https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94035
--- Comment #6 from rguenther at suse dot de <rguenther at suse dot de> --- On Thu, 5 Mar 2020, ch3root at openwall dot com wrote: > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94035 > > --- Comment #5 from Alexander Cherepanov <ch3root at openwall dot com> --- > I see. But the problem with decimal floating-point formats remains... > > Based on bug 93806, comment 41, here is an example with equal but different > values: > > ---------------------------------------------------------------------- > #include <stdio.h> > > __attribute__((noipa)) // imagine it in a separate TU > static _Decimal32 opaque(_Decimal32 d) { return d; } > > int main() > { > _Decimal32 x = opaque(1.0DF); > unsigned char *p = (unsigned char *)&x; > > if (x == 1.00DF) > printf("%d\n", p[0]); > printf("%d\n", p[0]); > } > ---------------------------------------------------------------------- > $ gcc -std=c2x -pedantic -Wall -Wextra test.c && ./a.out > 10 > 10 > $ gcc -std=c2x -pedantic -Wall -Wextra -O3 test.c && ./a.out > 100 > 10 > ---------------------------------------------------------------------- > gcc x86-64 version: gcc (GCC) 10.0.1 20200305 (experimental) > ---------------------------------------------------------------------- > > And then there are non-canonical representations for DFP values... From an implementation perspective it should be a FP format helper that tells us whether the format has multiple representations for values considered equal (eventually even for a specific representation of one value), MODE_HAS_AMBIGUITY_FOR(MODE, VAL) or so (where a NULL VAL should return whether there's any such value). Then we can easily test that before doing equivalence substitution.
