On Cortex-M4F microcontrollers: is fixed point math faster or floating point?

Andy Neil said:

An issue often noted with fixed point is that, aside from the actual calculations, it adds overhead & complexity to the code which needs to supply the input data and/or use the results.

I agree.

Another problem in using fixed point is that code won't be portable due to the use of non-standard intrinsic functions like __SMMLA etc.

One problem in using float is that it will increase ISR entry and exit times, due to the need of saving and restoring FPU registers.

Andy Neil said:

An issue often noted with fixed point is that, aside from the actual calculations, it adds overhead & complexity to the code which needs to supply the input data and/or use the results.

I agree.

Another problem in using fixed point is that code won't be portable due to the use of non-standard intrinsic functions like __SMMLA etc.

One problem in using float is that it will increase ISR entry and exit times, due to the need of saving and restoring FPU registers.

Hi Pramod,

Hypothetically yes, however do your ISRs use the FPU registers?

Ronan Synnott said:

Hypothetically yes, however do your ISRs use the FPU registers?

Yes!

Intrinsic functions are more commonly used for 'non C' type actions, such as barrier instructions. In higher order code, the compiler will generate VFP instructions automatically when compiled for VFP. If you really want to hand craft a function, you would use assembler, rather than intrinsics. For example:

float foo(float a, float b){
  return (a+b);
}

Compiled with:

armclang -c -O2 --target=arm-arm-none-eabi -mcpu=cortex-m4 -mfpu=vfpv3 float.c

outputs:

    foo
        0x00000000:    ee001a10    ....    VMOV     s0,r1
        0x00000004:    ee010a10    ....    VMOV     s2,r0
        0x00000008:    ee310a00    1...    VADD.F32 s0,s2,s0
        0x0000000c:    ee100a10    ....    VMOV     r0,s0
        0x00000010:    4770        pG      BX       lr

For completeness, when compiled without VFP it calls a library function, which will take many cycles

    foo
        0x00000000:    f7ffbffe    ....    B.W      __aeabi_fadd