framepointer

A frame pointer is a dedicated processor register used to mark the start of the current stack frame in a program’s call stack. On many x86 and x86-64 platforms, the frame pointer register (EBP/RBP) provides a stable reference from which function parameters and local variables can be accessed at fixed offsets, independently of the stack pointer.

In a typical function prologue, the callee-saved frame pointer is pushed or saved, the frame pointer register is set to the current stack pointer, and space is allocated for locals. For example, a common pattern is push ebp; mov ebp, esp; sub esp, N. Local variables live at negative offsets from ebp, while parameters are found at positive offsets after the return address. The epilogue restores the previous frame pointer and stack state before returning.

Benefits of using a frame pointer include straightforward stack unwinding for debugging and exception handling, easier inspection of function locals and parameters, and compatibility with certain runtime and debugging tools that rely on a stable base reference.

Trade-offs involve potential limits on register availability and performance. Many modern optimizing compilers omit the frame pointer (frame pointer omission, FPO) to free a register for better register allocation and faster code. This is often controlled by compiler options and can be disabled with flags like -fno-omit-frame-pointer in some toolchains. Some environments, especially older 32-bit code or certain debugging or runtime scenarios, may retain the frame pointer by default.

Variations exist across architectures and ABIs. While x86-64 and ARM can use a frame pointer, the extent to which it is preserved or omitted depends on the target platform, compiler, and optimization level. Unwind information and debugging formats (DWARF, SEH) can also aid stack reconstruction with or without a frame pointer.