https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81906
--- Comment #4 from Andrew Pinski <pinskia at gcc dot gnu.org> --- (In reply to Vadim Zeitlin from comment #3) > (In reply to Andrew Pinski from comment #1) > > You want -frounding-math > > OK, thanks, this does indeed solve my immediate problem. However is it > really normal that the compiler behaviour silently (!) changes like this, in > backwards incompatible way between versions? I didn't remember seeing this > change mentioned in gcc 6 release notes and I just reread them again and > can't find it anywhere. I think changes silently breaking existing working > (and standard conforming) code should be at the very least prominently > documented (and, of course, ideally completely avoided IMO, but this ship > has probably already sailed). Yes and no. Since this behavior has been documented this way since 4.4.0: https://gcc.gnu.org/onlinedocs/gcc-4.4.0/gcc/Optimize-Options.html -frounding-math Disable transformations and optimizations that assume default floating point rounding behavior. This is round-to-zero for all floating point to integer conversions, and round-to-nearest for all other arithmetic truncations. ___This option should be specified for programs that change the FP rounding mode dynamically, or that may be executed with a non-default rounding mode.____ This option disables constant folding of floating point expressions at compile-time (which may be affected by rounding mode) and arithmetic transformations that are unsafe in the presence of sign-dependent rounding modes. Note I add ___. So basically what changed between 5 and 6 was constant folding of rint. Since that is covered in the documentation, the change itself does not need to be documented as happening.
