https://gcc.gnu.org/bugzilla/show_bug.cgi?id=79720
Thomas Koenig <tkoenig at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Keywords| |wrong-code
Status|WAITING |NEW
Target Milestone|--- |7.0
Summary|floating point result |[6/7 Regression] complex
|different at compile time / |division different at
|runtime |compile time / runtime
--- Comment #5 from Thomas Koenig <tkoenig at gcc dot gnu.org> ---
Here's a C test case. I chose the %a format for
printf to better show what is going on:
#include <complex.h>
#include <stdio.h>
#include <math.h>
char input[] = "1.2e10 -3.2";
int main()
{
float complex c1, c2, r1, r2;
float re, im;
c1 = 1.2e10f - 3.2f*I;
sscanf(input,"%f %f", &re, &im);
c2 = re + im*I;
printf("c1 = (%a, %a) c2 = (%a, %a)\n", crealf(c1), cimagf(c1),
crealf(c2), cimagf(c2));
printf("c1 - c2 = (%a, %a)\n", crealf(c1)-crealf(c2),
cimagf(c1)-cimagf(c2));
r1 = 1/c1;
r2 = 1/c2;
printf("r1 = (%a, %a) r2 = (%a, %a)\n", crealf(r1), cimagf(r1),
crealf(r2), cimagf(r2));
printf("r1 - r2 = (%a, %a)\n", crealf(r1)-crealf(r2),
cimagf(r1)-cimagf(r2));
return 0;
}
$ gcc -O re.c && ./a.out
c1 = (0x1.65a0bcp+33, -0x1.99999ap+1) c2 = (0x1.65a0bcp+33, -0x1.99999ap+1)
c1 - c2 = (0x0p+0, 0x0p+0)
r1 = (0x1.6e80fep-34, 0x1.a3c414p-66) r2 = (0x1.6e80fep-34, 0x1.a3c412p-66)
r1 - r2 = (0x0p+0, 0x1p-89)
So, two ULP difference.
There is no difference with gcc 4.3.2:
tkoenig@gcc12:~$ gcc -O re.c
tkoenig@gcc12:~$ ./a.out
c1 = (0x1.65a0bcp+33, -0x1.99999ap+1) c2 = (0x1.65a0bcp+33, -0x1.99999ap+1)
c1 - c2 = (0x0p+0, 0x0p+0)
r1 = (0x1.6e80fep-34, 0x1.a3c412p-66) r2 = (0x1.6e80fep-34, 0x1.a3c412p-66)
r1 - r2 = (0x0p+0, 0x0p+0)
tkoenig@gcc12:~$ gcc -v
Using built-in specs.
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Debian 4.3.2-1.1'
--with-bugurl=file:///usr/share/doc/gcc-4.3/README.Bugs
--enable-languages=c,c++,fortran,objc,obj-c++ --prefix=/usr --enable-shared
--with-system-zlib --libexecdir=/usr/lib --without-included-gettext
--enable-threads=posix --enable-nls --with-gxx-include-dir=/usr/include/c++/4.3
--program-suffix=-4.3 --enable-clocale=gnu --enable-libstdcxx-debug
--enable-objc-gc --enable-mpfr --enable-cld --enable-checking=release
--build=x86_64-linux-gnu --host=x86_64-linux-gnu --target=x86_64-linux-gnu
Thread model: posix
gcc version 4.3.2 (Debian 4.3.2-1.1)
but gcc 6 and 7 fail.
