https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85957
--- Comment #12 from Rich Felker <bugdal at aerifal dot cx> --- Note that -fexcess-precision=standard is not available in C++ mode to fix this. However, -ffloat-store should also ensure consistency to the optimizer (necessary to prevent this bug, and other variants of it, from happening) at the expense of some extreme performance and code size costs and making the floating point results even more semantically incorrect (double-rounding all over the place, mismatching FLT_EVAL_METHOD==2) and -ffloat-store is available in C++ mode. Despite all these nasty effects, it may be a suitable workaround, and at least it avoids letting the optimizer prove 0==1, thereby effectively treating any affected code as if it contained UB. Note that in code written to be excess-precision-aware, making use of float_t and double_t for intermediate operands and only using float and double for in-memory storage, -ffloat-store should yield behavior equivalent to -fexcess-precision=standard.
