Re: [9fans] BUG!!! in Plan9 compiler!

[Bakul Shah <bakul+plan9@bitblocks.com>]

On Fri, 23 Apr 2010 18:34:56 EDT erik quanstrom <quanstro@labs.coraid.com>  wrote:
> > On Fri, Apr 23, 2010 at 1:08 PM, Bakul Shah <bakul+plan9@bitblocks.com> wro
> te:
> >
> > >
> > > If so, I consider it a bug; particularly as there is no
> > > overflow involved anywhere.
> > >
> > >
> >
> > r1 2147482110 r2 -1537
>
> on arm the difference is interesting.  the first
> / is translated:
> 	main+0x20 0x00001040	MOVW	(R4>>1),R4
> and the second:
> 	main+0x34 0x00001054	ADD.MI	$#0x1,R2,R2
> 	main+0x38 0x00001058	MOVW	(R2->1),R2
>
> (5c -S has it SRL and SRA, respectively.)
> if we are unsigned preserving, wouldn't this make sense, since
> (x + y + z) is an unsigned expression but (x + y) is not.

It is clear that there is no one *right* answer for how mixed
signed/unsigned number operations should be treated but c89
rules do seem a bit more sensible to me as they avoid
surprises like (x + y + 0u)/z being different from (x + y)/z,
where x,y,z are signed ints.

Is this behaviour really useful for anything? Is there
anything in plan9 code that relies on this behaviour in a
critical way? I suspect this rule can be changed without
impacting plan 9 code much (which as a rule is of much higher
quality than most open source code) and we already know
programs ported to p9p work fine.

Anyway, I think I've said all I want to on this topic!
Probably more than.