Will bit-shift by zero bits work correctly?

c++ bit-manipulation bit-shift
sharptooth picture sharptooth · Jun 11, 2009 · Viewed 25k times · Source

Say I have a function like this:

inline int shift( int what, int bitCount )
{
    return what >> bitCount;
}

It will be called from different sites each time bitCount will be non-negative and within the number of bits in int. I'm particularly concerned about call with bitCount equal to zero - will it work correctly then?

Also is there a chance that a compiler seeing the whole code of the function when compiling its call site will reduce calls with bitCount equal to zero to a no-op?

Answer

plinth picture plinth · Jun 11, 2009

According to K&R "The result is undefined if the right operand is negative, or greater than or equal to the number of bits in the left expression's type." (A.7.8) Therefore >> 0 is the identity right shift and perfectly legal.

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