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Date: Wed, 20 Jan 2021 14:24:57 +0100 (CET)
From: Richard Biener <rguenther@suse.de>
To: Jakub Jelinek <jakub@redhat.com>
cc: gcc-patches@gcc.gnu.org, richard.sandiford@arm.com
Subject: Re: [PATCH] Handle overflow in dependence analysis lambda ops
 gracefully
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On Wed, 20 Jan 2021, Jakub Jelinek wrote:

> On Wed, Jan 20, 2021 at 01:15:12PM +0100, Richard Biener wrote:
> > OK, fixed.  Guess we could also use __builtin_mul_overflow directly
> > if supported via a GCC_VERSION check.  Looks like it's present
> > since GCC 5 at least.  So sth like (incremental)
> > 
> > diff --git a/gcc/hwint.h b/gcc/hwint.h
> > index 53f4ed5dcad..6d2d491acfa 100644
> > --- a/gcc/hwint.h
> > +++ b/gcc/hwint.h
> > @@ -354,12 +354,19 @@ add_hwi (HOST_WIDE_INT a, HOST_WIDE_INT b, bool 
> > *overflow)
> >  inline HOST_WIDE_INT
> >  mul_hwi (HOST_WIDE_INT a, HOST_WIDE_INT b, bool *overflow)
> >  {
> > +#if GCC_VERSION < 5001
> >    unsigned HOST_WIDE_INT result = a * (unsigned HOST_WIDE_INT)b;
> > -  if (a != 0 && (HOST_WIDE_INT)result / a != b)
> > +  if ((a == -1 && b == HOST_WIDE_INT_MIN)
> > +      || (a != 0 && (HOST_WIDE_INT)result / a != b))
> >      *overflow = true;
> >    else
> >      *overflow = false;
> >    return result;
> > +#else
> > +  HOST_WIDE_INT result;
> > +  *overflow = __builtin_mul_overflow (a, b, &result);
> > +  return result;
> > +#endif
> >  }
> > 
> > for the add case we should match all of the function I guess.
> 
> Yeah, sure.
> Maybe bump somewhat the GCC_VERSION number in there, we've had bugs in
> __builtin_mul_overflow expansion too.

Wow, a bugzilla search finds wrong-code that's only fixed in 9.3+.
So I guess I'll check for GCC 11+ then and not worry for stage1,
non-bootstrapped GCC or crosses with host != GCC 11.

Richard.