Re: [PATCH] [PR tree-optimization/18639] Compare nonzero bits in irange with widest_int.

[Jakub Jelinek <jakub@redhat.com>]

On Fri, Feb 03, 2023 at 09:50:43AM +0100, Aldy Hernandez wrote:
> [PR tree-optimization/18639] Compare nonzero bits in irange with widest_int.

0 missing in the bug number in the subject line, though the current
recommended formatting of the subject is I think:
value-range: Compare nonzero bits in irange with widest_int [PR180639]
                                                                                                                                                                                      
        PR 108639/tree-optimization

Reversed component and number

> --- a/gcc/value-range.cc
> +++ b/gcc/value-range.cc
> @@ -1259,7 +1259,10 @@ irange::legacy_equal_p (const irange &other) const
>  			       other.tree_lower_bound (0))
>  	  && vrp_operand_equal_p (tree_upper_bound (0),
>  				  other.tree_upper_bound (0))
> -	  && get_nonzero_bits () == other.get_nonzero_bits ());
> +	  && (widest_int::from (get_nonzero_bits (),
> +				TYPE_SIGN (type ()))
> +	      == widest_int::from (other.get_nonzero_bits (),
> +				   TYPE_SIGN (other.type ()))));
>  }
>  
>  bool
> @@ -1294,7 +1297,11 @@ irange::operator== (const irange &other) const
>  	  || !operand_equal_p (ub, ub_other, 0))
>  	return false;
>      }
> -  return get_nonzero_bits () == other.get_nonzero_bits ();
> +  widest_int nz1 = widest_int::from (get_nonzero_bits (),
> +				     TYPE_SIGN (type ()));
> +  widest_int nz2 = widest_int::from (other.get_nonzero_bits (),
> +				     TYPE_SIGN (other.type ()));
> +  return nz1 == nz2;
>  }

While the above avoids the ICE (and would be certainly correct for
the bounds, depending on the sign of their type sign or zero extended
to widest int), but is the above what we want for non-zero bits
to be considered equal?  The wide_ints (which ought to have precision
of the corresponding type) don't represent normal numbers but bitmasks,
0 - this bit is known to be zero, 1 - nothing is known about this bit).
So, if there are different precisions and the narrower value has 0
in the MSB of the bitmask (so MSB is known to be zero), the above requires
for equality that in the other range all upper bits are known to be zero
too for both signed and unsigned.  That is ok.  Similarly for MSB set
if TYPE_SIGN of the narrower is unsigned, the MSB value is unknown, but we
require on the wider to have all the upper bits cleared.  But for signed
narrower type with MSB set, i.e. it is unknown if it is positive or
negative, the above requires that all the above bits are unknown too.
And that is the case I'm not sure about, whether in that case the
upper bits of the wider wide_int should be checked at all.
Though, perhaps from the POV of nonzero bits derived from the sign-extended
values in the ranges sign bit copies (so all above bits 1) is what one would
get, so maybe it is ok.  Just food for thought.

As for retesting, if you have done full bootstrap/regtest with the patch
without the testcases in it, it should be more than enough to test just
make check-gcc \
RUNTESTFLAGS='--target_board=unix\{-m32,-m64\} compile.exp=pr10863*.c'
You don't really need to rerun all tests just for it.

	Jakub