Re: discouraging register use

[Ian Lance Taylor <ian at airs dot com>]

Richard Henderson <rth@redhat.com> writes:

> On Thu, Dec 30, 2004 at 06:54:44PM -0500, DJ Delorie wrote:
> > Do I have to do something like this?  If so, I'm going to have some
> > *really* long lines in my md file (for this chip).  Or is there some
> > other way of changing gcc's register preferences, without precluding
> > the registers completely?
> > 
> >   [(set (match_operand:SI 0 "" "c,c,?m,?m")
> >         (plus:SI (match_operand:SI 1 "" "0,0,0,0")
> > 		 (match_operand:SI 1 "" "c,?m,c,?m")))]
> 
> Yes, that looks plausible.  The other way isn't going to work because
> you penalize c at the same time as m by using "?cm".

My suspicion is that that is not enough, because find_reg() in
global.c will allocate any ordinary register which appears in
REG_ALLOC_ORDER and for which the value passes HARD_REGNO_MODE_OK.  So
the compiler will still allocate the expensive registers as soon as it
has used the cheap registers for something.

But it's not like I've tried it.

Ian