extract-asm · diff
git:20260309.5fd6737 to git:20260309.3e3a523
34 added, 45 removed. Audit A to A.
---
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:
+ ## Arguments
- `$1` = input file (`.o`, `.obj`, `.s`, or `.asm`)
- `$2` = function name
Examples:
- `build/CMakeFiles/zlib-ng.dir/trees.c.o compress_block`
- `/tmp/output.s my_function`
- `build/CMakeFiles/zlib-ng.dir/Release/trees.obj compress_block`
## Steps
- 1. Extract assembly for the function from the input:
+ 1. **Extract assembly** for the function based on file type:
- **If `.o` object file (Mach-O / ELF):**
- - On macOS, use `llvm-objdump` (ships with Xcode). Mach-O symbols have a `_` prefix:
- ```
- llvm-objdump --disassemble-symbols=_<function_name> --no-show-raw-insn <file>.o
- ```
- - On Linux, use GNU objdump (binutils 2.32+):
- ```
- objdump --disassemble=<function_name> --no-show-raw-insn <file>.o
- ```
+ | Platform | File | Command |
+ |----------|------|---------|
+ | macOS | `.o` | `llvm-objdump --disassemble-symbols=_<name> --no-show-raw-insn <file>` |
+ | Linux | `.o` | `objdump --disassemble=<name> --no-show-raw-insn <file>` |
+ | Windows | `.obj` | `llvm-objdump --disassemble-symbols=<name> --no-show-raw-insn <file>` |
- **If `.obj` object file (PE/COFF — Windows):**
- - Prefer `llvm-objdump` (ships with VS LLVM/clang workload or standalone LLVM install):
- ```
- llvm-objdump --disassemble-symbols=<function_name> --no-show-raw-insn <file>.obj
- ```
- - Fallback to `dumpbin` if `llvm-objdump` is not available. `dumpbin` requires the MSVC environment:
+ Symbol names differ by platform — use this when passing to `--disassemble-symbols` or searching in `.s` files:
+
+ | Platform | C function `foo` | C++ function `foo` |
+ |----------|------------------|--------------------|
+ | macOS (Mach-O) | `_foo` | `_Z3foov` (Itanium mangled, `_` prefixed) |
+ | Linux (ELF) | `foo` | `_Z3foov` (Itanium mangled) |
+ | Windows (PE/COFF) | `foo` | `?foo@@YAXXZ` (MSVC decorated) |
+
+ Notes:
+ - macOS `llvm-objdump` ships with Xcode.
+ - Linux requires GNU binutils 2.32+.
+ - Windows: if `llvm-objdump` is not available, fall back to `dumpbin`:
```
VSPATH=$(vswhere -latest -property installationPath)
cmd.exe /c "call \"${VSPATH}\VC\Auxiliary\Build\vcvarsall.bat\" amd64 >nul 2>&1 && dumpbin /disasm <file>.obj"
```
- Then extract the named function: find `<function_name>:` label, collect until the next label or summary line.
+ Then manually extract the function: find `<function_name>:` label, collect until the next label or summary line.
- **If `.s` or `.asm` assembly file:**
- - Read the file and extract the named function manually:
- - **Mach-O:** Find `_<function_name>:`, collect until next `^_[a-zA-Z]` label (excluding `_Ltmp`, `_LCFI`, `_LBB` prefixes).
- - **ELF/other:** Find `<function_name>:`, collect until the next function label.
+ **For `.s` or `.asm` files:** Read the file and extract manually:
+ - **Mach-O:** Find `_<function_name>:`, collect until next `^_[a-zA-Z]` label (excluding `_Ltmp`, `_LCFI`, `_LBB` prefixes).
+ - **ELF/other:** Find `<function_name>:`, collect until the next function label.
- 2. From the extracted output:
+ 2. **Clean up** the extracted output:
- 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.
+ 3. **Output** the assembly to stdout with a summary line showing 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.
+ | Architecture | Key details |
+ |-------------|-------------|
+ | **x86-64** | `offset(%reg)` memory syntax. `leaq (%rip)` is PC-relative, not a load. |
+ | **AArch64** | `[reg, #offset]` memory syntax. Loads: `ldr`, `ldur`, `ldp`, `ldrb`, `ldrh`, `ldrsw`. Stores: `str`, `stur`, `stp`, `strb`, `strh`. Apple Silicon compiles natively. |
+ | **x86-64 (MSVC)** | Intel syntax (dest first): `mov rax, [rcx+168]`. Args in `rcx`, `rdx`, `r8`, `r9`. C++ names may be decorated (`?name@@...`). |
- ## General
- - Object files from CMake builds are typically at:
- - Unix: `build/CMakeFiles/<target>.dir/<source>.o`
- - Windows (Ninja): `build/CMakeFiles/<target>.dir/<source>.obj`
- - Windows (VS generator): `build/CMakeFiles/<target>.dir/<config>/<source>.obj`
+ ## CMake Build Paths
+ - Unix: `build/CMakeFiles/<target>.dir/<source>.o`
+ - Windows (Ninja): `build/CMakeFiles/<target>.dir/<source>.obj`
+ - Windows (VS): `build/CMakeFiles/<target>.dir/<config>/<source>.obj`