The s390x port was based on the ppc64 port and, because of the way the
port was done, inherited some instructions from it. ppc64 supports
3-operand (4-operand for FMADD etc.) floating point instructions
but s390x doesn't (the destination register is always an input) and
so these were emulated.
There is a bug in the emulation of FMADD whereby if the destination
register is also a source for the multiplication it will be
clobbered. This doesn't break any assembly code in the std lib but
could affect future work.
To fix this I have gone through the floating point instructions and
removed all unnecessary 3-/4-operand emulation. The compiler doesn't
need it and assembly writers don't need it, it's just a source of
bugs.
I've also deleted the FNMADD family of emulated instructions. They
aren't used anywhere.