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2 changed files with 130 additions and 84 deletions
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@ -85,22 +85,12 @@
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</section>
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<section>
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<h2>it all starts at the source</h2>
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<pre><code data-trim>
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<pre><code data-trim class="language-rust">
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pub fn add(a: u8, b: u8) -> u8 {
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a.wrapping_add(b)
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}
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</code></pre>
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</section>
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<section>
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<h2>it gets processed</h2>
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<pre><code data-trim>
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#[attr = MacroUse {arguments: UseAll}]
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extern crate std;
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#[prelude_import]
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use std::prelude::rust_2024::*;
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fn add(a: u8, b: u8) -> u8 { a.wrapping_add(b) }
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</code></pre>
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</section>
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<section>
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<h2>until it doesn't even look like Rust anymore</h2>
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<p>MIR</p>
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@ -176,13 +166,12 @@
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crate --> cgu1["Codegen-Unit 1"]
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crate --> cgu2["Codegen-Unit 2"]
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crate --> cgu3["Codegen-Unit 3"]
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crate --> cgu4["Codegen-Unit 4"]
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</pre>
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</div>
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</section>
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<section>
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<h2>codegen units</h2>
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<pre><code>
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<pre><code data-trim class="language-rust">
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fn main() {}
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</code></pre>
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<div class="mermaid">
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@ -199,17 +188,31 @@ fn main() {}
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mainmir --> mainll
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mycgu1.rcgu.o --> my_binary
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std["std (and others)"] --> my_binary
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mycgu1.rcgu.o --> |link| my_binary
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std["std (and others)"] --> |link| my_binary
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</pre>
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</div>
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</section>
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<section data-markdown>
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<textarea data-template>
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## the linker
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can be a slow part for incremental builds
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- LLD (Linux (default for x86-64), Windows)
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- [mold (Linux)](https://github.com/rui314/mold)
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- [wild (Linux, experimental)](https://github.com/davidlattimore/wild)
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- MacOS default ld64 is fast already
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</textarea>
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</section>
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<section>
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<h2>codegen units (but more)</h2>
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<pre><code>
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<pre><code data-trim class="language-rust">
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fn main() {}
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mod foos {
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fn foo1() {}
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fn foo2() {}
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}
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</code></pre>
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<div class="mermaid">
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<pre>
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@ -243,7 +246,7 @@ fn foo2() {}
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<section>
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<h2>codegen units (cross-crate)</h2>
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<div style="display: flex; flex-direction: row; gap: 16px">
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<pre><code>
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<pre><code data-trim class="language-rust">
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fn add() {}
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</code></pre>
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<pre><code>
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@ -394,6 +397,7 @@ fn main() { math::add() }
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- spend N times optimizing the function
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- and there's duplicate instances!
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- `cargo-llvm-lines`
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- share-generics helps for non-release builds
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</textarea>
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</section>
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<section data-markdown>
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@ -552,67 +556,6 @@ fn main() { math::add() }
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- comes in many forms
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</textarea>
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</section>
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<!--
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# r-a
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base:
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Benchmark 1: cargo build --release
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Time (mean ± σ): 58.150 s ± 0.163 s [User: 758.211 s, System: 37.637 s]
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Range (min … max): 57.936 s … 58.321 s 5 runs
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thin:
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Benchmark 1: cargo build --release
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Time (mean ± σ): 63.999 s ± 0.105 s [User: 879.703 s, System: 40.045 s]
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Range (min … max): 63.921 s … 64.182 s 5 runs
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fat:
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Time (mean ± σ): 264.606 s ± 2.238 s [User: 570.800 s, System: 31.826 s]
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Range (min … max): 261.573 s … 267.297 s 5 runs
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# cargo
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base:
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Benchmark 1: cargo build --release
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Time (mean ± σ): 89.381 s ± 0.460 s [User: 689.874 s, System: 55.347 s]
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Range (min … max): 88.605 s … 89.696 s 5 runs
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thin:
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Benchmark 1: cargo build --release
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Time (mean ± σ): 91.208 s ± 0.610 s [User: 757.353 s, System: 58.558 s]
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Range (min … max): 90.415 s … 92.112 s 5 runs
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fat:
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Time (mean ± σ): 212.215 s ± 2.062 s [User: 576.259 s, System: 50.961 s]
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Range (min … max): 208.662 s … 213.818 s 5 runs
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# ripgrep
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base:
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Time (mean ± σ): 7.507 s ± 0.223 s [User: 64.115 s, System: 4.514 s]
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Range (min … max): 7.357 s … 7.882 s 5 runs
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thin:
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Time (mean ± σ): 9.285 s ± 0.019 s [User: 81.101 s, System: 5.241 s]
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Range (min … max): 9.262 s … 9.308 s 5 runs
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fat:
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Time (mean ± σ): 29.202 s ± 0.279 s [User: 51.015 s, System: 3.652 s]
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Range (min … max): 28.860 s … 29.574 s 5 runs
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# triagebot
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base:
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Time (mean ± σ): 74.532 s ± 0.378 s [User: 766.778 s, System: 58.719 s]
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Range (min … max): 74.105 s … 75.109 s 5 runs
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thin:
