Attempting to use -fexcess-precision=standard with g++ produces:
cc1plus: sorry, unimplemented: '-fexcess-precision=standard' for C++
In light of eldritch horrors like pr 85957 this means floating point is essentially catastrophically broken on i386 and m68k.
This came to my attention while analyzing https://github.com/OSGeo/PROJ/issues/1906. Most of the problems are g++ incorrectly handling excess precision, and they're having to put awful hacks with volatile objects in place to work around it.
Yes this is known. See PR 323.
Where even this is mentioned that C++ is not enabled yet.
*** This bug has been marked as a duplicate of bug 323 ***