🚀 go-pugleaf

   This behaviour is not correct, becauses there are two values of x which
   equal 0.0 in IEEE fp math, and the optimizer erroneously thinks there is
   only one.  The IEEE defn also asserts that -0. < 0.

No, the IEEE 754 standard says that -0.0 == 0.0, and that !(-0.0 < 0.0).
You cannot directly distinguish -0.0 from 0.0 with a standard comparison op;
you can distinguish them only by printing them out, or by performing some
other arithmetic operations on them (e.g. 1/x) and comparing the results.
So GCC is acting correctly in the cases that you cite.