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Time (mean ± σ): 89.505 s ± 0.299 s [User: 1523.951 s, System: 102.429 s]
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Range (min … max): 89.024 s … 89.796 s 5 runs
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fat:
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Time (mean ± σ): 273.275 s ± 1.694 s [User: 929.604 s, System: 65.856 s]
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Range (min … max): 271.007 s … 275.619 s 5 runs
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-->
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<section>
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<h2>lto = "fat" (monolithic)</h2>
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<div class="mermaid">
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@ -645,7 +588,7 @@ Range (min … max): 271.007 s … 275.619 s 5 runs
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end
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</pre>
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</div>
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<p>compiles r-a 237583957% more slowly</p>
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<p>easily compiles 2-4x more slowly</p>
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</section>
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<section>
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<h2>lto = "thin" (sharded)</h2>
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@ -665,14 +608,16 @@ Range (min … max): 271.007 s … 275.619 s 5 runs
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end
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subgraph my crate
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subgraph thinltosummary[ThinLTO Summary]
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subgraph thinltosummary[ThinLTO Index]
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end
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subgraph thinlto1[ThinLTO 1]
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addll["add (LLVM IR)"]
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subll["sub (LLVM IR)"]
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end
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subgraph thinlto2[ThinLTO 2]
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subll["sub (LLVM IR)"]
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end
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subgraph thinlto3[ThinLTO 3]
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mainll["main (LLVM IR)"]
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end
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@ -686,10 +631,14 @@ Range (min … max): 271.007 s … 275.619 s 5 runs
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thinlto1 --> my_binary
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thinlto2 --> my_binary
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thinlto3 --> my_binary
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end
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</pre>
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</div>
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<p>compiles r-a 70% more slowly</p>
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<p>
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compiles ~1.1x-1.2x more slowly |
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<a href="https://www.youtube.com/watch?v=p9nH2vZ2mNo">ThinLTO Talk</a>
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</p>
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</section>
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<section data-markdown>
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<textarea data-template>
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97
slides/2025-10-10-how-rust-compiles/lto-speed.py
Normal file
97
slides/2025-10-10-how-rust-compiles/lto-speed.py
Normal file
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@ -0,0 +1,97 @@
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"""
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base:
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Benchmark 1: cargo build --release
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Time (mean ± σ): 58.150 s ± 0.163 s [User: 758.211 s, System: 37.637 s]
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Range (min … max): 57.936 s … 58.321 s 5 runs
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thin:
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Benchmark 1: cargo build --release
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Time (mean ± σ): 63.999 s ± 0.105 s [User: 879.703 s, System: 40.045 s]
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Range (min … max): 63.921 s … 64.182 s 5 runs
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fat:
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Time (mean ± σ): 264.606 s ± 2.238 s [User: 570.800 s, System: 31.826 s]
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Range (min … max): 261.573 s … 267.297 s 5 runs
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# cargo
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base:
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Benchmark 1: cargo build --release
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Time (mean ± σ): 89.381 s ± 0.460 s [User: 689.874 s, System: 55.347 s]
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Range (min … max): 88.605 s … 89.696 s 5 runs
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thin:
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Benchmark 1: cargo build --release
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Time (mean ± σ): 91.208 s ± 0.610 s [User: 757.353 s, System: 58.558 s]
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Range (min … max): 90.415 s … 92.112 s 5 runs
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fat:
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Time (mean ± σ): 212.215 s ± 2.062 s [User: 576.259 s, System: 50.961 s]
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Range (min … max): 208.662 s … 213.818 s 5 runs
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# ripgrep
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base:
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Time (mean ± σ): 7.507 s ± 0.223 s [User: 64.115 s, System: 4.514 s]
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Range (min … max): 7.357 s … 7.882 s 5 runs
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thin:
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Time (mean ± σ): 9.285 s ± 0.019 s [User: 81.101 s, System: 5.241 s]
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Range (min … max): 9.262 s … 9.308 s 5 runs
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fat:
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Time (mean ± σ): 29.202 s ± 0.279 s [User: 51.015 s, System: 3.652 s]
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Range (min … max): 28.860 s … 29.574 s 5 runs
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# triagebot
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base:
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Time (mean ± σ): 74.532 s ± 0.378 s [User: 766.778 s, System: 58.719 s]
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Range (min … max): 74.105 s … 75.109 s 5 runs
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thin:
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Time (mean ± σ): 89.505 s ± 0.299 s [User: 1523.951 s, System: 102.429 s]
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Range (min … max): 89.024 s … 89.796 s 5 runs
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fat:
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Time (mean ± σ): 273.275 s ± 1.694 s [User: 929.604 s, System: 65.856 s]
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Range (min … max): 271.007 s … 275.619 s 5 runs
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"""
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data = [
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{
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"name": "r-a",
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"base": 58.150,
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"thin": 63.999,
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"fat": 264.606,
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},
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{
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"name": "cargo",
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"base": 89.381,
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"thin": 91.208,
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"fat": 212.215,
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},
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{
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"name": "ripgrep",
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"base": 7.507,
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"thin": 9.285,
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"fat": 29.202,
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},
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{
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"name": "triagebot",
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"base": 74.532,
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"thin": 89.505,
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"fat": 273.275,
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}
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]
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for bench in data:
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print(f"{bench["name"]} ThinLTO: {bench["thin"] / bench["base"]}")
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print(f"{bench["name"]} Fat LTO: {bench["fat"] / bench["base"]}")
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def avg_of(scenario: str) -> float:
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avg_percentage = sum([bench[scenario] / bench["base"] for bench in data]) / len(data)
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return avg_percentage
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print(f"ThinLTO: {avg_of("thin")}")
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print(f"Fat LTO: {avg_of("fat")}")
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