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From: Aldy Hernandez <aldyh@redhat.com>
To: Jakub Jelinek <jakub@redhat.com>, Andrew MacLeod <amacleod@redhat.com>
Cc: gcc-patches@gcc.gnu.org
Subject: Re: [PATCH] range-op-float: Fix up -frounding-math frange_arithmetic +- handling [PR110755]
Date: Tue, 25 Jul 2023 21:15:52 +0200	[thread overview]
Message-ID: <e160d3a6-8808-cad8-e04c-67268acc4c32@redhat.com> (raw)
In-Reply-To: <ZL6gc8/8THf1gN18@tucnak>

The frange bits look fine to me, so if you feel confident in the math 
logic, go right ahead :).

Thanks.
Aldy

On 7/24/23 18:01, Jakub Jelinek wrote:
> Hi!
> 
> IEEE754 says that x + (-x) and x - x result in +0 in all rounding modes
> but rounding towards negative infinity, in which case the result is -0
> for all finite x.  x + x and x - (-x) if it is zero retain sign of x.
> Now, range_arithmetic implements the normal rounds to even rounding,
> and as the addition or subtraction in those cases is exact, we don't do any
> further rounding etc. and e.g. on the testcase below distilled from glibc
> compute a range [+0, +INF], which is fine for -fno-rounding-math or
> if we'd have a guarantee that those statements aren't executed with rounding
> towards negative infinity.
> 
> I believe it is only +- which has this problematic behavior and I think
> it is best to deal with it in frange_arithmetic; if we know -frounding-math
> is on, it is x + (-x) or x - x and we are asked to round to negative
> infinity (i.e. want low bound rather than high bound), change +0 result to
> -0.
> 
> Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk and
> after a while for 13.3?  I'm afraid rushing this so late into 13.2...
> 
> 2023-07-24  Jakub Jelinek  <jakub@redhat.com>
> 
> 	PR tree-optimization/110755
> 	* range-op-float.cc (frange_arithmetic): Change +0 result to -0
> 	for PLUS_EXPR or MINUS_EXPR if -frounding-math, inf is negative and
> 	it is exact op1 + (-op1) or op1 - op1.
> 
> 	* gcc.dg/pr110755.c: New test.
> 
> --- gcc/range-op-float.cc.jj	2023-07-23 19:32:20.832434105 +0200
> +++ gcc/range-op-float.cc	2023-07-24 09:41:26.231030258 +0200
> @@ -324,6 +324,24 @@ frange_arithmetic (enum tree_code code,
>     bool inexact = real_arithmetic (&value, code, &op1, &op2);
>     real_convert (&result, mode, &value);
>   
> +  /* When rounding towards negative infinity, x + (-x) and
> +     x - x is -0 rather than +0 real_arithmetic computes.
> +     So, when we are looking for lower bound (inf is negative),
> +     use -0 rather than +0.  */
> +  if (flag_rounding_math
> +      && (code == PLUS_EXPR || code == MINUS_EXPR)
> +      && !inexact
> +      && real_iszero (&result)
> +      && !real_isneg (&result)
> +      && real_isneg (&inf))
> +    {
> +      REAL_VALUE_TYPE op2a = op2;
> +      if (code == PLUS_EXPR)
> +	op2a.sign ^= 1;
> +      if (real_isneg (&op1) == real_isneg (&op2a) && real_equal (&op1, &op2a))
> +	result.sign = 1;
> +    }
> +
>     // Be extra careful if there may be discrepancies between the
>     // compile and runtime results.
>     bool round = false;
> --- gcc/testsuite/gcc.dg/pr110755.c.jj	2023-07-21 10:34:05.037251433 +0200
> +++ gcc/testsuite/gcc.dg/pr110755.c	2023-07-21 10:35:10.986326816 +0200
> @@ -0,0 +1,29 @@
> +/* PR tree-optimization/110755 */
> +/* { dg-do run } */
> +/* { dg-require-effective-target fenv } */
> +/* { dg-require-effective-target hard_float } */
> +/* { dg-options "-O2 -frounding-math" } */
> +
> +#include <fenv.h>
> +
> +__attribute__((noipa)) float
> +foo (float x)
> +{
> +  if (x > 0.0)
> +    {
> +      x += 0x1p+23;
> +      x -= 0x1p+23;
> +      x = __builtin_fabsf (x);
> +    }
> +  return x;
> +}
> +
> +int
> +main ()
> +{
> +#ifdef FE_DOWNWARD
> +  fesetround (FE_DOWNWARD);
> +  if (__builtin_signbit (foo (0.5)))
> +    __builtin_abort ();
> +#endif
> +}
> 
> 	Jakub
> 


      parent reply	other threads:[~2023-07-25 19:15 UTC|newest]

Thread overview: 4+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2023-07-24 16:01 Jakub Jelinek
2023-07-24 19:39 ` Joseph Myers
2023-07-24 19:45   ` Jakub Jelinek
2023-07-25 19:15 ` Aldy Hernandez [this message]

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