1. A method for executing program code in a processor that includes a mechanism for executing vector instructions, comprising: while executing program code, encountering at least one vector instruction for performing at least one operation in a loop in the program code, wherein each element in a set of elements in the vector instruction is used to perform the operation for a corresponding iteration of the loop, and at least one vector-control instruction that uses runtime data dependency information to identify elements in the vector instruction that may be operated on in parallel without causing an error due to a runtime data dependency between the iterations of the loop; and executing the loop by repeatedly: executing the vector-control instruction to identify a next group of elements that can be operated on in the vector instruction; and selectively executing the vector instruction to perform the operation for the next group of elements in the vector instruction, until the operation has been performed for all elements of the vector instruction.