I'm not aware of any time GCC does that except due to C++'s promotion rules (e.g. float + double -> double), which is a problem with C++, not the compiler. I write deterministic software using floating point in C++.
Yes, it's not replacing 64-bit math with 80-bit math, x87 just doesn't have proper 64-bit floats with correctly sized exponent-field and the flags here interpret float literals as "long double" if I remember correctly. It's just the 80-bit x87 problem referred to earlier in the thread. The workarounds mentioned in the github aren't actually enough. Compilers cannot do IEEE-compliant computation on x87 without relatively large performance penalties (I made a library that did it).
