Unexpected C/C++ bitwise shift operators outcome

Question

I think I\'m going insane with this.

I have a a piece of code that needs to create an (unsigned) integer with N consequent bits set to 1. To be exact I

Answer 1

My 32 cents:

#include 

#define INT_BIT     (CHAR_BIT * sizeof(int))

unsigned int set_bit_range(unsigned int n, int frm, int cnt)
{
        return n | ((~0u >> (INT_BIT - cnt)) << frm);
}

^{List 1.}

A safe version with bogus / semi-circular result could be:

unsigned int set_bit_range(unsigned int n, int f, int c)
{
        return n | (~0u >> (c > INT_BIT ? 0 : INT_BIT - c)) << (f % INT_BIT);
}

^{List 2.}

Doing this without branching, or local variables, could be something like;

return n | (~0u >> ((INT_BIT - c) % INT_BIT)) << (f % INT_BIT);

^{List 3.}

List 2 and List 3 This would give "correct" result as long as from is less then INT_BIT and >= 0. I.e.:

./bs 1761 26 810
Setting bits from 26 count 810 in 1761 -- of 32 bits
Trying to set bits out of range, set bits from 26 to 836 in 32 sized range
x = ~0u       =  1111 1111 1111 1111 1111 1111 1111 1111

Unsafe version:
x = x >> -778 =  0000 0000 0000 0000 0000 0011 1111 1111
x = x <<  26  =  1111 1100 0000 0000 0000 0000 0000 0000
x v1 Result   =  1111 1100 0000 0000 0000 0110 1110 0001
Original:        0000 0000 0000 0000 0000 0110 1110 0001    

Safe version, branching:
x = x >>   0  =  1111 1111 1111 1111 1111 1111 1111 1111
x = x <<  26  =  1111 1100 0000 0000 0000 0000 0000 0000
x v2 Result   =  1111 1100 0000 0000 0000 0110 1110 0001
Original:        0000 0000 0000 0000 0000 0110 1110 0001    

Safe version, modulo:
x = x >>  22  =  0000 0000 0000 0000 0000 0011 1111 1111
x = x <<  26  =  1111 1100 0000 0000 0000 0000 0000 0000
x v3 Result   =  1111 1100 0000 0000 0000 0110 1110 0001
Original:        0000 0000 0000 0000 0000 0110 1110 0001