Variable-Length Data on the Stack

Return Sequence
1. The callee stores the return value near the parameters. Note that this address can be determined by the caller using the old (soon-to-be-restored) sp.
2. The callee restores sp and the registers.
3. The callee jumps to the return address.
Note that varagrs are supported.

Variable-Length Data on the Stack

There are two flavors of variable-length data.
• Data obtained by malloc/new have hard to determine lifetimes and are stored in the heap instead of the stack.
• Data, such as arrays with bounds determined by the parameters are still stack like in their lifetimes (if A calls B, these variables of A are allocated before and released after the corresponding variables of B).







It is the second flavor that we wish to allocate on the stack. The goal is for the (called) procedure to be able to access these arrays using addresses determinable at compile time even though the size of the arrays (and hence the location of all but the first element) is not known until the program is called, and indeed often differs from one call to the next (even when the two calls correspond to the same source statement). The solution is to leave room for pointers to the arrays in the AR. These are fixed size and can thus be accessed using static offsets. Then when the procedure is invoked and the sizes are known, the pointers are filled in and the space allocated.
A small change caused by storing these variable size items on the stack is that it no longer is obvious where the real top of the stack is located relative to sp. Consequently another pointer (call it real-top-of-stack) is also kept. This is used on a call to tell where the new allocation record should begin.