How to force GCC to generate specific floating-point instructions [duplicate]

Given the following C code,

    double a = 4.00;
    double b = 123.456789;
    double c = 15.123;

    double res = a*b + c;

    return res;

compiling with riscv64-linux-gcc -g -o2 generates the following

    double res = a*b + c;
   102e8:   fe843707            fld fa4,-24(s0)
   102ec:   fe043787            fld fa5,-32(s0)
   102f0:   12f777d3            fmul.d  fa5,fa4,fa5
   102f4:   fd843707            fld fa4,-40(s0)
   102f8:   02f777d3            fadd.d  fa5,fa4,fa5
   102fc:   fcf43827            fsd fa5,-48(s0)

Meaning that the compiler did not recognize that this is a fused multiplication/addition operation (FMADD).

So, I have two questions:

• Is there a means to force the compiler to generate FMADD.D instead of the separate MUL.D and ADD.D instructions?

• What mad the compiler choose to implement it this way?

(from what I know, FMA is generally more efficient that separate MUL and ADD, and also, 1 instruction is generally more efficient than 2 from a power and speed point of view, so I'm a little bit confused about the choices being made by GCC)