Function Prologue and Epilogue

Topic Overview

Function Entry and Exit Sequences

Every compiled function needs a prologue to set up the stack frame and an epilogue to tear it down. x86-64 SysV ABI: push rbp; mov rbp, rsp; sub rsp, N in prologue; mov rsp, rbp; pop rbp; ret in epilogue. Clang and GCC emit these around every function body.

Stack Frame Setup

Save caller's frame pointer. Establish new frame pointer. Allocate space for locals and spilled registers. On return, reverse the sequence and restore callee-saved registers.

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• Callee-saved: rbx, rbp, r12-r15 on x86-64
• Caller-saved argument registers may be clobbered
• Local size must be 16-byte aligned on macOS/Linux
• Epilogue must run on every exit path, including early return

Connecting to real compilers

Production compilers embed this step inside a longer pipeline. GCC flows through cpp, cc1, assembly, and ld; Clang uses the driver, Sema, LLVM IR passes, and a target backend. LLVM bitcode, JVM bytecode, and WASM are other familiar IRs at the same layer. The exercise isolates one pass so you can test it alone before chaining it to the next stage.

Why for this exercise

You will generate function prologue and epilogue assembly. This exercise asks you to emit stack frame setup and teardown instructions that bracket the function body per the target ABI.

Your Task

Generate the prologue and epilogue of x86-64 functions under the System V ABI: save the frame pointer and any callee-saved registers, reserve an aligned frame, spill register arguments to their home slots, and undo it all on the way out.

Input

One function per line:

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locals(...) and saves(...) may be empty or missing. Every parameter and local is 8 bytes. Callee-saved registers come from rbx r12 r13 r14 r15.

Frame layout (follow it exactly)

1. push rbp, mov rbp, rsp. At this point rsp is 16-byte aligned.
2. Push each saved register in the order listed: the k-th one lives at [rbp-8k].
3. The first six parameters arrive in rdi rsi rdx rcx r8 r9 and get home slots, then the locals get slots, continuing downward: the first slot is just below the last saved register.
4. Parameters seven and up are already on the caller's stack at [rbp+16], [rbp+24], … and are not copied.
5. Reserve N bytes with sub rsp, N, where N is the smallest value ≥ 8 × (slots) such that 8 × saves + N is a multiple of 16, so rsp is aligned for any call the body makes. Omit the sub when N is 0.
6. Store each register parameter to its slot: mov [rbp-OFF], REG.

Epilogue: if registers were saved, lea rsp, [rbp-8×saves] then pop them in reverse order; otherwise mov rsp, rbp. Then pop rbp and ret.

Output

For each function (a blank line between functions):

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The frame line lists parameters in order, then locals; S is the number of bytes below rbp (8 × saves + N). Instructions are indented 4 spaces.

Example

Input:

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Output:

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(4 slots = 32 bytes; 8 + 32 = 40 is not a multiple of 16, so N becomes 40.)

Requirements

1. Compute the whole layout (offsets, N) before printing anything.
2. The epilogue must restore rsp from rbp before popping, so it is correct whatever the body did to the stack.

Hidden tests cover more than six parameters, functions with no locals or no saved registers, several saved registers, and a function with nothing to save or store at all.