Guile generally assumes that the underlying C implementation will be
standards compliant. -Ofast, -ffast-math and related options disregard
standards compliance. Among other things, -ffast-math implies
-ffinite-math-only, which generates code that assumes that all floating
point values are finite. However, there's code in Guile's compiler that
uses +inf.0 and -inf.0 and expects that they will behave as specified in
IEEE-754.
So, in summary, I would say that Guile does not support being compiled
with -Ofast, -ffast-math, or many of the options that -ffast-math
implies.