what is call frame in swift ?

In Swift, a call frame is a portion of memory allocated on the stack when a function is called. It provides the function with the necessary space to execute, including storing local variables and other state information. You can think of the call frame as having a layout similar to a C struct, where all the local state of the function becomes fields within this frame.

When a function is called, the stack pointer is adjusted to allocate the call frame. For example, if a function needs 208 bytes, the stack pointer is decremented by 208 bytes at the start of the function to allocate this space. Before the function returns, the stack pointer is incremented by the same amount to deallocate the memory.

This allocation is very efficient because it involves simple arithmetic operations on the stack pointer. The call frame is ideal for storing local state because the compiler always emits the necessary instructions to adjust the stack pointer, making this form of memory allocation almost free in terms of performance.

For more details, you can refer to the session Explore Swift performance (07:38).