Re: Funny results with long double denorms on m68k

[Laurent Vivier <laurent@vivier.eu>]

Le 20/08/2023 à 22:55, Keith Packard a écrit :
> #include <stdio.h>
> #include <stdint.h>
> 
> #define X       0x1p+16383l
> #define Y       0x1p-16446l
> 
> static long double build_mul = X * Y;
> static volatile long double x = X;
> static volatile long double y = Y;
> 
> static void
> dump_ld(const char *label, long double ld)
> {
>      union {
>          long double     d;
>          struct {
>              uint32_t    exp:16;
>              uint32_t    space:16;
>              uint32_t    h;
>              uint32_t    l;
>          };
>      } u;
> 
>      u.d = ld;
>      printf("%12s: % -27La 0x%04x 0x%08x 0x%08x\n", label, u.d, u.exp, u.h, u.l);
> }
> 
> int main(void)
> {
>      long double runtime_mul = x * y;
> 
>      dump_ld("x", x);
>      dump_ld("y", y);
>      dump_ld("build_mul", build_mul);
>      dump_ld("runtime_mul", runtime_mul);
>      return 0;
> }

FYI, result of this program on a real hardware (q800/m68040) is:

            x:  0x8p+16380                 0x7ffe 0x80000000 0x00000000
            y:  0x0.000000000000001p-16386 0x0000 0x00000000 0x00000001
    build_mul:  0x8p-66                    0x3fc0 0x80000000 0x00000000
  runtime_mul:  0x8p-66                    0x3fc0 0x80000000 0x00000000

Thanks,
Laurent