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| <h1 id="profiling-the-compiler"><a class="header" href="#profiling-the-compiler">Profiling the compiler</a></h1> |
| <p>This section talks about how to profile the compiler and find out where it spends its time.</p> |
| <p>Depending on what you’re trying to measure, there are several different approaches:</p> |
| <ul> |
| <li> |
| <p>If you want to see if a PR improves or regresses compiler performance, |
| see the <a href="tests/perf.html">rustc-perf chapter</a> for requesting a benchmarking run.</p> |
| </li> |
| <li> |
| <p>If you want a medium-to-high level overview of where <code>rustc</code> is spending its time:</p> |
| <ul> |
| <li>The <code>-Z self-profile</code> flag and <a href="https://github.com/rust-lang/measureme">measureme</a> tools offer a query-based approach to profiling. |
| See <a href="https://github.com/rust-lang/measureme/blob/master/summarize/README.md">their docs</a> for more information.</li> |
| </ul> |
| </li> |
| <li> |
| <p>If you want function level performance data or even just more details than the above approaches:</p> |
| <ul> |
| <li>Consider using a native code profiler such as <a href="profiling/with_perf.html">perf</a></li> |
| <li>or <a href="https://github.com/nagisa/rust_tracy_client">tracy</a> for a nanosecond-precision, |
| full-featured graphical interface.</li> |
| </ul> |
| </li> |
| <li> |
| <p>If you want a nice visual representation of the compile times of your crate graph, |
| you can use <a href="https://doc.rust-lang.org/nightly/cargo/reference/timings.html">cargo’s <code>--timings</code> flag</a>, |
| e.g. <code>cargo build --timings</code>. |
| You can use this flag on the compiler itself with <code>CARGOFLAGS="--timings" ./x build</code></p> |
| </li> |
| <li> |
| <p>If you want to profile memory usage, you can use various tools depending on what operating system |
| you are using.</p> |
| <ul> |
| <li>For Windows, read our <a href="profiling/wpa_profiling.html">WPA guide</a>.</li> |
| </ul> |
| </li> |
| </ul> |
| <h2 id="optimizing-rustcs-bootstrap-times-with-cargo-llvm-lines"><a class="header" href="#optimizing-rustcs-bootstrap-times-with-cargo-llvm-lines">Optimizing rustc’s bootstrap times with <code>cargo-llvm-lines</code></a></h2> |
| <p>Using <a href="https://github.com/dtolnay/cargo-llvm-lines">cargo-llvm-lines</a> 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.</p> |
| <p>To use <code>cargo-llvm-lines</code> together with somewhat custom rustc build process, you can use |
| <code>-C save-temps</code> 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 <code>*.no-opt.bc</code> |
| extension in LLVM bitcode format.</p> |
| <p>Example usage:</p> |
| <pre><code>cargo install cargo-llvm-lines |
| # On a normal crate you could now run `cargo llvm-lines`, but `x` isn't normal :P |
| |
| # Do a clean before every run, to not mix in the results from previous runs. |
| ./x clean |
| env RUSTFLAGS=-Csave-temps ./x 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 |
| </code></pre> |
| <p>Example output for the compiler:</p> |
| <pre><code> 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 |
| </code></pre> |
| <p>Since this doesn’t seem to work with incremental compilation or <code>./x check</code>, |
| you will be compiling rustc <em>a lot</em>. |
| I recommend changing a few settings in <code>bootstrap.toml</code> to make it bearable:</p> |
| <pre><code>[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 |
| </code></pre> |
| <p>The llvm-lines output is affected by several options. |
| <code>optimize = false</code> increases it from 2.1GB to 3.5GB and <code>codegen-units = 0</code> to 4.1GB.</p> |
| <p>MIR optimizations have little impact. Compared to the default <code>RUSTFLAGS="-Z mir-opt-level=1"</code>, level 0 adds 0.3GB and level 2 removes 0.2GB. |
| As of <!-- date-check --> July 2022, |
| inlining happens in LLVM and GCC codegen backends, |
| missing only in the Cranelift one.</p> |
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