Re: [PATCH] ppc32 use simplified mmenonics

Albert Cahalan
2004-08-22T13:17:07Z

On Sun, 2004-08-22 at 05:43, Vincent Hanquez wrote:
> On Sat, Aug 21, 2004 at 08:45:26PM -0400, Albert Cahalan wrote:
> > I'd rather you went the other way, replacing these
> > barely-documented instructions with ones that are
> > easy to look up. Motorola has about a zillion of
> > these "simplified" instructions. I guess Motorola
> > and IBM were jealous of Intel's CISC instructions.
> >
> > The big problem is this:
> > THESE ARE NOT IN THE INDEX!!!!!!
> >
> > So, if I forget what one of these many instructions
> > does, I'll have quite the time paging through the
> > manual trying to find it.
> >
> > If it's not in the index, please avoid it.
>
> Well,
>
> let's analyse 'mr R1,R2'. Which is simplified instruction for moving register,
> which represent the following instruction 'or R2,0,R1'

The "or" is much easier, because it follows the
standard 3-register pattern. With "mr", which way
does it go? That's one more thing to remember.
In fact I don't know, but the "or" is obvious.

The 0 is your hint that the "or" isn't a plain "or".

> bne target = bc 4,2,target
>
> When I see first form, I know exactly what the program do, whereas on
> the second form : What the hell is 4,2 ?

This is a slightly better example, but still, it's
lots easier to look up "bc" to see the selection of
options that are available.

Plus, yeah, "what the hell is 4,2", but those numbers
replace a _lot_ of other things you'd need to remember.
There are 456 of these "simplified" branch instructions.
If you use those, you'll tend to restrict your code to
those few common ones you remember. There's bdnzltrl,
bdnzfla, bunla... That's madness.

That's 114 opcodes to 1.

> So I'ld rather go with simplified instruction, even if that index
> doesn't contain them (which I agree with you, is very bad).
> There are still in Appendix F of my pdf and you can search with the find
> utility include in your reader (xpdf does)

I have the physical book. Other than being unsearchable,
it's much nicer than the pdf. It's easy on the eyes,
doesn't occupy screen space, and I can read it on the can.