https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110622
--- Comment #16 from Mathieu Malaterre <malat at debian dot org> ---
(In reply to Xi Ruoyao from comment #15)
> (In reply to Mathieu Malaterre from comment #14)
> > (In reply to Andrew Pinski from comment #13)
> > > (In reply to Mathieu Malaterre from comment #12)
> > > > I am seeing a difference in result (log1p computation) in the range:
> > > >
> > > > 4318952042648305665 - 0x1.0000000000001p-64
> > > > 4368493837572636672 - 0x1.002p-53
> > > >
> > > > the other values seems to match expectation of log1p computation.
> > >
> > > But you used excess-precision=fast
> > >
> > > *** This bug has been marked as a duplicate of bug 323 ***
> >
> > AFAIK bug #323 does not mention my trick:
> >
> > asm volatile("" : "+r"(y.raw[0]) : : "memory");
> >
> > That simple line totally changed the optimizer code generation.
>
> Because in x87 the excessive precision only exists in x87 stack-like
> registers. The "memory" clobber forces a store and reload for all
> non-register variables, thus the value is truncated into a normal double
> value and the excessive precision is lost.
>
> There are infinite ways to work around an issue, but it does not mean PR 323
> must mention all of them.
Oh, I see. Basically my trick is convoluted `-ffloat-store`.
I finally took myself by the hand and convinced me with a simple code:
```
// gcc -m32 -fexcess-precision=fast -O2 t.c
#include <inttypes.h>
#include <stdint.h>
#include <stdio.h>
#include <string.h>
[[gnu::noipa]] void test(uint64_t v, double x, double y) {
const double y2 = x + 1.0;
if (y != y2)
printf("error %" PRIu64 " %.17g %a\n", v, x, x);
else
printf("ok %" PRIu64 " %.17g %a\n", v, x, x);
}
void main() {
uint64_t kSamplesPerRange = 4000, start = 0, stop = 9218868437227405311;
uint64_t step = (stop / kSamplesPerRange);
for (uint64_t value_bits = start; value_bits <= stop; value_bits += step) {
double value;
memcpy(&value, &value_bits, sizeof value);
double x = value;
double y = x + 1.0;
test(value_bits, x, y);
}
}
```
please accept my apologies for the noise.
