How to use fused multiply–add with MKL?

[Konovalov__Pavel]

I want to do basic a*x + b operation, where a, x and b are the vectors(or matrixes) with utilization of FMA processor capabilities. I think I am using v?Mul + v?Add I will get two separate operations. How to use FMA with the help of MKL and Intel compiler? Must I use FMA Intrinsics only?

[TimP]

Do ?axpy work for you?

[Konovalov__Pavel]

No, Tim. in ?axpy a is a scalar, not a vector and there is no vector b.

[McCalpinJohn · Accepted solution]

The C and Fortran compilers will generate FMA instructions from ordinary source code loops for which the FMA operation is appropriate, provided that

• the target instruction set includes FMA (AVX2 or newer -- note that the default is SSE, which does not support FMA), and
• the optimization level is high enough (at least O1, but preferably O2), and
• you have not prohibited FMA with a different compiler flag (-no-fma or some options to the -fp-model flag).

There are a few cases involving reduction operations where the compiler will choose not to use FMA operations because it estimates that there will be a shorter critical path by splitting the operation (doing the multiplication earlier and the addition later).

[Konovalov__Pavel]

Thank you, John! Is there any indication from the compiler ouput that loop is FMAsed :) ? like with vectorisation report