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, see the rustc-perf chapter for requesting a benchmarking run.
If you want a medium-to-high level overview of where
rustcis spending its time:
-Z self-profileflag 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:
If you want a nice visual representation of the compile times of your crate graph, you can use cargo's
cargo build --timings. You can use this flag on the compiler itself with
CARGOFLAGS="--timings" ./x.py build
If you want to profile memory usage, you can use various tools depending on what operating system you are using.
- For Windows, read our WPA guide.
Optimizing rustc's bootstrap times with
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.
cargo-llvm-lines together with somewhat custom rustc build process, you can use
-C save-temps to obtain required LLVM IR. The option preserves temporary work products
created during compilation. Among those is LLVM IR that represents an input to the
optimization pipeline; ideal for our purposes. It is stored in files with
extension in LLVM bitcode format.
cargo install cargo-llvm-lines # On a normal crate you could now run `cargo llvm-lines`, but `x.py` isn't normal :P # Do a clean before every run, to not mix in the results from previous runs. ./x.py clean env RUSTFLAGS=-Csave-temps ./x.py build --stage 0 compiler/rustc # Single crate, e.g., rustc_middle. (Relies on the glob support of your shell.) # Convert unoptimized LLVM bitcode into a human readable LLVM assembly accepted by cargo-llvm-lines. for f in build/x86_64-unknown-linux-gnu/stage0-rustc/x86_64-unknown-linux-gnu/release/deps/rustc_middle-*.no-opt.bc; do ./build/x86_64-unknown-linux-gnu/llvm/bin/llvm-dis "$f" done cargo llvm-lines --files ./build/x86_64-unknown-linux-gnu/stage0-rustc/x86_64-unknown-linux-gnu/release/deps/rustc_middle-*.ll > llvm-lines-middle.txt # Specify all crates of the compiler. for f in build/x86_64-unknown-linux-gnu/stage0-rustc/x86_64-unknown-linux-gnu/release/deps/*.no-opt.bc; do ./build/x86_64-unknown-linux-gnu/llvm/bin/llvm-dis "$f" done cargo llvm-lines --files ./build/x86_64-unknown-linux-gnu/stage0-rustc/x86_64-unknown-linux-gnu/release/deps/*.ll > llvm-lines.txt
Example output for the compiler:
Lines Copies Function name ----- ------ ------------- 45207720 (100%) 1583774 (100%) (TOTAL) 2102350 (4.7%) 146650 (9.3%) core::ptr::drop_in_place 615080 (1.4%) 8392 (0.5%) std::thread::local::LocalKey<T>::try_with 594296 (1.3%) 1780 (0.1%) hashbrown::raw::RawTable<T>::rehash_in_place 592071 (1.3%) 9691 (0.6%) core::option::Option<T>::map 528172 (1.2%) 5741 (0.4%) core::alloc::layout::Layout::array 466854 (1.0%) 8863 (0.6%) core::ptr::swap_nonoverlapping_one 412736 (0.9%) 1780 (0.1%) hashbrown::raw::RawTable<T>::resize 367776 (0.8%) 2554 (0.2%) alloc::raw_vec::RawVec<T,A>::grow_amortized 367507 (0.8%) 643 (0.0%) rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl 355882 (0.8%) 6332 (0.4%) alloc::alloc::box_free 354556 (0.8%) 14213 (0.9%) core::ptr::write 354361 (0.8%) 3590 (0.2%) core::iter::traits::iterator::Iterator::fold 347761 (0.8%) 3873 (0.2%) rustc_middle::ty::context::tls::set_tlv 337534 (0.7%) 2377 (0.2%) alloc::raw_vec::RawVec<T,A>::allocate_in 331690 (0.7%) 3192 (0.2%) hashbrown::raw::RawTable<T>::find 328756 (0.7%) 3978 (0.3%) rustc_middle::ty::context::tls::with_context_opt 326903 (0.7%) 642 (0.0%) rustc_query_system::query::plumbing::try_execute_query
Since this doesn't seem to work with incremental compilation or
you will be compiling rustc a lot.
I recommend changing a few settings in
config.toml to make it bearable:
[rust] # 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 July 2022,
inlining happens in LLVM and GCC codegen backends,
missing only in the Cranelift one.