[PATCH, combine] Try REG_EQUAL for nonzero_bits

[Eric Botcazou]

> 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?

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?

-- 
Eric Botcazou