Re: [PATCH, combine] Try REG_EQUAL for nonzero_bits

[Jeff Law <law at redhat dot com>]

On 02/09/2015 07:00 PM, Thomas Preud'homme wrote:
> And this is part 2.
>
> > From: gcc-patches-owner@gcc.gnu.org [mailto:gcc-patches-
> > owner@gcc.gnu.org] On Behalf Of Eric Botcazou
> >
> > Once this is done, the same thing needs to be applied to XEXP
> > (reg_equal, 0)
> > before it is sent to nonzero_bits.
> >
> >
> > > -	  /* Don't call nonzero_bits if it cannot change anything.  */
> > > -	  if (rsp->nonzero_bits != ~(unsigned HOST_WIDE_INT) 0)
> > > -	    rsp->nonzero_bits |= nonzero_bits (src, nonzero_bits_mode);
> > >   	  num = num_sign_bit_copies (SET_SRC (set), GET_MODE (x));
> > >   	  if (rsp->sign_bit_copies == 0
> > >
> > >   	      || rsp->sign_bit_copies > num)
> > >
> > >   	    rsp->sign_bit_copies = num;
> > > +
> > > +	  /* Don't call nonzero_bits if it cannot change anything.  */
> > > +	  if (rsp->nonzero_bits != ~(unsigned HOST_WIDE_INT) 0)
> > > +	    update_rsp_from_reg_equal (rsp, insn, src, x);
> >
> > Can't we improve on this?  rsp->sign_bit_copies is modified both here
> > and in
> > update_rsp_from_reg_equal, but rsp->nonzero_bits is modified only in
> > the
> > latter function.  There is no reason for this discrepancy, so they ought to
> > be
> > handled the same way, either entirely here or entirely in the function.
>
> So I moved all the handling inside the new function and also added a check
> before calling num_sign_bit_copies about whether it would give any more
> information to be consistent with what is done for nonzero_bits.
>
> ChangeLog entry is as follows:
>
> 2015-02-09  Thomas Preud'homme  <thomas.preudhomme@arm.com>
>
>          * combine.c i(set_nonzero_bits_and_sign_copies): Split code updating
>          rsp->sign_bit_copies and rsp->nonzero_bits into ...
>          (update_rsp_from_reg_equal): This.  Also use REG_EQUAL note on src if
>          present to get more accurate information about the number of sign bit
>          copies and non zero bits.
Do you have a testcase where this change can result in better generated 
code.  If so please add that testcase.  It's OK if it's ARM specific.

OK with a testcase.

jeff