Re: CANNOT_CHANGE_MODE changes for df/ra*

[Michael Matz <matz at suse dot de>]

Hi,

On 3 Feb 2003, Geoff Keating wrote:

> > this changes df.c/ra*.c to make use of the new interface of
> > CANNOT_CHANGE_MODE_CLASS.  It additionally adds a new flag to df.h which
> > will be of use later, and a fix to read_modify_subreg_p (to make GCC
> > bootstrap with activated new allocator).
> >
> > Bootstraps without regressions on i686-linux (Ada untested).  I
> > additionally tested, if the new ra is still working somewhat, so it also
> > bootstraps without regressions when the new ra is activated by default,
> > when Java is deactivated.
> >
> > The changes to df.* need approval.  OK for mainline?
>
> This patch is OK, except that in this code:
>
> +  /* Paradoxical subreg writes don't leave a trace of the old content.  */
>    if (isize <= osize)
> -    return true;
> +    return false;
>    if (isize <= UNITS_PER_WORD)
>      return false;
> -  if (osize > UNITS_PER_WORD)
> +  if (osize >= UNITS_PER_WORD)
>      return false;
>    return true;
>  }
>
> the logic is wrong.  Consider when word_mode is SImode:
>
> (set (subreg:HI (reg:DI ...)) ...)
> (set (subreg:SI (reg:DI ...)) ...)
> (set (subreg:DI (reg:TI ...)) ...)

Ahh, one of my favourite topics, subregs ;-)  In the new register
allocator they are tracked precisely (at least wordwise), so there partial
writes never result in an additional use.  The reason is, that e.g.
(sub:HI (reg:SI ))  clobbers the whole word (SImode), ergo is a complete
def.  Similar for (sub:SI (reg:DI)).  This sets the complete half, but
doesn't need the other one (and it clobbers nothing).  I.e for the new RA
I've deactivated that function at all.  Hmm, I guess for other users of
df.c that don't do that, the definition of read_modify_subreg_p() is now
wrong indeed.  OK, I'll change the function to do, what you suggested, and
introduce a flag for df_flags, which can be set by the new RA to not call
that function.

> return (isize > osize && isize > UNITS_PER_WORD)
>
> rather than using double negatives.


Ciao,
Michael.