On Sun, Apr 26, 2020 at 10:43:53PM +0200, Iain Buclaw wrote:
> >>>> + // The layout of the type is:
> >>>> + //
> >>>> + // [1| 7 | 56 ][ 8 | 56 ]
> >>>> + // [S| Exp | Fraction (hi) ][ Unused | Fraction (low) ]
> >>>> + //
> >>>> + // We can get the least significant bits by subtracting the
> >>>> IEEE
> >>>> + // double precision portion from the real value.
> >>>
> >>> That's not correct. There is no "Unused" field, and the lower fraction
> >>> is not always an immediate extension of the higher fraction.
> >>>
> >>> (It's not 1,7,56 -- it is 1,11,52).
> > All bits are significant to the value, there are no unused bits, for
> > most values. The sign and exponent of the second number are very much
> > relevant, in general.
> >
> > I didn't look at your actual implementation, just this comment, but it
> > sounds like your tests miss a lot of cases, if no problems were found?
>
> All these tests pass. That is, the computed compile-time byte layout
> (the result of this function) is the same as the layout at run-time.
Then either it tests not nearly enough, or it does not implement what the
comment says.
> // check subnormal storage edge case for Quadruple
> testNumberConvert!("real.min_normal/2UL^^56");
> testNumberConvert!("real.min_normal/19");
> testNumberConvert!("real.min_normal/17");
IBM long double has quite different edge cases than IEEE QP float.
> /**True random values*/
> testNumberConvert!("-0x9.0f7ee55df77618fp-13829L");
> testNumberConvert!("0x7.36e6e2640120d28p+8797L");
> testNumberConvert!("-0x1.05df6ce4702ccf8p+15835L");
> testNumberConvert!("0x9.54bb0d88806f714p-7088L");
None of these are valid double-double numbers (they all underflow or
overflow).
> /**Big overflow or underflow*/
> testNumberConvert!("cast(double)-0x9.0f7ee55df77618fp-13829L");
> testNumberConvert!("cast(double)0x7.36e6e2640120d28p+8797L");
> testNumberConvert!("cast(double)-0x1.05df6ce4702ccf8p+15835L");
> testNumberConvert!("cast(double)0x9.54bb0d88806f714p-7088L");
> testNumberConvert!("cast(float)-0x9.0f7ee55df77618fp-13829L");
> testNumberConvert!("cast(float)0x7.36e6e2640120d28p+8797L");
(Exactly like these).
Segher
