Bitfield structs and suggestion

[bearophile]

0ffh:

> OTOH, the accessor templates from the D.learn thread look very nice.

I think they aren't good enough yet for practical usage, they do too much upcasting (to 64 bit numbers, in that thread I have shown it that using 32 bit makes those bitfields faster), so they require improvement. And they need more testing.


> I think the answer may in part depend on the result of the planned
> "GCC bitfields" vs. "GDC templates" shootout as planned on D.learn
> by bearophile.
> Hi bearophile, any results yet? :)

Hello, but sadly os far I haven't succed yet to install GDC (I'll try again), despite I already have MinGW installed. So I have tried with just the MinGW, the following:

#include <stdio.h>
#define N 25

typedef union {
  unsigned int i;
  struct {
    unsigned int i;

    unsigned int sign(void) { return (i >> 0x0) & 0x1; }
    void sign(unsigned int val) { i = (i & 0xfffffffffffffffeLL) | ((val & 0x1) << 0x0); }

    unsigned int biasedexponent(void) { return (i >> 0x1) & 0xff; }
    void biasedexponent(unsigned int val) { i = (i & 0xfffffffffffffe01LL) | ((val & 0xff) << 0x1); }

    unsigned int significand(void) { return (i >> 0x9) & 0x7fffff; }
    void significand(unsigned int val) { i = (i & 0x1ff) | ((val & 0x7fffff) << 0x9); }
  } bits;
} INTORFLOAT;

int main(void) {
  int i, j;
  unsigned long long tot = 0;
  INTORFLOAT f;

  unsigned int v[N] = {1628676761, 1620574103, 1237153253, 1098880307, 87513741,
             13181925, 14686126, 7429435, 16286706, 6474381,
             4879794, 7734725, 3745958, 13353858, 4236193,
             7587, 4309, 28846, 7313, 14516,
             126, 143, 171, 221, 156};

  for(j = 0; j < 4000000; j++)
    for(i = 0; i < N; i++) {
      f.i = v[i];
      f.bits.biasedexponent(f.bits.biasedexponent() / 2);
      tot += f.bits.biasedexponent() + f.bits.sign();
    }

   printf("%d\n", tot);
}

It runs even a bit faster than the normal C bitfields ;-) So they may run fast on GDC too.

Bye,
bearophile