git:20260219.c78d333 to git:20260228.0092da9

---

name: extract-asm

description: Extract the assembly of a specific function from an object file or assembly file

argument-hint: "<file> <function-name>"

---

# Extract Assembly for a Function

Extract the assembly of a specific function from an object file or assembly file.

## Prerequisites

The input must be a compiled object file (`.o` or `.obj`) or assembly file (`.s` or `.asm`). If you have source code, compile it first (e.g. via CMake build or standalone `clang -S`).

## Usage

Invoke with: the input file and function name.

Arguments:

- `$1` = input file (`.o`, `.obj`, `.s`, or `.asm`)

- `$2` = function name

Examples:

- `build/CMakeFiles/zlib-ng.dir/trees.c.o compress_block`

- `/tmp/output.s my_function`

- `build/CMakeFiles/zlib-ng.dir/Release/trees.obj compress_block`

## Steps

1. Obtain assembly from the input:

**If `.o` object file (Mach-O / ELF):**

- Disassemble with: `objdump -d --no-show-raw-insn <file>.o`

**If `.obj` object file (PE/COFF — Windows):**

**If `.s` or `.asm` assembly file:**

- Use directly.

2. Extract the named function:

**Mach-O:** Find `_<function_name>:`, collect until next `^_[a-zA-Z]` label (excluding `_Ltmp`, `_LCFI`, `_LBB` prefixes).

**ELF:** Find `<function_name>:` in objdump output, collect until the next function header.

**PE/COFF (dumpbin):** Find `<function_name>:` label, collect until the next label or summary line.

Then for all formats:

- Filter out assembler directives (`.` prefix), comments (`#`, `;`, `//`, `@`), and blank lines

- Keep only instruction lines

3. Output the extracted assembly to stdout. Print a summary line with the instruction count.

## Architecture Notes

### x86-64

- `leaq` instructions with `(%rip)` are PC-relative address computations, not memory loads.

- Memory operands use `offset(%reg)` syntax (e.g. `168(%rdi)` loads from rdi+168).

### AArch64

- Memory operands use `[reg, #offset]` syntax (e.g. `[x0, #168]` loads from x0+168).

- Load instructions: `ldr`, `ldur`, `ldp`, `ldrb`, `ldrh`, `ldrsw`. Store: `str`, `stur`, `stp`, `strb`, `strh`.

- On Apple Silicon this compiles natively — no cross-compilation needed.

### x86-64 (Windows/MSVC)

- Uses Intel syntax by default (destination first): `mov rax, [rcx+168]`.

- Calling convention: first four args in `rcx`, `rdx`, `r8`, `r9` (not `rdi`, `rsi` like SysV).

- Function names may be decorated (e.g. `?name@@...` for C++). C functions are undecorated.

## General

- Object files from CMake builds are typically at:

- Unix: `build/CMakeFiles/<target>.dir/<source>.o`