cargo-llvm-lines

Count the number of lines of LLVM IR across all instantiations of a generic function. Based on a suggestion from @eddyb on how to count monomorphized functions in order to debug compiler memory usage, executable size and compile time.

<eddyb> unoptimized LLVM IR <eddyb> first used grep '^define' to get only the lines defining function bodies <eddyb> then regex replace in my editor to remove everything before @ and everything after ( <eddyb> then sort | uniq -c

Installation

Install with cargo install cargo-llvm-lines.

Output

One line per function with three columns of output:

1. Total number of lines of LLVM IR generated across all instantiations of the function (and the percentage of the total).
2. Number of instantiations of the function (and the percentage of the total). For a generic function, roughly the number of distinct combinations of generic type parameters it is called with.
3. Name of the function.

$ cargo llvm-lines | head -20

  Lines         Copies       Function name
  -----         ------       -------------
  30737 (100%)  1107 (100%)  (TOTAL)
   1395 (4.5%)    83 (7.5%)  core::ptr::drop_in_place
    760 (2.5%)     2 (0.2%)  alloc::slice::merge_sort
    734 (2.4%)     2 (0.2%)  alloc::raw_vec::RawVec<T,A>::reserve_internal
    666 (2.2%)     1 (0.1%)  cargo_llvm_lines::count_lines
    490 (1.6%)     1 (0.1%)  <std::process::Command as cargo_llvm_lines::PipeTo>::pipe_to
    476 (1.5%)     6 (0.5%)  core::result::Result<T,E>::map
    440 (1.4%)     1 (0.1%)  cargo_llvm_lines::read_llvm_ir
    422 (1.4%)     2 (0.2%)  alloc::slice::merge
    399 (1.3%)     4 (0.4%)  alloc::vec::Vec<T>::extend_desugared
    388 (1.3%)     2 (0.2%)  alloc::slice::insert_head
    366 (1.2%)     5 (0.5%)  core::option::Option<T>::map
    304 (1.0%)     6 (0.5%)  alloc::alloc::box_free
    296 (1.0%)     4 (0.4%)  core::result::Result<T,E>::map_err
    295 (1.0%)     1 (0.1%)  cargo_llvm_lines::wrap_args
    291 (0.9%)     1 (0.1%)  core::char::methods::<impl char>::encode_utf8
    286 (0.9%)     1 (0.1%)  cargo_llvm_lines::run_cargo_rustc
    284 (0.9%)     4 (0.4%)  core::option::Option<T>::ok_or_else

Multicrate Projects

Interpreting the output in the presence of multiple crates and generics can be tricky. cargo llvm-lines only shows the contribution of the root crate; dependencies are not included. To assess the contribution of an intermediate crate, use the -p flag:

$ cargo llvm-lines -p some-depenency

Note however, that Rust generics are monomorphised — a generic function will be accounted for in the crates that use it, rather than in the defining crate.

There is a trick to get a holistic view: enabling link time optimization causes all code generation to happen in the root crate. So you can use the following invocation to get a full picture:

$ CARGO_PROFILE_RELEASE_LTO=fat cargo llvm-lines --release

License