[PATCH] combine: Convert subreg-of-lshiftrt to zero_extract properly (PR78390)

[Segher Boessenkool]

On Wed, Nov 23, 2016 at 04:24:37PM +0100, Michael Matz wrote:
> > r242414, for PR77881, introduces some bugs (PR78390, PR78438, PR78477).
> > It all has the same root cause: that patch makes combine convert every
> > lowpart subreg of a logical shift right to a zero_extract.  This cannot
> > work at all if it is not a constant shift,
> 
> Even with non-constant shifts it remains an extract and make_extraction 
> does support variable start positions (which is the shift amount), as does 
> the zero_extract pattern (depending on target of course).

Sure, but the extraction length is non-constant as well then!  And not
just not-constant, with a conditional inside it, even.

> >  	if (GET_CODE (inner) == LSHIFTRT
> > +	    && CONST_INT_P (XEXP (inner, 1))
> >  	    && GET_MODE_SIZE (mode) < GET_MODE_SIZE (GET_MODE (inner))
> >  	    && subreg_lowpart_p (x))
> >  	  {
> >  	    new_rtx = make_compound_operation (XEXP (inner, 0), next_code);
> > +	    int width = GET_MODE_PRECISION (GET_MODE (inner))
> > +			- INTVAL (XEXP (inner, 1));
> 
> GET_MODE (new_rtx), because that's the object you're giving to 
> make_extraction, not inner (and not XEXP(inner, 0)).

This is about the *original* rtx, the lshiftrt: how many non-zero bits
does that leave.  The bits zeroed out by the lshiftrt have to be zeroed
out by the zero_extract as well, to keep the same semantics in the
resulting rtl.

> > +	    if (width > mode_width)
> > +	      width = mode_width;
> >  	    new_rtx = make_extraction (mode, new_rtx, 0, XEXP (inner, 1),
> > -				       mode_width, 1, 0, in_code == COMPARE);
> > +				       width, 1, 0, in_code == COMPARE);
> >  	    break;
> >  	  }


Segher