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| <main> |
| <h1 id="mir-construction"><a class="header" href="#mir-construction">MIR construction</a></h1> |
| <p>The lowering of <a href="../hir.html">HIR</a> to <a href="./index.html">MIR</a> occurs for the following (probably incomplete) |
| list of items:</p> |
| <ul> |
| <li>Function and closure bodies</li> |
| <li>Initializers of <code>static</code> and <code>const</code> items</li> |
| <li>Initializers of enum discriminants</li> |
| <li>Glue and shims of any kind |
| <ul> |
| <li>Tuple struct initializer functions</li> |
| <li>Drop code (the <code>Drop::drop</code> function is not called directly)</li> |
| <li>Drop implementations of types without an explicit <code>Drop</code> implementation</li> |
| </ul> |
| </li> |
| </ul> |
| <p>The lowering is triggered by calling the <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_mir_transform/fn.mir_built.html"><code>mir_built</code></a> query. The MIR builder does |
| not actually use the HIR but operates on the <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_mir_build/thir/index.html">THIR</a> instead, processing THIR |
| expressions recursively.</p> |
| <p>The lowering creates local variables for every argument as specified in the signature. |
| Next, it creates local variables for every binding specified (e.g. <code>(a, b): (i32, String)</code>) |
| produces 3 bindings, one for the argument, and two for the bindings. Next, it generates |
| field accesses that read the fields from the argument and writes the value to the binding |
| variable.</p> |
| <p>With this initialization out of the way, the lowering triggers a recursive call |
| to a function that generates the MIR for the body (a <code>Block</code> expression) and |
| writes the result into the <code>RETURN_PLACE</code>.</p> |
| <h2 id="unpack-all-the-things"><a class="header" href="#unpack-all-the-things"><code>unpack!</code> all the things</a></h2> |
| <p>Functions that generate MIR tend to fall into one of two patterns. |
| First, if the function generates only statements, then it will take a |
| basic block as argument onto which those statements should be appended. |
| It can then return a result as normal:</p> |
| <pre><code class="language-rust ignore">fn generate_some_mir(&mut self, block: BasicBlock) -> ResultType { |
| ... |
| }</code></pre> |
| <p>But there are other functions that may generate new basic blocks as well. |
| For example, lowering an expression like <code>if foo { 22 } else { 44 }</code> |
| requires generating a small “diamond-shaped graph”. |
| In this case, the functions take a basic block where their code starts |
| and return a (potentially) new basic block where the code generation ends. |
| The <code>BlockAnd</code> type is used to represent this:</p> |
| <pre><code class="language-rust ignore">fn generate_more_mir(&mut self, block: BasicBlock) -> BlockAnd<ResultType> { |
| ... |
| }</code></pre> |
| <p>When you invoke these functions, it is common to have a local variable <code>block</code> |
| that is effectively a “cursor”. It represents the point at which we are adding new MIR. |
| When you invoke <code>generate_more_mir</code>, you want to update this cursor. |
| You can do this manually, but it’s tedious:</p> |
| <pre><code class="language-rust ignore">let mut block; |
| let v = match self.generate_more_mir(..) { |
| BlockAnd { block: new_block, value: v } => { |
| block = new_block; |
| v |
| } |
| };</code></pre> |
| <p>For this reason, we offer a macro that lets you write |
| <code>let v = unpack!(block = self.generate_more_mir(...))</code>. |
| It simply extracts the new block and overwrites the |
| variable <code>block</code> that you named in the <code>unpack!</code>.</p> |
| <h2 id="lowering-expressions-into-the-desired-mir"><a class="header" href="#lowering-expressions-into-the-desired-mir">Lowering expressions into the desired MIR</a></h2> |
| <p>There are essentially four kinds of representations one might want of an expression:</p> |
| <ul> |
| <li><code>Place</code> refers to a (or part of a) preexisting memory location (local, static, promoted)</li> |
| <li><code>Rvalue</code> is something that can be assigned to a <code>Place</code></li> |
| <li><code>Operand</code> is an argument to e.g. a <code>+</code> operation or a function call</li> |
| <li>a temporary variable containing a copy of the value</li> |
| </ul> |
| <p>The following image depicts a general overview of the interactions between the |
| representations:</p> |
| <img src="mir_overview.svg"> |
| <p><a href="mir_detailed.svg">Click here for a more detailed view</a></p> |
| <p>We start out with lowering the function body to an <code>Rvalue</code> so we can create an |
| assignment to <code>RETURN_PLACE</code>, This <code>Rvalue</code> lowering will in turn trigger lowering to |
| <code>Operand</code> for its arguments (if any). <code>Operand</code> lowering either produces a <code>const</code> |
| operand, or moves/copies out of a <code>Place</code>, thus triggering a <code>Place</code> lowering. An |
| expression being lowered to a <code>Place</code> can in turn trigger a temporary to be created |
| if the expression being lowered contains operations. This is where the snake bites its |
| own tail and we need to trigger an <code>Rvalue</code> lowering for the expression to be written |
| into the local.</p> |
| <h2 id="operator-lowering"><a class="header" href="#operator-lowering">Operator lowering</a></h2> |
| <p>Operators on builtin types are not lowered to function calls (which would end up being |
| infinite recursion calls, because the trait impls just contain the operation itself |
| again). Instead there are <code>Rvalue</code>s for binary and unary operators and index operations. |
| These <code>Rvalue</code>s later get codegened to llvm primitive operations or llvm intrinsics.</p> |
| <p>Operators on all other types get lowered to a function call to their <code>impl</code> of the |
| operator’s corresponding trait.</p> |
| <p>Regardless of the lowering kind, the arguments to the operator are lowered to <code>Operand</code>s. |
| This means all arguments are either constants, or refer to an already existing value |
| somewhere in a local or static.</p> |
| <h2 id="method-call-lowering"><a class="header" href="#method-call-lowering">Method call lowering</a></h2> |
| <p>Method calls are lowered to the same <code>TerminatorKind</code> that function calls are. |
| In <a href="./index.html">MIR</a> there is no difference between method calls and function calls anymore.</p> |
| <h2 id="conditions"><a class="header" href="#conditions">Conditions</a></h2> |
| <p><code>if</code> conditions and <code>match</code> statements for <code>enum</code>s with variants that have no fields are |
| lowered to <code>TerminatorKind::SwitchInt</code>. Each possible value (so <code>0</code> and <code>1</code> for <code>if</code> |
| conditions) has a corresponding <code>BasicBlock</code> to which the code continues. |
| The argument being branched on is (again) an <code>Operand</code> representing the value of |
| the if condition.</p> |
| <h3 id="pattern-matching"><a class="header" href="#pattern-matching">Pattern matching</a></h3> |
| <p><code>match</code> statements for <code>enum</code>s with variants that have fields are lowered to |
| <code>TerminatorKind::SwitchInt</code>, too, but the <code>Operand</code> refers to a <code>Place</code> where the |
| discriminant of the value can be found. This often involves reading the discriminant |
| to a new temporary variable.</p> |
| <h2 id="aggregate-construction"><a class="header" href="#aggregate-construction">Aggregate construction</a></h2> |
| <p>Aggregate values of any kind (e.g. structs or tuples) are built via <code>Rvalue::Aggregate</code>. |
| All fields are |
| lowered to <code>Operator</code>s. This is essentially equivalent to one assignment |
| statement per aggregate field plus an assignment to the discriminant in the |
| case of <code>enum</code>s.</p> |
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