https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110045
--- Comment #7 from Frank J. T. Wojcik <gccbugs at elkpod dot com> ---
After playing with this a bit more, I was able to find a Generator which
produces a slightly wider bound, but still nowhere near 0x1.fffffep+127.
This also includes the requested change to SimpleGen.
#include <cstdio>
#include <cstdint>
#include <random>
#define MAXOFFSET 3
// Return 25-bit values centered around 0x1000000 (the halfway-point of all
// possible 25-bit outputs). Widening the Generator does not alter the
// program's output.
class SimpleGen {
public:
using result_type = uint32_t;
result_type val, offset, ctr = 0;
static constexpr result_type min() { return 0; }
static constexpr result_type max() { return 0x1ffffff; }
result_type operator()() { offset = (ctr & 1) ?
((ctr / 2) / (2 * MAXOFFSET)) :
((ctr / 2) % (2 * MAXOFFSET));
val = 0x1000000 + offset - MAXOFFSET;
printf("\tG 0x%07x\n", val);
++ctr; return val; }
};
int main(void) {
SimpleGen gen;
std::normal_distribution<float> norm {0, 1};
printf("min: %+f %a\nmax: %+f %a\n\n", norm.min(), norm.min(),
norm.max(), norm.max());
for (int i = 0; i < ((2 * MAXOFFSET) * (2 * MAXOFFSET - 1) * 2) ; i++) {
float r = norm(gen);
printf("%d %f %a\n", i, r, r);
}
}
Build output:
$ g++-12.2 -Wall -Wextra -o normdist2 normdist2.cpp -fno-strict-aliasing
-fwrapv -fno-aggressive-loop-optimizations -fsanitize=undefined
$ echo $?
0
Actual outputs (excerpts only!):
min: -340282346638528859811704183484516925440.000000 -0x1.fffffep+127
max: +340282346638528859811704183484516925440.000000 0x1.fffffep+127
G 0x1000000
G 0x0ffffff
30 -8.157336 -0x1.0508e6p+3
31 0.000000 0x0p+0
G 0x1000001
G 0x0ffffff
32 -8.157336 -0x1.0508e6p+3
33 0.000000 0x0p+0
G 0x0ffffff
G 0x1000000
40 0.000000 0x0p+0
41 -8.157336 -0x1.0508e6p+3
G 0x1000002
G 0x1000000
42 0.000000 0x0p+0
43 7.985583 0x1.ff13ccp+2
G 0x0ffffff
G 0x1000001
48 0.000000 0x0p+0
49 -8.157336 -0x1.0508e6p+3
G 0x1000002
G 0x1000001
50 0.000000 0x0p+0
51 7.985583 0x1.ff13ccp+2
G 0x1000000
G 0x1000002
58 7.985583 0x1.ff13ccp+2
59 0.000000 0x0p+0
Expected outputs (excerpt only!):
min: -8.157336 -0x1.0508e6p+3
max: +7.985583 0x1.ff13ccp+2
