaHost
 HostExtendedRoot
 -D scriptable
+-D HXCPP_GC_GENERATIONAL
 -D dll_export=host_classes.info
 -L utest
 --dce no