On Tue, 26 Aug 2025, Zhou Zhao wrote:

> 
> 在 2025/8/26 下午3:37, Richard Biener 写道:
> > On Thu, 21 Aug 2025, Zhou Zhao wrote:
> >
> >> This patch is a respond of the patch posted at
> >> https://gcc.gnu.org/pipermail/gcc-patches/2025-January/673051.html
> >> as some suggestion by Richard Biener, I have adopted these suggestions
> >> and regenerated the patch.
> >>
> >> In the 538.imagick_r benchmark of Spec2017, I find these pattern from
> >> MagickRound function. This patch implements these pattern in match.pd
> >> for 4 rules under -funsafe-math-optimizations:
> >> 1) (x-floor(x)) < (ceil(x)-x) ? floor(x) : ceil(x) -> floor(x+0.5)
> >> 2) (x-floor(x)) >= (ceil(x)-x) ? ceil(x) : floor(x) -> floor(x+0.5)
> >> 3) (ceil(x)-x) > (x-floor(x)) ? floor(x) : ceil(x) -> floor(x+0.5)
> >> 4) (ceil(x)-x) <= (x-floor(x)) ? ceil(x) : floor(x) -> floor(x+0.5)
> >>
> >> The patch implements floor(x+0.5) operation to replace these pattern
> >> that semantics of round(x) function.
> > Can you say why you particularly chose floor (x + 0.5) as result
> > while describing it to have the semantics of round (x)?  The reasonable
> > other choice is round(x) itself?
> >
> > What exact differences prompt you do gate this with
> > -funsafe-math-optimizations?  I can see signed zeros have
> > different behavior, for x == -0.0 all forms in original form
> > return -0.0 while the simplification will return 0.0.  The behavior
> > for Infs and NaNs looks unchanged.  0.5 and -0.5 seem to compute
> > to the same value when using floor(x+0.5) as simplification (unless
> > I made a mistake).  floor or ceil do not raise IEEE exceptions,
> > so wouldn't -fno-signed-zeros be enough as a gate?
> >
> > Thanks,
> > Richard.
> >
> Thank you for your reply. The time interval since the last patch
> submission might be too long, so I will re-describe our discussion on
> the above issues:
> 
> 1. I consider the round functions are round(x) and rint(x). In round
> halfway cases, round(x) away from zero, rint(x) rounds x to the nearest
> even integer. When the pattern input is x=-2.5, return -2.0, but
> round(-2.5) return -3.0. When the pattern input is x=2.5, it return
> 3.0, but rint(2.5) return 2.0. Therefore, using floor(x + 0.5) is the
> best matches expression I think. Do you have any other functions with
> semantics of round that could be used to represent this pattern?

No, I think that covers it.  rint() also is affected by the rounding
mode so I think cannot be used here.

> 2. As you mentioned, I need to add the -funsafe-math-optimizations
> option to protect the (+0.5) operation. With the -Ofast option, which
> enables -fno-signed-zeros, I observed that when x = -0.4, the pattern
> returns -0.0 on aarch64-linux-gnu but returns 0.0 on x86_64-linux-gnu.
> floor(x + 0.5) will return 0.0 on all the above targets. Additionally,
> the pattern behaves the same for all double values, including INFs
> and NaNs.
> 
> 3. Indeed, I cannot guarantee that (+0.5) will always yield the
> expected value, so I use -funsafe-math-optimizations  for protection.
> I think this is better than checking HONOR_NANS/INFS/SIGNED_ZEROS,
> because when the true result of x + 0.5 cannot be exactly represented
> in the target floating-point format, and happens to be halfway between
> two adjacent floating-point numbers, different rounding rules will
> make different choices, leading to different final results.

I think 0.5 can be always exactly represented, but there might be
a special 'x' where + 0.5 triggers a one-ulp difference depending
on rounding mode.

That said, HONOR_SIGN_DEPENDENT_ROUNDING might be also an issue
because of that.

But other than that the transform should be value-preserving?
So I'd gate with !HONOR_SIGNED_ZEROS && !HONOR_SIGN_DEPENDENT_ROUNDING
instead?

Richard.

