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| <h1 id="monomorphization"><a class="header" href="#monomorphization">Monomorphization</a></h1> |
| <p>As you probably know, Rust has a very expressive type system that has extensive |
| support for generic types. But of course, assembly is not generic, so we need |
| to figure out the concrete types of all the generics before the code can |
| execute.</p> |
| <p>Different languages handle this problem differently. For example, in some |
| languages, such as Java, we may not know the most precise type of value until |
| runtime. In the case of Java, this is ok because (almost) all variables are |
| reference values anyway (i.e. pointers to a heap allocated object). This |
| flexibility comes at the cost of performance, since all accesses to an object |
| must dereference a pointer.</p> |
| <p>Rust takes a different approach: it <em>monomorphizes</em> all generic types. This |
| means that compiler stamps out a different copy of the code of a generic |
| function for each concrete type needed. For example, if I use a <code>Vec<u64></code> and |
| a <code>Vec<String></code> in my code, then the generated binary will have two copies of |
| the generated code for <code>Vec</code>: one for <code>Vec<u64></code> and another for <code>Vec<String></code>. |
| The result is fast programs, but it comes at the cost of compile time (creating |
| all those copies can take a while) and binary size (all those copies might take |
| a lot of space).</p> |
| <p>Monomorphization is the first step in the backend of the Rust compiler.</p> |
| <h2 id="collection"><a class="header" href="#collection">Collection</a></h2> |
| <p>First, we need to figure out what concrete types we need for all the generic |
| things in our program. This is called <em>collection</em>, and the code that does this |
| is called the <em>monomorphization collector</em>.</p> |
| <p>Take this example:</p> |
| <pre><pre class="playground"><code class="language-rust">fn banana() { |
| peach::<u64>(); |
| } |
| |
| fn main() { |
| banana(); |
| }</code></pre></pre> |
| <p>The monomorphization collector will give you a list of <code>[main, banana, peach::<u64>]</code>. These are the functions that will have machine code generated |
| for them. Collector will also add things like statics to that list.</p> |
| <p>See <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_monomorphize/collector/index.html">the collector rustdocs</a> for more info.</p> |
| <p>The monomorphization collector is run just before MIR lowering and codegen. |
| <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_codegen_ssa/base/fn.codegen_crate.html"><code>rustc_codegen_ssa::base::codegen_crate</code></a> calls the |
| <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_monomorphize/partitioning/fn.collect_and_partition_mono_items.html"><code>collect_and_partition_mono_items</code></a> query, which does monomorphization |
| collection and then partitions them into <a href="../appendix/glossary.html#codegen-unit">codegen |
| units</a>.</p> |
| <h2 id="codegen-unit-cgu-partitioning"><a class="header" href="#codegen-unit-cgu-partitioning">Codegen Unit (CGU) partitioning</a></h2> |
| <p>For better incremental build times, the CGU partitioner creates two CGU for each source level |
| modules. One is for "stable" i.e. non-generic code and the other is more volatile code i.e. |
| monomorphized/specialized instances.</p> |
| <p>For dependencies, consider Crate A and Crate B, such that Crate B depends on Crate A. |
| The following table lists different scenarios for a function in Crate A that might be used by one |
| or more modules in Crate B.</p> |
| <div class="table-wrapper"><table><thead><tr><th>Crate A function</th><th>Behavior</th></tr></thead><tbody> |
| <tr><td>Non-generic function</td><td>Crate A function doesn't appear in any codegen units of Crate B</td></tr> |
| <tr><td>Non-generic <code>#[inline]</code> function</td><td>Crate A function appears within a single CGU of Crate B, and exists even after post-inlining stage</td></tr> |
| <tr><td>Generic function</td><td>Regardless of inlining, all monomorphized (specialized) functions <br> from Crate A appear within a single codegen unit for Crate B. <br> The codegen unit exists even after the post inlining stage.</td></tr> |
| <tr><td>Generic <code>#[inline]</code> function</td><td>- same -</td></tr> |
| </tbody></table> |
| </div> |
| <p>For more details about the partitioner read the module level <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_monomorphize/partitioning/index.html">documentation</a>.</p> |
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