On Fri, 1 Sept 2023 at 11:14, Jonathan Wakely <jwakely.gcc@xxxxxxxxx> wrote: > > On Fri, 1 Sept 2023 at 10:57, Mathieu Malaterre via Gcc-help > <gcc-help@xxxxxxxxxxx> wrote: > > > > Hi all, > > > > I am reading a previous post from Pascal Cuoq(*), and it seems things > > have changed quite a bit in GCC nowadays. I fail to understand how > > FLT_EVAL_METHOD relates to fexcess-precision. Did I miss something? > > See https://gcc.gnu.org/gcc-13/changes.html#cxx Oh but you're asking about C, which hasn't changed in a while. I'm not sure what your question is then. FLT_EVAL_METHOD tells you what precision is used for floating-point constants and arithmetic, and -fexcess-precision says whether GCC strictly follows the standard's rules about when those excess precision bits are used. If FLT_EVAL_METHOD==2 then float and double constants are represented in 80-bit long double format, and arithmetic operations are done in 80-bit long double format. To remove the excess precision and get a 32-bit float or 64-bit double you need to use an explicit cast to float or double. If FLT_EVAL_METHOD==0 then there are no excess precision bits, and so -fexcess-precision makes no difference. Does that answer the question? > > > > > --- > > > > Here is what I see on my Debian/sid/i386 system: > > > > % gcc -O2 -fexcess-precision=fast m.c && ./a.out > > float eps = 1.192093e-07 > > 2 > > > > while: > > > > % gcc -O2 -fexcess-precision=standard m.c && ./a.out > > float eps = 1.084202e-19 > > 2 > > > > with: > > > > % cat m.c > > #include <stdio.h> > > #include <float.h> > > > > int main(void) > > { > > float floatEps = 1; > > > > while (1 + floatEps / 2 != 1) > > floatEps /= 2; > > > > printf("float eps = %e\n", floatEps); > > printf("%d\n", FLT_EVAL_METHOD); > > } > > > > For reference: > > > > % gcc --version > > gcc (Debian 13.2.0-2) 13.2.0 > > > > (*) https://stackoverflow.com/a/16064515/136285 > > -- > > Mathieu
