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@ -54,35 +54,22 @@
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</div>
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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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# speed 🚀
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</textarea>
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</section>
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<section>
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<h2>behind cargo build</h2>
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<p>cargo vs rustc</p>
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<div style="display: grid; grid-template-columns: 2fr 1fr 2fr">
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<pre><code data-trim>
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$ cargo build -v
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</code></pre>
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<div style="display: flex; align-items: center">cargo</div>
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<div style="display: flex; flex-direction: column; align-items: flex-start">
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<div>rustc (clap)</div>
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<div>rustc (tokio)</div>
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<div>rustc (tracing)</div>
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<div>rustc (yourcrate)</div>
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</div>
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<div>
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asciinema of cargo build -v
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</div>
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</section>
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<section>
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<h2>what does rustc like, do?</h2>
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<h4>a quick overview of the compilation phases</h4>
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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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#[no_mangle]
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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>the frontend and the backend</h2>
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<div class="mermaid">
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@ -98,36 +85,35 @@
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</div>
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</section>
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<section>
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<h2>lex, parse, resolve, typecheck, all these fancy things</h2>
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<div class="mermaid">
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<pre>
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%%{init: {'theme': 'dark', 'themeVariables': { 'darkMode': true, 'fontSize': '25px' }}}%%
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flowchart TB
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function --> return
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function --> params
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params --> a_def[a]
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params --> b_def[b]
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function --> body
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body --> cl["method call"]
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cl --> a_use[a]
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cl --> wrapping_add
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cl --> b_use[b]
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</pre>
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</div>
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<h2>it all starts at the source</h2>
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<pre><code data-trim>
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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>so you want to compile a crate</h2>
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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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<img src="add-runtime-mir.svg" />
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</section>
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<section data-markdown>
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<textarea data-template>
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## so you want to compile a crate
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LLVM IR
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## further going to LLVM IR
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```
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; meow::add
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define noundef i8 @add(i8 noundef %a, i8 noundef %b) unnamed_addr #0 {
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define noundef i8 @add(i8 noundef %a, i8 noundef %b) #0 {
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start:
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%_0 = add i8 %b, %a
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ret i8 %_0
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@ -137,8 +123,7 @@
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</section>
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<section data-markdown>
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<textarea data-template>
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## so you want to compile a crate
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Assembly
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## and then you're done
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```
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<add>:
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@ -149,9 +134,7 @@
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</section>
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<section data-markdown>
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<textarea data-template>
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## but compiling a ton of crates can't be that simple!
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- yes
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## ok but why does my program compile so slowly now?
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</textarea>
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</section>
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<section data-markdown>
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@ -161,6 +144,29 @@
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- the how, what, and when of invoking LLVM
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</textarea>
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</section>
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<section>
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<h2>a crate - the compilation unit</h2>
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<p>quite big</p>
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<p>in C it's just a single file</p>
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</section>
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<section>
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<h2>a codegen unit</h2>
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<p>LLVM is single-threaded</p>
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<p>rustc: hi LLVM, look we are like a C file, now be fast</p>
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<p>~1-256 depending on size and configuration (⚙️)</p>
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<div class="mermaid">
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<pre>
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%%{init: {'theme': 'dark', 'themeVariables': { 'darkMode': true, 'fontSize': '25px' }}}%%
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flowchart LR
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crate
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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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@ -267,6 +273,14 @@ fn main() { math::add() }
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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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# so compile times just depend on the amount of functions?
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- yes...
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- but not source functions!
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</textarea>
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</section>
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<section data-markdown>
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<textarea data-template>
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## generics
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@ -358,6 +372,19 @@ fn main() { math::add() }
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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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# generics are slow to compile
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- spend N times optimizing the function
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- and there's duplicate instances!
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</textarea>
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</section>
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<section data-markdown>
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<textarea data-template>
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# and the duplicates get worse
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</textarea>
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</section>
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<section data-markdown>
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<textarea data-template>
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## inlining
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@ -473,17 +500,29 @@ fn main() { math::add() }
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</section>
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<section data-markdown>
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<textarea data-template>
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## `#[inline]`
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- `#[inline]` enables cross-crate inlining of non-generic functions
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## `#[inline]` (⚙️)
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- for non-generic functions
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- for very small functions, this happens automatically
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- for other functions, it doesn't, because it would be slow (try with `-Zcross-crate-inline-threshold=always`)
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- for other functions, it doesn't, because it would be slow
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- don't over-apply it in a library, but also don't forget about it
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- benchmark!
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</textarea>
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</section>
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<section data-markdown>
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<textarea data-template>
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## but i want maximal performance...
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## being lazy has advantages
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- that's why i wrote most of this talk last week
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- `#[inline]` means that the function is *never* instantiated if it's never used!
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https://blog.rust-lang.org/inside-rust/2025/07/15/call-for-testing-hint-mostly-unused/
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</textarea>
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</section>
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<section data-markdown>
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<textarea data-template>
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## but performance is great, i love performance
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- its ok i can wait forever
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</textarea>
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</section>
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<section data-markdown>
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@ -586,20 +625,40 @@ fn main() { math::add() }
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</section>
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<section data-markdown>
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<textarea data-template>
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## inlining across codegen units in the same crate
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## there was some LTO all along
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- ThinLTO across different codegen units by default
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- in release mode, automatic ThinLTO across codegen units in the same crate
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</textarea>
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</section>
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<section data-markdown>
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<textarea data-template>
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## `Cargo.toml` config
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## how do i make my program run quickly?
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```toml
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[profile.release]
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lto = "thin"
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codegen-units = 1
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```
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- let the compiler inline functions
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- libraries: remember `#[inline]`
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- binaries: you want LTO
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- really, it really needs to inline your functions
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- without it, it's so over
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- and read this: https://nnethercote.github.io/perf-book
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</textarea>
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</section>
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<section data-markdown>
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<textarea data-template>
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## how do i make my program compile quickly?
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- reduce the amount and size of functions
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- importantly: in LLVM IR, not necessarily source!
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- duplicate and frequent instantiations are bad
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- and read this:
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- https://corrode.dev/blog/tips-for-faster-rust-compile-times
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- https://doc.rust-lang.org/nightly/cargo/guide/build-performance.html
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</textarea>
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</section>
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<section data-markdown>
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<textarea data-template>
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## and both? 🥺👉👈
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- no
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</textarea>
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</section>
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<section data-markdown>
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