RE: [PATCH, combine] Try REG_EQUAL for nonzero_bits

["Zhenqiang Chen" <zhenqiang dot chen at arm dot com>]

> -----Original Message-----
> From: Eric Botcazou [mailto:ebotcazou@adacore.com]
> Sent: Saturday, November 22, 2014 6:15 PM
> To: Zhenqiang Chen
> Cc: gcc-patches@gcc.gnu.org
> Subject: Re: [PATCH, combine] Try REG_EQUAL for nonzero_bits
> 
> > The patch tries to use REG_EQUAL to get more precise info for
> > nonzero_bits, which helps to remove unnecessary zero_extend.
> >
> > Here is an example when compiling Coremark, we have rtx like,
> >
> > (insn 1244 386 388 47 (set (reg:SI 263 [ D.5767 ])
> >         (reg:SI 384 [ D.5767 ])) 786 {*thumb2_movsi_insn}
> >      (expr_list:REG_EQUAL (zero_extend:SI (mem:QI (reg/v/f:SI 271 [
> > memblock
> > ]) [0 *memblock_13(D)+0 S1 A8]))
> >         (nil)))
> >
> > from "reg:SI 384", we can only know it is a 32-bit value. But from
> > REG_EQUAL, we can know it is an 8-bit value. Then for the following
> > rtx seq,
> >
> > (insn 409 407 410 50 (set (reg:SI 308)
> >         (plus:SI (reg:SI 263 [ D.5767 ])
> >             (const_int -48 [0xffffffffffffffd0]))) core_state.c:170 4
> > {*arm_addsi3}
> >      (nil))
> > (insn 410 409 411 50 (set (reg:SI 309)
> >         (zero_extend:SI (subreg:QI (reg:SI 308) 0))) core_state.c:170
> > 812 {thumb2_zero_extendqisi2_v6}
> >      (expr_list:REG_DEAD (reg:SI 308)
> >         (nil)))
> >
> > the zero_extend for r309 can be optimized by combine pass.
> 
> This sounds like a good idea.
> 
> > 2014-11-21  Zhenqiang Chen  <zhenqiang.chen@arm.com>
> >
> > 	* combine.c (set_nonzero_bits_and_sign_copies): Try REG_EQUAL
> note.
> >
> > diff --git a/gcc/combine.c b/gcc/combine.c index 6a7d16b..68a883b
> > 100644
> > --- a/gcc/combine.c
> > +++ b/gcc/combine.c
> > @@ -1713,7 +1713,15 @@ set_nonzero_bits_and_sign_copies (rtx x,
> > const_rtx set, void *data)
> >
> >  	  /* 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);
> > +	    {
> > +	      rtx reg_equal = insn ? find_reg_note (insn, REG_EQUAL,
> > NULL_RTX)
> > +				     : NULL_RTX;
> > +	      if (reg_equal)
> > +		rsp->nonzero_bits |= nonzero_bits (XEXP (reg_equal, 0),
> > +						   nonzero_bits_mode);
> > +	      else
> > +		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)
> 
> Use find_reg_equal_equiv_note instead.  Are you sure that this won't yield
> inferior results in very peculiar cases?  IOW, why not use both sources?

Thanks for the comments. I will compare the two nonzero_bits from src and
REG_EQUAL. Then select the smaller one.
 
> Note that 'src' is massaged just above if SHORT_IMMEDIATES_SIGN_EXTEND
> is defined so we should probably do it for the note datum too, for example
> by factoring the code into a function and invoking it.
> 
> Why not do the same for num_sign_bit_copies?

Do you know why it use " SET_SRC (set)" other than "src" for
num_sign_bit_copies?

If it is "src", I should do the same for num_sign_bit_copies with REG_EQUAL
info.

Thanks!
-Zhenqiang