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carminteger-arithmeticcmsis

Shift 1 and multiply vs direct shift

I came across code similar to this in the ARM CMSIS-NN library:

int32_t val, shift;

// code that initializes val and shift

int32_t result = val * (1 << shift);

which aims to multiply val by some power of two value x, with shift being the exponent, i.e. x=pow(2,shift). Why are they not simply shifting? Like so

int32_t result = val << shift;

Is there something special that I am missing? Can the compiler optimize the former operation in some special way?

EDIT: What confuses me is that they are using "simple shifts" throughout the code. Additionally, the code should be highly optimized. But I guess modern compilers will figure out by themselves, that shifting is the way to go (instead of multiplying)?

Solution

  • It is always sign correct and forces the use of the proper FPU instructions and works with any type of data.