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Viewing assembly of Rust code

Generating assembly text

To see the assembly code that Rust compiles down to, do the following:

First, install the cargo-show-asm crate (NOT the outdated cargo-asm crate)

cargo install cargo-show-asm

Then, do:

cargo asm \
-p emu86rs \
--profile=release-with-debug \
--color \
--rust \
--att \
--lib \
--simplify \
--everything \
> everything.lst
  • -p emu86rs selects the emulator package
  • --profile=release-with-debug uses the release-with-debug profile, which will hopefully correlate with the build profile we will use for the debugger.
  • --color adds ANSI color codes, which makes the output look nice.
  • --rust interleaves Rust code in the assembly, so it's easy to see what corresponds to what.
  • --att outputs assembly in the AT&T syntax instead of Intel syntax.
  • --lib selects all code that falls under the lib module.
  • --simplify removes various non-assembly directives that just clutter things.
  • --everything will generate assembly for the entire program.

Other options:

  • --intel (the default) outputs assembly in the Intel syntax instead of AT&T syntax. However, this will make it not match up with the debugger disassembly output, so it may be easier to stick not specify this when trying to match it up with the debugger.

You can also change --lib for --bin to see the executable startup code. See cargo asm --help for more options.

Viewing the assembly

To view the listing file with colors in VSCode, be sure to install the ANSI Colors extension and then do ANSI TEXT: Open Preview.

Alternatively, do less -R main.lst in a terminal to see the colorized listing file.

x86 Assembly Syntax

There are basically two flavors of x86 assembly: AT&T/Linux, and Intel. For a quick refresher on the syntax, see x86 assembly language - Syntax.

Other x86-64 assembly tips:

  • TODO

llvm-mca

llvm-mca is a performance analysis tool that can be used to statically guess the performance of machine code for a specific CPU. It guesses Instructions Per Cycle (IPC) as well as hardware resource pressure.

To use it, first install the llvm Debian package:

sudo apt install llvm

This will install many llvm-based tools, including llvm-mca.

Next, run it like so:

 cargo asm --mca-intel --lib decode 9 > decode.mca.txt