c - Left shifting with a negative shift count

Question

What exactly happens here?

a << -5

Obviously it doesn't right shift. But the book I'm reading states:

On one machine, this expression actually does a left shift of 27 bits

My question is; why? What causes a left shift of 27 bits? And what exactly happens when shifting with a negative shift count? Thank you.

Answer 1

Negative integers on right-hand side is undefined behavior in the C language.

ISO 9899:2011 6.5.7 Bit-wise shift operators:

The integer promotions are performed on each of the operands. The type of the result is that of the promoted left operand. If the value of the right operand is negative or is greater than or equal to the width of the promoted left operand, the behavior is undefined.