Two decades of live coding and debugging of virtual machines through simulation

Abstract

OpenSmalltalk-VM is a virtual machine (VM) for languages in the Smalltalk family (eg, Squeak and Pharo), which is itself written in a subset of Smalltalk that can easily be translated to C. VM development is done in Smalltalk, an activity we call "simulation." The production VM is then derived by translating the core VM code to C. As a result, two execution models coexist: simulation, where the Smalltalk code is executed on top of a Smalltalk VM, and production, where the same code is compiled to an executable through a C compiler. The whole VM execution can be simulated: the heap is represented as a huge byte array, the VM code is executed as Smalltalk, and the native code generated by the just-in-time (JIT) compiler is executed by a processor simulator. All the Smalltalk development tools, such as the debugger, are then available while simulating. In addition, in simulation, it is also possible to use debugging features such as single stepping in the machine code generated by the JIT compiler. The Smalltalk development tools combined with the simulation debugging features provide developers with a productive environment in which to extend and debug the VM. In this article, we detail the VM simulation infrastructure and report our experiences developing and debugging VM features within it such as the garbage collector and the JIT compiler.

Automatic inspection of program state in an uncooperative environment

“…11 It relies on the compiler only-it does not depend on the operating system or the architecture. Thus, Whiro can be used even in embedded devices that lack support of inspection tools like Valgrind, 12 which is architecture-dependent, or GDB, 13 which is operating system-dependent, or the OpenSmalltalk debugger, 14 which relies on a virtual machine simulator.…”

Ease Virtual Machine Level Tooling with Language Level Ordinary Object Pointers

“…It is written in Pharo itself and generated to C using a VMspecific translator [19]. Executing the Pharo VM in Pharo is called simulating the VM [20,26]. Pharo VM developers work mostly in the simulation [29].…”

“…The Pharo VM simulation has been used to debug memory corruptions [20,29]. This was possible because they had a reproducible case.…”

“…Testing and debugging a VM is a laborious task without proper tooling [29]. Many VMs are developed in low level languages [40] or in low level style [7,19,20]. Additionally they are often used to support high level languages such as object-oriented programming languages [6].…”

“…Pharo [4] general purpose language derived from Smalltalk [13]. The Pharo VM is written in a low level style in a restricted Pharo translated to C code [19,20,26]. This is an abstraction gap for Pharo developers wanting to tool the VM level.…”