[COFF] [TUHS] Re: machine code translation, as mental architecture models

[Dan Cross]

[TUHS to Bcc:, +COFF]

On Wed, Jul 10, 2024 at 5:26 PM John Levine <johnl at taugh.com> wrote:
> It appears that Noel Chiappa <jnc at mercury.lcs.mit.edu> said:
> >    > From: Dan Cross
> >
> >    > These techniques are rather old, and I think go back much further than
> >    > we're suggesting. Knuth mentions nested translations in TAOCP ..
> >    > suggesting the technique was well-known as early as the mid-1960s.
>
> Knuth was talking about simulating one machine on another, interpreting
> one instruction at a time.  As he notes, the performance is generally awful,
> although IBM did microcode emulation of many of their second generation
> machines on S/360 which all (for business reasons) ran faster than the
> real machines.  Unsurprisingly, you couldn't emulate a 7094 on anything
> smaller than a 360/65.

It's not clear to me why you suggest with such evident authority that
Knuth was referring only to serialized instruction emulation and not
something like JIT'ed code; true, he doesn't specify one way or the
other, but I find it specious to conclude that that implies the
technique wasn't already in use, or at least known.  But certainly by
then JIT'ing techniques for "interpreted" programming languages were
known; it doesn't seem like a great leap to extend that to binary
translation.  Of course, that's speculation on my part, and I could
certainly be wrong.

> We've been discussing batch or JIT translation of code which gives
> much better performance without a lot of hardware help.

JIT'd performance of binary transliteration is certainly going to be
_better_ than strict emulation, but it is unlikely to be _as good_ as
native code. Indeed, this is still an active area of research; e.g.,
luajit; https://www.mattkeeter.com/blog/2022-10-04-ssra/ (disclaimer:
Matt's a colleague of mine), etc.

        - Dan C.