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);
     }
 

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)