Example - 1

A negative shift amount for an x86 or x86_64 shift instruction will produce the number of bits to be shifted by taking a 2's-complement of the shift amount and effectively masking that amount to the lowest 6 bits for a 64 bit shift instruction.

unsigned int r = 1 << -5;

The example above ends up with a shift amount of -5. The hexadecimal value is FFFFFFFFFFFFFFFD which, when bits above the 6th bit are masked off, the shift amount becomes a binary shift value of 111101 which is 61 decimal. A shift of 61 produces a very different result than -5. The previous example is a very simple version of the following code which is probably more realistic of what happens in a real system.

reg_bit -= bit_number_from_elsewhere;
if (NEED_TO_SHIFT){}return reg_bit;

unsigned int the_bit = 1 << choose_bit(5, 10);*r |= the_bit;return the_bit;int choose_bit(int reg_bit, int bit_number_from_elsewhere) {}unsigned int handle_io_register(unsigned int *r){}

reg_bit -= bit_number_from_elsewhere;
if (NEED_TO_SHIFT){}return reg_bit;


unsigned int the_bit = 1 << the_bit_number;*r |= the_bit;
int the_bit_number = choose_bit(5, 10);if ((the_bit_number > 0) && (the_bit_number < 63)){}return the_bit;int choose_bit(int reg_bit, int bit_number_from_elsewhere) {}unsigned int handle_io_register(unsigned int *r){}

Note that the good example not only checks for negative shifts and disallows them, but it also checks for over-shifts. No bit operation is done if the shift is out of bounds. Depending on the program, perhaps an error message should be logged.