extract-asm git:20260219.c78d333A

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---
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):**
   - Disassemble with: `dumpbin /disasm <file>.obj`

   **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`
  - Windows: `build/CMakeFiles/<target>.dir/<config>/<source>.obj`