Re: [PATCH] range-op-float: Extend lhs by 0.5ulp rather than 1ulp if not -frounding-math [PR109008]

[Richard Biener <rguenther@suse.de>]

On Fri, 10 Mar 2023, Jakub Jelinek wrote:

> Hi!
> 
> This patch, incremental to the just posted one, improves the reverse
> operation ranges significantly by widening just by 0.5ulp in each
> direction rather than 1ulp.  Again, REAL_VALUE_TYPE has both wider
> exponent range and wider mantissa precision (160 bits) than any
> supported type, this patch uses the latter property.
> 
> The patch doesn't do it if -frounding-math, because then the rounding
> can be +-1ulp in each direction depending on the rounding mode which
> we don't know, or for IBM double double because that type is just weird
> and we can't trust in sane properties.
> 
> I've performed testing of these 2 patches on 300000 random tests as with
> yesterday's patch, exact numbers are in the PR, but I see very significant
> improvement in the precision of the ranges while keeping it conservatively
> correct.
> 
> Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?

The flow is a little bit obfuscated, but OK.

Thanks,
Richard.

> 2023-03-10  Jakub Jelinek  <jakub@redhat.com>
> 
> 	PR tree-optimization/109008
> 	* range-op-float.cc (float_widen_lhs_range): If not
> 	-frounding-math and not IBM double double format, extend lhs
> 	range just by 0.5ulp rather than 1ulp in each direction.
> 
> --- gcc/range-op-float.cc.jj	2023-03-09 12:13:57.189790814 +0100
> +++ gcc/range-op-float.cc	2023-03-09 13:12:05.248873234 +0100
> @@ -2205,8 +2205,8 @@ zero_to_inf_range (REAL_VALUE_TYPE &lb,
>     [1., 1.] = op1 + [1., 1.].  op1's range is not [0., 0.], but
>     [-0x1.0p-54, 0x1.0p-53] (when not -frounding-math), any value for
>     which adding 1. to it results in 1. after rounding to nearest.
> -   So, for op1_range/op2_range extend the lhs range by 1ulp in each
> -   direction.  See PR109008 for more details.  */
> +   So, for op1_range/op2_range extend the lhs range by 1ulp (or 0.5ulp)
> +   in each direction.  See PR109008 for more details.  */
>  
>  static frange
>  float_widen_lhs_range (tree type, const frange &lhs)
> @@ -2230,6 +2230,14 @@ float_widen_lhs_range (tree type, const
>  	  lb = dconstm1;
>  	  SET_REAL_EXP (&lb, FLOAT_MODE_FORMAT (TYPE_MODE (type))->emax + 1);
>  	}
> +      if (!flag_rounding_math && !MODE_COMPOSITE_P (TYPE_MODE (type)))
> +	{
> +	  /* If not -frounding-math nor IBM double double, actually widen
> +	     just by 0.5ulp rather than 1ulp.  */
> +	  REAL_VALUE_TYPE tem;
> +	  real_arithmetic (&tem, PLUS_EXPR, &lhs.lower_bound (), &lb);
> +	  real_arithmetic (&lb, RDIV_EXPR, &tem, &dconst2);
> +	}
>      }
>    if (real_isfinite (&ub))
>      {
> @@ -2240,6 +2248,14 @@ float_widen_lhs_range (tree type, const
>  	  ub = dconst1;
>  	  SET_REAL_EXP (&ub, FLOAT_MODE_FORMAT (TYPE_MODE (type))->emax + 1);
>  	}
> +      if (!flag_rounding_math && !MODE_COMPOSITE_P (TYPE_MODE (type)))
> +	{
> +	  /* If not -frounding-math nor IBM double double, actually widen
> +	     just by 0.5ulp rather than 1ulp.  */
> +	  REAL_VALUE_TYPE tem;
> +	  real_arithmetic (&tem, PLUS_EXPR, &lhs.upper_bound (), &ub);
> +	  real_arithmetic (&ub, RDIV_EXPR, &tem, &dconst2);
> +	}
>      }
>    /* Temporarily disable -ffinite-math-only, so that frange::set doesn't
>       reduce the range back to real_min_representable (type) as lower bound
> 
> 	Jakub
> 
> 

-- 
Richard Biener <rguenther@suse.de>
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