Profiling the compiler

This section talks about how to profile the compiler and find out where it spends its time.

Depending on what you're trying to measure, there are several different approaches:

  • If you want to see if a PR improves or regresses compiler performance:

    • The rustc-perf project makes this easy and can be triggered to run on a PR via the @rustc-perf bot.
  • If you want a medium-to-high level overview of where rustc is spending its time:

    • The -Z self-profile flag and measureme tools offer a query-based approach to profiling. See their docs for more information.
  • If you want function level performance data or even just more details than the above approaches:

    • Consider using a native code profiler such as perf
    • or tracy for a nanosecond-precision, full-featured graphical interface.
  • If you want a nice visual representation of the compile times of your crate graph, you can use cargo's -Z timings flag, eg. cargo -Z timings build. You can use this flag on the compiler itself with CARGOFLAGS="-Z timings" ./ build

Optimizing rustc's bootstrap times with cargo-llvm-lines

Using cargo-llvm-lines you can count the number of lines of LLVM IR across all instantiations of a generic function. Since most of the time compiling rustc is spent in LLVM, the idea is that by reducing the amount of code passed to LLVM, compiling rustc gets faster.

Example usage:

cargo install cargo-llvm-lines
# On a normal crate you could now run `cargo llvm-lines`, but isn't normal :P

# Do a clean before every run, to not mix in the results from previous runs.
./ clean
RUSTFLAGS="--emit=llvm-ir" ./ build --stage 0 compiler/rustc

# Single crate, eg. rustc_middle
cargo llvm-lines --files ./build/x86_64-unknown-linux-gnu/stage0-rustc/x86_64-unknown-linux-gnu/debug/deps/rustc_middle-a539a639bdab6513.ll > llvm-lines-middle.txt
# Specify all crates of the compiler. (Relies on the glob support of your shell.)
cargo llvm-lines --files ./build/x86_64-unknown-linux-gnu/stage0-rustc/x86_64-unknown-linux-gnu/debug/deps/*.ll > llvm-lines.txt

Example output:

  Lines            Copies        Function name
  -----            ------        -------------
  11802479 (100%)  52848 (100%)  (TOTAL)
   1663902 (14.1%)   400 (0.8%)  rustc_query_system::query::plumbing::get_query_impl::{{closure}}
    683526 (5.8%)  10579 (20.0%) core::ptr::drop_in_place
    568523 (4.8%)    528 (1.0%)  rustc_query_system::query::plumbing::get_query_impl
    472715 (4.0%)   1134 (2.1%)  hashbrown::raw::RawTable<T>::reserve_rehash
    306782 (2.6%)   1320 (2.5%)  rustc_middle::ty::query::plumbing::<impl rustc_query_system::query::QueryContext for rustc_middle::ty::context::TyCtxt>::start_query::{{closure}}::{{closure}}::{{closure}}
    212800 (1.8%)    514 (1.0%)  rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
    194813 (1.7%)    124 (0.2%)  rustc_query_system::query::plumbing::force_query_impl
    158488 (1.3%)      1 (0.0%)  rustc_middle::ty::query::<impl rustc_middle::ty::context::TyCtxt>::alloc_self_profile_query_strings
    119768 (1.0%)    418 (0.8%)  core::ops::function::FnOnce::call_once
    119644 (1.0%)      1 (0.0%)  rustc_target::spec::load_specific
    104153 (0.9%)      7 (0.0%)  rustc_middle::ty::context::_DERIVE_rustc_serialize_Decodable_D_FOR_TypeckResults::<impl rustc_serialize::serialize::Decodable<__D> for rustc_middle::ty::context::TypeckResults>::decode::{{closure}}
     81173 (0.7%)      1 (0.0%)  rustc_middle::ty::query::stats::query_stats
     80306 (0.7%)   2029 (3.8%)  core::ops::function::FnOnce::call_once{{vtable.shim}}
     78019 (0.7%)   1611 (3.0%)  stacker::grow::{{closure}}
     69720 (0.6%)   3286 (6.2%)  <&T as core::fmt::Debug>::fmt
     56327 (0.5%)    186 (0.4%)  rustc_query_system::query::plumbing::incremental_verify_ich
     49714 (0.4%)     14 (0.0%)  rustc_mir::dataflow::framework::graphviz::BlockFormatter<A>::write_node_label

Since this doesn't seem to work with incremental compilation or check, you will be compiling rustc a lot. I recommend changing a few settings in config.toml to make it bearable:

# A debug build takes _a third_ as long on my machine,
# but compiling more than stage0 rustc becomes unbearably slow.
optimize = false

# We can't use incremental anyway, so we disable it for a little speed boost.
incremental = false
# We won't be running it, so no point in compiling debug checks.
debug = false

# Using a single codegen unit gives less output, but is slower to compile.
codegen-units = 0  # num_cpus

The llvm-lines output is affected by several options. optimize = false increases it from 2.1GB to 3.5GB and codegen-units = 0 to 4.1GB.

MIR optimizations have little impact. Compared to the default RUSTFLAGS="-Z mir-opt-level=1", level 0 adds 0.3GB and level 2 removes 0.2GB. As of January 2021, inlining currently only happens in LLVM but this might change in the future.