Hindsight: Reversible Computing

One of the more interesting tech pieces that came out this week has been Hindsight [PDF]. Hindsight is made by Virtutech and is billed as the "the first complete, general-purpose tool for reverse execution and debugging of arbitrary electronic systems." The demos were received extremely well and it just looks cool.

Comments

UI

[GigsVT]

They say the way they accomplish this is running the program in some sort of sandbox and taking checkpoints every so often and then when you step back, it actually runs forward from the closest checkpoint and stops one instruction short.

My question is how UI interactions are handled. If the execution between the checkpoint and current-1 instruction includes a UI interaction, it might be very confusing to the programmer to know what or how many UI interactions need to be carried out to accomplish the backstep.

But what about external events

[bangzilla]

It's all very well to be able to run code backwards/forwards/slo-mo/etc, but how to handle non deterministic external events coming in from the network? Does this tool presume that all applications to which it will be applied live in isolation?

Not by a decade.

[Murmer]

This technology has existed, in GPL form, for ten years. It's just had exactly zero uptake.

I read this usenet post every now and then when I'm trying to fix something, and it makes me want to cry every time I do.

OCaml anyone?

[fab13n]

OCaml as been offering timestamps and backward debugging for years, in addition of a great programming language (backward debugger's implementation is based on Unix's forking and copy-on-write, so running it on windows requires cygwin). Simply compile your stuff to byte-code rather than with the native optimizing compiler, run the debugger and use backstep/backward just as you used to do with step/forward. Breakpoints block execution in both directions.

And what about GUI and other side effects? Debugging a program in which such side-effects are deeply interleaved with algorithmics can be tricky indeed, although smart timestamping from the debugger will reduce glitches. But if you don't know better than randomly mixing algo and front-end in the first place, then you'd better fix the programmer than the program...

Any relation to ReVirt?

[eddy]

ReVirt:

The ability to replay the execution of a virtual machine is useful in many ways besides intrusion analysis. For example, it enables one to replay and debug any portion of a prior execution. We have built an extension to gdb that uses virtual-machine replay to provide the illusion of time travel. In particular, we provide the ability to do reverse debugging, though commands such as reverse watchpoint and reverse breakpoint. graph. See our paper in USENIX 2005 for details.

Elephants never forget

[same_old_story]

John McCarthy has been talking about giving programming langues the notion of time for quite some time (no pun intended).

In this paper, he proposes the Elephant language that can refer to the past in computer programs.

Pretty cool stuff!