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Subject:	Re: log2() is not called for C code, a bug into math.h ?
To:	KENNON J CONRAD <kennonconrad AT comcast DOT net>
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From:	"Carlo B. via Cygwin" <cygwin AT cygwin DOT com>
Reply-To:	"Carlo B." <carlo DOT bramini AT gmail DOT com>
Cc:	KENNON J CONRAD via Cygwin <cygwin AT cygwin DOT com>
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Hello,
actually, we have to say that the source code of log2() into newlib:

https://sourceware.org/git/?p=newlib-cygwin.git;a=blob;f=newlib/libm/common/log2.c;h=e48c16cf8014b5ba4b3a89af261f54472d643f65;hb=HEAD

and the source code of log2() into glibc:

https://sourceware.org/git/?p=glibc.git;a=blob;f=sysdeps/ieee754/dbl-64/e_log2.c;h=872cebddd8b4283edd2e653167035c0fd527f3ea;hb=HEAD

are identical, so we can assume that this code is correct and probably
the best we can get in terms of speed.
However, here the problem is different.
Using log() and the property of the logarithms for changing the base
is mathematically correct.
But here, it is WRONG.
Unfortunately, this method doesn't provide the right result because
the intrinsic format of IEEE-754 numbers propagates a precision error.
Calculating the log2 of 2^1023 is a perfect integer number and it's
1023, not 1023.00000000000011.
So, in my opinion that macro MUST be removed and the genuine log2()
function must be used instead.

Thank you very much for your time.
Sincerely,

Carlo Bramini.


Il giorno dom 7 giu 2026 alle ore 01:24 KENNON J CONRAD
<kennonconrad AT comcast DOT net> ha scritto:
>
> Should have written "I found that using log2 directly on my computer was slower than using log
> with the divide".
>
> > On 06/06/2026 4:06 PM PDT KENNON J CONRAD via Cygwin <cygwin AT cygwin DOT com> wrote:
> >
> >
> > Hello Carlo B.,
> >
> > There was another discussion of this issue back in January.  Beware, at the time I found that
> > using log2 directly on my computer was slower than using log2 with the divide (by removing the
> > same lines you mention in math.h).  It seems to me that in a perfect world, using log2 should
> > be both more accurate and faster than calling log and dividing the result by a constant.
> >
> > Best Regards,
> >
> > Kennon Conrad
> >
> > https://cygwin.com/pipermail/cygwin/2026-January/259314.html
> >
> > > On 06/06/2026 9:53 AM PDT Carlo B. via Cygwin <cygwin AT cygwin DOT com> wrote:
> > >
> > >
> > > Hello,
> > >
> > > Il Sab 6 Giu 2026, 14:19 Dimitry Andric <dimitry AT unified-streaming DOT com> ha
> > > scritto:
> > >
> > > >
> > > > It's been defined like that since the "import newlib-2000-02-17 snapshot",
> > > > so for 26 years:
> > > >
> > > >
> > > > https://sourceware.org/git/?p=newlib-cygwin.git;a=blob;f=newlib/libc/include/math.h;h=d3f2e99ae4d33596b608225f0349a6d581d265ca;hb=8a0efa53e44919bcf5ccb1d3353618a82afdf8bc#l103
> > > >
> > > > My guess is that at the time, there wasn't yet a separate log2()
> > > > implementation, so this was an easy way to cover that case.
> > > >
> > >
> > > Thank you very much for the reply.
> > > So, do you think that it would be possible to remove those lines from
> > > math.h and use the log2() now provided?
> > >
> > > Sincerely,
> > >
> > > Carlo Bramini
> > >
> > >
> > > > -Dimitry
> > > >
> > > >
> > >
> > > --
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