> Thanks,
> Zhou Zhao.
> >> The patch was regtested on aarch64-linux-gnu and x86_64-linux-gnu,
> >> SPEC 2017 and SPEC 2006 were run:
> >> As for SPEC 2017, 538.imagick_r benchmark performance increased by 3%+
> >> in base test of ratio mode.
> >> As for SPEC 2006, while the transform does not seem to be triggered,
> >> we also see no non-noise impact on performance.
> >> OK for mainline?
> >>
> >> gcc/ChangeLog:
> >>
> >>  * match.pd: Add new pattern for round.
> >>
> >> gcc/testsuite/ChangeLog:
> >>
> >>    * gcc.dg/fold-round-1.c: New test.
> >> ---
> >>   gcc/match.pd                        | 17 +++++++++
> >>   gcc/testsuite/gcc.dg/fold-round-1.c | 56 +++++++++++++++++++++++++++++
> >>   2 files changed, 73 insertions(+)
> >>   create mode 100644 gcc/testsuite/gcc.dg/fold-round-1.c
> >>
> >> diff --git a/gcc/match.pd b/gcc/match.pd
> >> index 66e8a787449..94036603e08 100644
> >> --- a/gcc/match.pd
> >> +++ b/gcc/match.pd
> >> @@ -794,6 +794,23 @@ DEFINE_INT_AND_FLOAT_ROUND_FN (RINT)
> >>    (rdiv @0 (negate @1))
> >>    (rdiv (negate @0) @1))
> >>   +(if (flag_unsafe_math_optimizations)
> >> +/* convert semantics of round(x) function to floor(x+0.5).  */
> >> +/* (x-floor(x)) < (ceil(x)-x) ? floor(x) : ceil(x) --> floor(x+0.5).  */
> >> +/* (x-floor(x)) >= (ceil(x)-x) ? ceil(x) : floor(x) --> floor(x+0.5).  */
> >> +/* (ceil(x)-x) > (x-floor(x)) ? floor(x) : ceil(x) --> floor(x+0.5).  */
> >> +/* (ceil(x)-x) <= (x-floor(x)) ? ceil(x) : floor(x) --> floor(x+0.5).  */
> >> +(for op (lt ge)
> >> +     bt (FLOOR CEIL)
> >> +     bf (CEIL FLOOR)
> >> +     floor (FLOOR FLOOR)
> >> +     ceil (CEIL CEIL)
> >> + (simplify
> >> +  (cond (op:c (minus:s SSA_NAME@0 (floor SSA_NAME@0))
> >> +        (minus:s (ceil SSA_NAME@0) SSA_NAME@0))
> >> +  (bt SSA_NAME@0) (bf SSA_NAME@0))
> >> +  (floor (plus @0 { build_real (type, dconsthalf); })))))
> >> +
> >>   (if (flag_unsafe_math_optimizations)
> >>    /* Simplify (C / x op 0.0) to x op 0.0 for C != 0, C != Inf/Nan.
> >>       Since C / x may underflow to zero, do this only for unsafe math.  */
> >> diff --git a/gcc/testsuite/gcc.dg/fold-round-1.c
> >> b/gcc/testsuite/gcc.dg/fold-round-1.c
> >> new file mode 100644
> >> index 00000000000..845d6d2e475
> >> --- /dev/null
> >> +++ b/gcc/testsuite/gcc.dg/fold-round-1.c
> >> @@ -0,0 +1,56 @@
> >> +/* { dg-do compile } */
> >> +/* { dg-options "-Ofast -funsafe-math-optimizations" } */
> >> +
> >> +extern void link_error (void);
> >> +
> >> +#define TEST_ROUND(TYPE, FFLOOR, FCEIL)
> >> \
> >> +  void round_##FFLOOR##_1 (TYPE x)
> >> \
> >> +  {
> >> \
> >> +    TYPE t1 = 0;
> >> \
> >> +    TYPE t2 = __builtin_##FFLOOR (x + 0.5);
> >> \
> >> +    if ((x - __builtin_##FFLOOR (x)) < (__builtin_##FCEIL (x) - x))
> >> \
> >> +      t1 = __builtin_##FFLOOR (x);
> >> \
> >> +    else
> >> \
> >> +      t1 = __builtin_##FCEIL (x);
> >> \
> >> +    if (t1 != t2)
> >> \
> >> +      link_error ();
> >> \
> >> +  }
> >> \
> >> +  void round_##FFLOOR##_2 (TYPE x)
> >> \
> >> +  {
> >> \
> >> +    TYPE t1 = 0;
> >> \
> >> +    TYPE t2 = __builtin_##FFLOOR (x + 0.5);
> >> \
> >> +    if ((__builtin_##FCEIL (x) - x) > (x - __builtin_##FFLOOR (x)))
> >> \
> >> +      t1 = __builtin_##FFLOOR (x);
> >> \
> >> +    else
> >> \
> >> +      t1 = __builtin_##FCEIL (x);
> >> \
> >> +    if (t1 != t2)
> >> \
> >> +      link_error ();
> >> \
> >> +  }
> >> \
> >> +  void round_##FFLOOR##_3 (TYPE x)
> >> \
> >> +  {
> >> \
> >> +    TYPE t1 = 0;
> >> \
> >> +    TYPE t2 = __builtin_##FFLOOR (x + 0.5);
> >> \
> >> +    if ((__builtin_##FCEIL (x) - x) <= (x - __builtin_##FFLOOR (x)))
> >> \
> >> +      t1 = __builtin_##FCEIL (x);
> >> \
> >> +    else
> >> \
> >> +      t1 = __builtin_##FFLOOR (x);
> >> \
> >> +    if (t1 != t2)
> >> \
> >> +      link_error ();
> >> \
> >> +  }
> >> \
> >> +  void round_##FFLOOR##_4 (TYPE x)
> >> \
> >> +  {
> >> \
> >> +    TYPE t1 = 0;
> >> \
> >> +    TYPE t2 = __builtin_##FFLOOR (x + 0.5);
> >> \
> >> +    if ((x - __builtin_##FFLOOR (x)) >= (__builtin_##FCEIL (x) - x))
> >> \
> >> +      t1 = __builtin_##FCEIL (x);
> >> \
> >> +    else
> >> \
> >> +      t1 = __builtin_##FFLOOR (x);
> >> \
> >> +    if (t1 != t2)
> >> \
> >> +      link_error ();
> >> \
> >> +  }
> >> +
> >> +TEST_ROUND (float, floorf, ceilf)
> >> +TEST_ROUND (double, floor, ceil)
> >> +TEST_ROUND (long double, floorl, ceill)
> >> +
> >> +/* { dg-final { scan-assembler-not "link_error" } } */
> >>
> 
> 
> 

-- 
Richard Biener <rguent...@suse.de>
SUSE Software Solutions Germany GmbH,
Frankenstrasse 146, 90461 Nuernberg, Germany;
GF: Ivo Totev, Andrew McDonald, Werner Knoblich; (HRB 36809, AG Nuernberg)

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