Re: [Tinycc-devel] Syncing caches

[Rob Landley]

On Monday 11 November 2002 20:14, Jim Peters wrote:
> Thanks for the work on TinyCC -- it has moved on a long way since I
> last looked at it, now being able to output object files too.
>
> Inspired by this style of code generation, I'm thinking of using
> something similar to generate fast custom digital filters routines at
> run-time for a filter library I'm writing.  I know I could use libtcc,
> but inspired by the code-generation of tcc, I think that my filter
> code is simple enough to be written straight out without going via C
> or libtcc.
>
> The only concern I have is about processor caches.  I remember a long
> long time ago (Z80/6502/68000) when it was okay to write self-
> modifying code, but then caches came along and it was no longer okay,
> because the instruction cache could get out of sync.  Does this still
> apply?

On x86 at least, yes it does.  It's an erratum, meaning it'll bite you very 
intermittently, differently on different processors, and you'll have a heck 
of a time tracking it down (if you ever do).

> I looked through the code, and I couldn't find any OS call to
> synchronize caches or anything like that.  Is it okay to just write
> code to memory and then call it?

Lots of people use interpreted bytecode.  Python, Java, dot-nyet...  It turns 
out to be faster than you'd think because it's 8 bit code and a surprising 
amount of it fits in cache (along with your entire interpreter most times), 
which is your real bottleneck.  (Stop and think: people are writing action 
games in perl.  Modern computers are FAST.)

A dirt simple way to fake up some bytecode is an array of function pointers 
with a loop calling elements out of the array.  Something vaguely like (ooh, 
it's been years, I'm going to get the array of function pointer syntax 
wrong):

*(int blah[])(args) = {func1, func2, func3...};

for (;;) blah[data[position++]](args);

Or have position be a pointer to save one level of indirection...

Rob

(That might be *(int blah)(args)[], try it and see...)

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