vm: load mega cache object before popping frame

Fixes #457 harder

vm/debug.cpp

@@ -254,6 +254,12 @@ void factor_vm::print_callstack()
 		std::cout << "*** Context not initialized" << std::endl;
 }
 
+void factor_vm::print_callstack_object(callstack *obj)
+{
+	stack_frame_printer printer(this);
+	iterate_callstack_object(obj,printer);
+}
+
 struct padded_address {
 	cell value;
 

vm/dispatch.cpp

@@ -148,8 +148,7 @@ void quotation_jit::emit_mega_cache_lookup(cell methods_, fixnum index, cell cac
 	data_root<array> methods(methods_,parent);
 	data_root<array> cache(cache_,parent);
 
-	/* Load the object from the datastack. */
-	emit_with_literal(parent->special_objects[PIC_LOAD],tag_fixnum(-index * sizeof(cell)));
+	/* The object must be on the top of the datastack at this point. */
 
 	/* Do a cache lookup. */
 	emit_with_literal(parent->special_objects[MEGA_LOOKUP],cache.value());

vm/errors.cpp

@@ -78,7 +78,7 @@ void factor_vm::general_error(vm_error_type error, cell arg1, cell arg2)
 		unwind_native_frames(special_objects[ERROR_HANDLER_QUOT],
 			ctx->callstack_top);
 	}
-	/* Error was thrown in early startup before error handler is set, just
+	/* Error was thrown in early startup before error handler is set, so just
 	crash. */
 	else
 	{

vm/quotations.cpp

@@ -281,11 +281,15 @@ void quotation_jit::iterate_quotation()
 			/* Method dispatch */
 			if(mega_lookup_p(i,length))
 			{
+				fixnum index = untag_fixnum(array_nth(elements.untagged(),i + 1));
+				/* Load the object from the datastack, then remove our stack frame. */
+				emit_with_literal(parent->special_objects[PIC_LOAD],tag_fixnum(-index * sizeof(cell)));
 				emit_epilog(safepoint, stack_frame);
 				tail_call = true;
+
 				emit_mega_cache_lookup(
 					array_nth(elements.untagged(),i),
-					untag_fixnum(array_nth(elements.untagged(),i + 1)),
+					index,
 					array_nth(elements.untagged(),i + 2));
 				i += 3;
 			}

vm/vm.hpp

@@ -406,6 +406,7 @@ struct factor_vm
 	void print_datastack();
 	void print_retainstack();
 	void print_callstack();
+	void print_callstack_object(callstack *obj);
 	void dump_cell(cell x);
 	void dump_memory(cell from, cell to);
 	template<typename Generation> void dump_generation(const char *name, Generation *gen);