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	Sat, 06 Jun 2026 05:19:41 -0700 (PDT)
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Subject:	Re: log2() is not called for C code, a bug into math.h ?
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Date:	Sat, 6 Jun 2026 14:19:41 +0200
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To:	"Carlo B." <carlo DOT bramini AT gmail DOT com>
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From:	Dimitry Andric via Cygwin <cygwin AT cygwin DOT com>
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On 6 Jun 2026, at 13:42, Carlo B. via Cygwin <cygwin AT cygwin DOT com> wrote:
> 
> Hello,
> look this tiny C source code:
> 
> ========
> 
> #include <stdio.h>
> #include <math.h>
> 
> int main()
> {
>    double value = 0x1p1023;
> 
>    printf("log2=%.14f\n", log2(value));
> #undef log2
>    printf("log2=%.14f\n", log2(value));
> 
>    return 0;
> }
> 
> ========
> 
> Compile it with a simple command:
> 
> $ gcc test_log2.c -o test_log2.exe
> 
> and run it.
> The result on the console is:
> 
> log2=1023.00000000000011
> log2=1023.00000000000000
> 
> If you look inside math.h from newlib, you will see this code:
> 
> extern double log2 (double);
> #if !defined(__cplusplus)
> #define log2(x) (log (x) / _M_LN2)
> #endif
> 
> As you can see, my simple test program just calculates log2() by using
> log() function and next, after undefining log2() macro, it uses the
> true log2() function.
> 
> Actually, that macro log2() "overloads" the log2() function.
> And as result, the calculation of "(log (x) / _M_LN2)" loses the
> precision on the last bits of the result.
> It is not clear to me if there is a particular reason for doing so,
> because this thing makes the Python's test suite to fail on the tests
> for the log2() function.
> What do you think?
> Because log2() function was perfectly able to pass the tests.

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.

-Dimitry


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