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Virtual code execution via IL interpretation

As Soma announced, we just shipped VS2010 Beta1. This includes dump debugging support for managed code and a very cool bonus feature tucked in there that I’ll blog about today.

Dump-debugging (aka post-mortem debugging) is very useful and a long-requested feature for managed code.  The downside is that with a dump-file, you don’t have a live process anymore, and so property-evaluation won’t work. That’s because property evaluation is implemented by hijacking a thread in the debuggee to run the function of interest, commonly a ToString() or property-getter. There’s no live thread to hijack in post-mortem debugging.

We have a mitigation for that in VS2010. In addition to loading the dump file, we can also interpret the IL opcodes of the function and simulate execution to show the results in the debugger.


Here, I’ll just blog about the end-user experience and some top-level points. I’ll save the technical drill down for future blogs.

Consider the following sample:

 using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Reflection;

public class Point 
    int m_x;
    int m_y;
    public Point(int x, int y)
        m_x = x;
        m_y = y;
    public override string ToString()
        return String.Format("({0},{1})", this.X, this.Y);

    public int X
            return m_x;
    public int Y
            return m_y;

public class Program
    static void Main(string[] args)
        Dictionary<int, string> dict = new Dictionary<int, string>();
        dict[5] = "Five";
        dict[3] = "three";

        Point p = new Point(3, 4);

    public static int  Dot(Point p1, Point p2)
        int r2 = p1.X * p2.X + p1.Y * p2.Y;
        return r2;



Suppose you have a dump-file from a thread stopped at the end of Main() (See newly added menu item “Debug | Save Dump As …”; load dump-file via “File | Open | File …”).

Normally, you could see the locals (dict, p) and their raw fields, but you wouldn’t be able to see the properties or ToString() values. So it would look something like this:


But with the interpreter, you can actually simulate execution. With the IL interpreter, here’s what it looks like in the watch window:


Which is exactly what you’d expect with live-debugging.  (In one sense, “everything still works like it worked before” is not a gratifying demo…)

The ‘*’ after the values are indications that they came from the interpreter.  Note you still need to ensure that property-evaluation is enabled in “Tools | options | Debugging”:






How does it work?
The Interpreter gets the raw IL opcodes via ICorDebug and then simulates execution of those opcodes. For example, when you inspect “p.X” in the watch window, the debugger can get the raw IL opcodes:

.method public hidebysig specialname instance int32
        get_X() cil managed
  // Code size       12 (0xc)
  .maxstack  1
  .locals init ([0] int32 CS$1$0000)
  IL_0000:  nop
  IL_0001:  ldarg.0
  IL_0002:  ldfld int32 Point::m_x
  IL_0007:  stloc.0
  IL_0008:  br.s       IL_000a
  IL_000a:  ldloc.0
  IL_000b:  ret
} // end of method Point::get_X

And then translate that ldfld opcode into a ICorDebug field fetch the same way it would fetch “p.m_x”. The problem gets a lot harder then that (eg, how does it interpret a newobj instruction?) but that’s the basic idea.


Other things it can interpret:

The immediate window is also wired up to use the interpreter when dump-debugging. Here are some sample things that work. Again, note the ‘*’ means the results are in the interpreter and the debuggee is not modified.

Simulating new objects:
? new Point(10,12).ToString()

Basic reflection:
? typeof(Point).FullName

Dynamic method invocation:
? typeof(Point).GetMethod("get_X").Invoke(new Point(6,7), null)

Calling functions, and even mixing debuggee data (the local variable ‘p’) with interpreter generated data (via the ‘new’ expression):
? Dot(p,new Point(10,20))


It even works for Visualizers

Notice that it can even load the Visualizer for the Dictionary (dict) and show you the contents as a pretty array view rather than just the raw view of buckets. Visualizers are their own dll,  and we can verify that the dll is not actually loaded into the debuggee. For example, the Dictionary visualizer dll is  Microsoft.VisualStudio.DebuggerVisualizers.dll, but that’s not in the module list:



That’s because the interpreter has virtualized loading the visualizer dll into its own “virtual interpreter” space and not the actual debuggee process space. That’s important because in a dump file, you can’t load a visualizer dll post-mortem.



Other issues:

There are lots of other details here that I’m skipping over, like:

  1. The interpreter is definitely not bullet proof. If it sees something it can’t interpreter (like a pinvoke or dangerous code), then it simply aborts the interpretation attempt.
  2. The intepreter is recursive, so it can handle functions that call other functions. (Notice that ToString call get_X.)
  3. How does it deal with side-effecting operations?
  4. How does it handle virtual dispatch call opcodes?
  5. How does it handle ecalls?
  6. How does it handle reflection


Other advantages?

There are other advantages of IL interpretation for function evaluation, mainly that it addresses the ”func-eval is evil” problems by essentially degenerating dangerous func-evals to safe field accesses.

  1. It is provably safe because it errs on the side of safety. The interpreter is completely non-invasive (it operates on a dump-file!).
  2. No accidentally executing dangerous code.
  3. Side-effect free func-evals. This is a natural consequence of it being non-invasive.
  4. Bullet proof func-eval abort.
  5. Bullet proof protection against recursive properties that stack-overflow.
  6. It allows func-eval to occur at places previously impossible, such as in dump-files, when the thread is in native code, retail code, or places where there is no thread to hijack.

Closing thoughts

We realize that the interpreter is definitely not perfect. That’s part of why we choose to have it active in dump-files but not replace func-eval in regular execution. For dump-file scenarios, it took something that would have been completely broken and made many things work.