Re: [PATCH] middle-end: If supported, use __builtin_bitreverse64 on reflect_hwi. [PR126625]

Likewise for ira-color.cc?

On Fri, Aug 7, 2026 at 1:46 AM Andrea Pinski <[email protected]>
wrote:

> On Thu, Aug 6, 2026 at 3:24 AM Kael Andrew Franco <[email protected]>
> wrote:
> >
> > From f3df22990341e3ff247160509c41e8b123aff0bd Mon Sep 17 00:00:00 2001
> > From: Kael Andrew Alonzo Franco <[email protected]>
> > Date: Wed, 5 Aug 2026 12:50:20 -0400
> > Subject: [PATCH] middle-end: If supported, use __builtin_bitreverse64 on
> reflect_hwi. [PR126625]
> >
> > reflect_hwi uses a naive for loop approach to emulate
> __builtin_bitreverse64 ().
> > This is slow compared to using __builtin_bitreverse64 () plus bitshift.
> > This is useful for bootstrapping GCC since r17-523.
> > Also add assert on BITWIDTH <= 64.
> >
> > Bootstrapped and regtested on x86_64-pc-linux-gnu.
> >
> > PR middle-end/126625
> >
> > gcc/ChangeLog:
> >
> > * hwint.cc (reflect_hwi): If supported, use __builtin_bitreverse64.
> >
> > Signed-off-by: Kael Andrew Franco <[email protected]>
> > ---
> >  gcc/hwint.cc | 19 ++++++++++++++++++-
> >  1 file changed, 18 insertions(+), 1 deletion(-)
> >
> > diff --git a/gcc/hwint.cc b/gcc/hwint.cc
> > index f3b3e7b8408..e87956ed3f5 100644
> > --- a/gcc/hwint.cc
> > +++ b/gcc/hwint.cc
> > @@ -189,12 +189,27 @@ least_common_multiple (HOST_WIDE_INT a,
> HOST_WIDE_INT b)
> >    return mul_hwi (abs_hwi (a) / gcd (a, b), abs_hwi (b));
> >  }
> >
> > -/* Reflect (reverse) the bits of a given VALUE within a specified
> BITWIDTH.  */
> > +#ifdef __has_builtin
> > +#if __has_builtin(__builtin_bitreverse64)
> > +#define HAVE_BITREVERSE64
> > +#endif
> > +#endif
>
> I think it is better to do:
> #define HAVE_BITREVERSE64 0
> #ifdef __has_builtin
> # if __has_builtin(__builtin_bitreverse64)
> #  define HAVE_BITREVERSE64 1
> #endif
> #endif
>
> > +
> > +/* Reflect (reverse) the bits of a given VALUE within a specified
> BITWIDTH <= 64.  */
> >
> >  unsigned HOST_WIDE_INT
> >  reflect_hwi (unsigned HOST_WIDE_INT value, unsigned bitwidth)
> >  {
> > +  if (bitwidth == 0)
> > +    return 0;
> > +
> > +  gcc_checking_assert (bitwidth <= 64);
> > +
> > +#ifdef HAS_BITREVERSE64
> #if HAVE_BITREVERSE64
> > +  return __builtin_bitreverse64 (value) >> (64 - bitwidth);
> > +#else
> >    unsigned HOST_WIDE_INT reflected_value = 0;
> > +
> >    /* Loop through each bit in the specified BITWIDTH.  */
> >    for (size_t i = 0; i < bitwidth; i++)
> >      {
> > @@ -204,5 +219,7 @@ reflect_hwi (unsigned HOST_WIDE_INT value, unsigned
> bitwidth)
> >        reflected_value |= (value & 1);
> >        value >>= 1;
> >      }
> > +
> >    return reflected_value;
> > +#endif
> >  }
> > --
> > 2.55.0
> >
> >
>