Root globals in toplevel exprs

src/codegen.cpp

@@ -6378,8 +6378,9 @@ static jl_cgval_t emit_expr(jl_codectx_t &ctx, jl_value_t *expr, ssize_t ssaidx_
         jl_value_t *val = expr;
         if (jl_is_quotenode(expr))
             val = jl_fieldref_noalloc(expr, 0);
-        if (jl_is_method(ctx.linfo->def.method)) // toplevel exprs are already rooted
-            val = jl_ensure_rooted(ctx, val);
+        // Toplevel exprs are rooted but because codegen assumes this is constant, it removes the write barriers for this code.
+        // This means we have to globally root the value here. (The other option would be to change how we optimize toplevel code)
+        val = jl_ensure_rooted(ctx, val);
         return mark_julia_const(ctx, val);
     }
 

src/gc-stock.c

@@ -305,6 +305,9 @@ STATIC_INLINE void gc_setmark_pool(jl_ptls_t ptls, jl_taggedvalue_t *o,
 STATIC_INLINE void gc_setmark(jl_ptls_t ptls, jl_taggedvalue_t *o,
                               uint8_t mark_mode, size_t sz) JL_NOTSAFEPOINT
 {
+    if (o->bits.in_image) {
+        return;
+    }
     if (sz <= GC_MAX_SZCLASS) {
         gc_setmark_pool(ptls, o, mark_mode);
     }
@@ -4013,6 +4016,36 @@ JL_DLLEXPORT void jl_gc_schedule_foreign_sweepfunc(jl_ptls_t ptls, jl_value_t *o
     arraylist_push(&ptls->gc_tls.sweep_objs, obj);
 }
 
+void jl_gc_force_mark_old(jl_ptls_t ptls, jl_value_t *v) JL_NOTSAFEPOINT
+{
+    jl_taggedvalue_t *o = jl_astaggedvalue(v);
+    jl_datatype_t *dt = (jl_datatype_t*)jl_typeof(v);
+    size_t dtsz = jl_datatype_size(dt);
+    if (o->bits.gc == GC_OLD_MARKED)
+        return;
+    o->bits.gc = GC_OLD_MARKED;
+    if (dt == jl_simplevector_type) {
+        size_t l = jl_svec_len(v);
+        dtsz = l * sizeof(void*) + sizeof(jl_svec_t);
+    }
+    else if (dt->name == jl_genericmemory_typename) {
+        dtsz = sizeof(jl_genericmemory_t);
+    }
+    else if (dt == jl_module_type) {
+        dtsz = sizeof(jl_module_t);
+    }
+    else if (dt == jl_task_type) {
+        dtsz = sizeof(jl_task_t);
+    }
+    else if (dt == jl_symbol_type) {
+        return;
+    }
+    gc_setmark(ptls, o, GC_OLD_MARKED, dtsz);
+    if (dt->layout->npointers != 0)
+        jl_gc_queue_root(v);
+}
+
+
 #ifdef __cplusplus
 }
 #endif

src/jl_exported_funcs.inc

@@ -170,6 +170,7 @@
     XX(jl_gc_small_alloc) \
     XX(jl_gc_queue_multiroot) \
     XX(jl_gc_queue_root) \
+    XX(jl_gc_force_mark_old) \
     XX(jl_gc_safepoint) \
     XX(jl_gc_schedule_foreign_sweepfunc) \
     XX(jl_gc_set_cb_notify_external_alloc) \

src/julia_internal.h

@@ -374,6 +374,7 @@ jl_value_t *jl_gc_small_alloc_noinline(jl_ptls_t ptls, int offset,
                                    int osize);
 jl_value_t *jl_gc_big_alloc_noinline(jl_ptls_t ptls, size_t allocsz);
 JL_DLLEXPORT int jl_gc_classify_pools(size_t sz, int *osize) JL_NOTSAFEPOINT;
+JL_DLLEXPORT void jl_gc_force_mark_old(jl_ptls_t ptls, jl_value_t *v);
 void gc_sweep_sysimg(void);
 
 

test/gc.jl

@@ -72,3 +72,17 @@ end
 @testset "Base.GC docstrings" begin
     @test isempty(Docs.undocumented_names(GC))
 end
+
+#testset doesn't work here because this needs to run in top level
+#Check that we ensure objects in toplevel exprs are rooted
+global dims54422 = [] # allocate the Binding
+GC.gc(); GC.gc(); # force the binding to be old
+GC.enable(false); # prevent new objects from being old
+@eval begin
+    Base.Experimental.@force_compile # use the compiler
+    dims54422 = $([])
+    nothing
+end
+GC.enable(true); GC.gc(false) # incremental collection
+@test typeof(dims54422) == Vector{Any}
+@test isempty(dims54422)