On Wed, Dec 14, 2016 at 01:32:48PM -0600, Segher Boessenkool wrote:
> On Wed, Dec 14, 2016 at 11:01:47AM +0100, Dominik Vogt wrote:
> > This is another micro-optimisation in change_zero_ext. If an
> >
> > (and (lshiftrt ... (N)) (M))
> >
> > generated by change_zero_ext is equivalent to just
> >
> > (lshiftrt ... (N))
> >
> > (because the AND constant selects the N rightmost bits of the
> > result), strip off the AND.
> >
> > _But_ I'm still not completely convinced whether this is a good
> > idea. It may become necessary to add md patterns to deal with
> > just the LSHIFTRT. On the other hand it saves the need for
> > another special case in change_zero_ext, and a less obvious, very
> > specific risbg pattern on s390
>
> For PowerPC we should already have all such patterns with a "bare" shift
> (they can be created in other ways, too).
>
> > Bootstrapped and regression tested on s390x and s390. (Targets
> > with risbg-like instructions (Power, others?) may need some
> > tuning.)
>
> But, it is also possible I missed some. So please wait until I have
> tested it.
>
>
> > diff --git a/gcc/combine.c b/gcc/combine.c
> > index 19851a2..5ebf31c 100644
> > --- a/gcc/combine.c
> > +++ b/gcc/combine.c
> > @@ -11280,8 +11280,13 @@ change_zero_ext (rtx pat)
> > else
> > continue;
> >
> > - wide_int mask = wi::mask (size, false, GET_MODE_PRECISION (mode));
> > - x = gen_rtx_AND (mode, x, immed_wide_int_const (mask, mode));
> > + if (!(GET_CODE (x) == LSHIFTRT
> > + && CONST_INT_P (XEXP (x, 1))
> > + && size + INTVAL (XEXP (x, 1)) == GET_MODE_PRECISION (mode)))
> > + {
> > + wide_int mask = wi::mask (size, false, GET_MODE_PRECISION (mode));
> > + x = gen_rtx_AND (mode, x, immed_wide_int_const (mask, mode));
> > + }
>
> One could argue that this should have been an lshiftrt in the first place
> then, not a zero_ext*. Hrm.
This one
void g2(ui64 *pl, i32 seed)
{
seed = 69607 * seed;
pl[0] = (seed >> 8) & 0xffffff;
}
generates
(zero_extract:DI (reg:SI 75 [ seed ])
(const_int 24 [0x18])
(const_int 0 [0]))
on s390x.
Ciao
Dominik ^_^ ^_^
--
Dominik Vogt
IBM Germany
