Re: long long / long long

[Jan Hubicka <jh@suse.cz>]

> Hi,
> 
> On Mon, 10 Sep 2001, Bernd Schmidt wrote:
> > > Well, the Linux kernel developers found that they couldn't let gcc
> > > do long long arithmetic because it did such a poor job, so they do
> > > it in assembly or in C on pairs of 32 bit values instead.  So at
> > > least some folks probably wouldn't mind seeing an improvement.
> >
> > The main problem probably is the register allocator's requirement that
> > DImode values be allocated to adjacent registers.
> 
> Yes, that requirement creates many constraints.
> 
> > Unfortunately this is not going to be easy to change.
> 
> The sad thing is, that it _is_ easy to change in the allocator, and in
> fact would make the algorithm simpler (I'm talking only about the
> new-regalloc) and the graph easier colorable.  The thing which horrifies
> me is the encoding of that requirement in the different machine
> descriptions.  A first step would be to define a new rtx code MREG
> ("multi" reg), which can possibly contain a set of (disjoint) REG
We sometimes represent such MREG as PARALLELs of multiple REGS, we also do have
CONCAT expression.  I think these should be used to represent register pairs
too.

All you need is to teach register_operand and simplify_subreg about it.  Then
to fix all those zillions of side effects the change will have (not so extremly
many I can think of - the flow analyzis and register renaming probably,
scheduler should be happy).  But having this as option controlable by target
macro (note that on i386, the 64bit multiply instruction requires the registers
in order), it can be trackable.  We will just convert those machine descriptions
where it makes sense.  There are still many, where the pairs must be consetuctive,
as hardware dictates that.

Honza
> or SUBREG expressions, including the then necessary handling of multi-reg
> moves (with cycle breaking).  The occurences of those MREG rtx's could
> probably be limited to few passes around the allocator.  Unfortunately
> nevertheless all .md files would need a good overhaul.  If we only had
> such a multi-reg rtx from the beginning ;-|
> 
> 
> Ciao,
> Michael.