Re: [Mingw-users] Unexpected inaccuracy in double to int conversion

[Martin W. <mai...@ma...>]

KHMan wrote:
> On 8/5/2012 6:21 AM, Martin Whitaker wrote:
>> After updating to the latest version of the MinGW GCC (4.7.0), one
>> of my regression tests started failing.
>
> Interesting, it fails for me on MinGW gcc 3.4.5, MinGW 4.5.2 and
> TDM gcc 4.5.2...
>
Hmmm, yes, the test case also fails for me if I go back to the
previous version. The real code is of course more complex, so
it's some other code generation change that has suddenly exposed
this issue.

> Search for 'pow' in your math.h header file and you will find the
> answer in a long comment block: "Excess precision when using a
> 64-bit mantissa for FPU math ops can cause unexpected results..."
>
> Adding:
>
>       #include <fenv.h>
>       fesetenv(FE_PC53_ENV);
>
> gives 123 for the second line. Or save it first. The assembly
> output around the pow() in the second line is:
>
>       flds   32(%esp)
>       fstpl  (%esp)
>       call   _pow
>       fldl   16(%esp)      ; loads 123000.0
>       fdivp  %st, %st(1)   ; divide with pow() on x87 stack
>
> So the intermediate result was kept on the x87 stack, and the
> final result isn't exact I suppose.
>
> glibc wraps the i386 pow instruction and test for various special
> cases and conditions, including appropriate integer arguments, in
> order to guarantee exact results where possible.
>
That explains it. I was misled by printing out the double value
and seeing it was an exact integer. Saving the intermediate
value in a temporary variable fixes the problem.

Thanks!

Martin