How to describe a FMAC insn

[Qing Wei]

Hi,
Could someone give some hints of how to describe a FMAC (float mult and
add) insn in machine description, it matches d = b*c+a, which is a four
operands float instrution. With a glimp through the array optabs[] in
genopinit.c, it seems no OP handler could match FMAC operation? And I
found a function gen_add_mult() in loops.c, but it also seems not very
helpful. And another my question is, the element of optabs[] are arrays
indexed by machine code, for example, add_optab[] indexed by SI, DI, QI,
FI machine mode, not by number of operands, it seems it only matches 3
operands add operation,if I want to add a four operands add operation,
what should I do?

Qing