[fpc-devel] Broken frac function in FPC3.1.1 / Windows x86_64

[Sven Barth]

Bart <bartjunk64 at gmail.com> schrieb am Fr., 27. Apr. 2018, 13:42:

> On Wed, Apr 25, 2018 at 11:04 AM,  <info at wolfgang-ehrhardt.de> wrote:
>
> > If you compile and run this 64-bit program on Win 64 you get a crash
>
> And AFAICS your analysis of the cause (see bugtracker) is correct as well.
>
> function fpc_frac_real(d: ValReal) : ValReal;compilerproc; assembler;
> nostackframe;
>   asm
>     cvttsd2si %xmm0,%rax
>     { Windows defines %xmm4 and %xmm5 as first non-parameter volatile
> registers;
>       on SYSV systems all are considered as such, so use %xmm4 }
>     cvtsi2sd %rax,%xmm4
>     subsd %xmm4,%xmm0
>   end;
>
> CVTTSD2SI — Convert with Truncation Scalar Double-Precision
> Floating-Point Value to Signed Integer
> This should not be used to get a ValReal result.
>

The code essentially does the following (instruction by instruction):

=== code begin ===

tmpi := int64(d - trunc(d));
tmpd := double(tmpi);
Result := d - tmpd;

=== code end ===

Though why it fails with the given value is a different topic...

Regards,
Sven

>
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