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| <main> |
| <h1 id="inline-assembly"><a class="header" href="#inline-assembly">Inline assembly</a></h1> |
| <h2 id="overview"><a class="header" href="#overview">Overview</a></h2> |
| <p>Inline assembly in rustc mostly revolves around taking an <code>asm!</code> macro invocation and plumbing it |
| through all of the compiler layers down to LLVM codegen. Throughout the various stages, an |
| <code>InlineAsm</code> generally consists of 3 components:</p> |
| <ul> |
| <li> |
| <p>The template string, which is stored as an array of <code>InlineAsmTemplatePiece</code>. Each piece |
| represents either a literal or a placeholder for an operand (just like format strings).</p> |
| <pre class="playground"><code class="language-rust"><span class="boring">#![allow(unused)] |
| </span><span class="boring">fn main() { |
| </span>pub enum InlineAsmTemplatePiece { |
| String(String), |
| Placeholder { operand_idx: usize, modifier: Option<char>, span: Span }, |
| } |
| <span class="boring">}</span></code></pre> |
| </li> |
| <li> |
| <p>The list of operands to the <code>asm!</code> (<code>in</code>, <code>[late]out</code>, <code>in[late]out</code>, <code>sym</code>, <code>const</code>). These are |
| represented differently at each stage of lowering, but follow a common pattern:</p> |
| <ul> |
| <li><code>in</code>, <code>out</code> and <code>inout</code> all have an associated register class (<code>reg</code>) or explicit register |
| (<code>"eax"</code>).</li> |
| <li><code>inout</code> has 2 forms: one with a single expression that is both read from and written to, and |
| one with two separate expressions for the input and output parts.</li> |
| <li><code>out</code> and <code>inout</code> have a <code>late</code> flag (<code>lateout</code> / <code>inlateout</code>) to indicate that the register |
| allocator is allowed to reuse an input register for this output.</li> |
| <li><code>out</code> and the split variant of <code>inout</code> allow <code>_</code> to be specified for an output, which means |
| that the output is discarded. This is used to allocate scratch registers for assembly code.</li> |
| <li><code>const</code> refers to an anonymous constants and generally works like an inline const.</li> |
| <li><code>sym</code> is a bit special since it only accepts a path expression, which must point to a <code>static</code> |
| or a <code>fn</code>.</li> |
| </ul> |
| </li> |
| <li> |
| <p>The options set at the end of the <code>asm!</code> macro. The only ones that are of particular interest to |
| rustc are <code>NORETURN</code> which makes <code>asm!</code> return <code>!</code> instead of <code>()</code>, and <code>RAW</code> which disables format |
| string parsing. The remaining options are mostly passed through to LLVM with little processing.</p> |
| <pre class="playground"><code class="language-rust"><span class="boring">#![allow(unused)] |
| </span><span class="boring">fn main() { |
| </span>bitflags::bitflags! { |
| pub struct InlineAsmOptions: u16 { |
| const PURE = 1 << 0; |
| const NOMEM = 1 << 1; |
| const READONLY = 1 << 2; |
| const PRESERVES_FLAGS = 1 << 3; |
| const NORETURN = 1 << 4; |
| const NOSTACK = 1 << 5; |
| const ATT_SYNTAX = 1 << 6; |
| const RAW = 1 << 7; |
| const MAY_UNWIND = 1 << 8; |
| } |
| } |
| <span class="boring">}</span></code></pre> |
| </li> |
| </ul> |
| <h2 id="ast"><a class="header" href="#ast">AST</a></h2> |
| <p><code>InlineAsm</code> is represented as an expression in the AST with the <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_ast/ast/struct.InlineAsm.html"><code>ast::InlineAsm</code> type</a>.</p> |
| <p>The <code>asm!</code> macro is implemented in <code>rustc_builtin_macros</code> and outputs an <code>InlineAsm</code> AST node. The |
| template string is parsed using <code>fmt_macros</code>, positional and named operands are resolved to |
| explicit operand indices. Since target information is not available to macro invocations, |
| validation of the registers and register classes is deferred to AST lowering.</p> |
| <h2 id="hir"><a class="header" href="#hir">HIR</a></h2> |
| <p><code>InlineAsm</code> is represented as an expression in the HIR with the <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_hir/hir/struct.InlineAsm.html"><code>hir::InlineAsm</code> type</a>.</p> |
| <p>AST lowering is where <code>InlineAsmRegOrRegClass</code> is converted from <code>Symbol</code>s to an actual register or |
| register class. If any modifiers are specified for a template string placeholder, these are |
| validated against the set allowed for that operand type. Finally, explicit registers for inputs and |
| outputs are checked for conflicts (same register used for different operands).</p> |
| <h2 id="type-checking"><a class="header" href="#type-checking">Type checking</a></h2> |
| <p>Each register class has a whitelist of types that it may be used with. After the types of all |
| operands have been determined, the <code>intrinsicck</code> pass will check that these types are in the |
| whitelist. It also checks that split <code>inout</code> operands have compatible types and that <code>const</code> |
| operands are integers or floats. Suggestions are emitted where needed if a template modifier should |
| be used for an operand based on the type that was passed into it.</p> |
| <h2 id="thir"><a class="header" href="#thir">THIR</a></h2> |
| <p><code>InlineAsm</code> is represented as an expression in the THIR with the <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/thir/struct.InlineAsmExpr.html"><code>InlineAsmExpr</code> type</a>.</p> |
| <p>The only significant change compared to HIR is that <code>Sym</code> has been lowered to either a <code>SymFn</code> |
| whose <code>expr</code> is a <code>Literal</code> ZST of the <code>fn</code>, or a <code>SymStatic</code> which points to the <code>DefId</code> of a |
| <code>static</code>.</p> |
| <h2 id="mir"><a class="header" href="#mir">MIR</a></h2> |
| <p><code>InlineAsm</code> is represented as a <code>Terminator</code> in the MIR with the <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/mir/enum.TerminatorKind.html#variant.InlineAsm"><code>TerminatorKind::InlineAsm</code> variant</a></p> |
| <p>As part of THIR lowering, <code>InOut</code> and <code>SplitInOut</code> operands are lowered to a split form with a |
| separate <code>in_value</code> and <code>out_place</code>.</p> |
| <p>Semantically, the <code>InlineAsm</code> terminator is similar to the <code>Call</code> terminator except that it has |
| multiple output places where a <code>Call</code> only has a single return place output.</p> |
| <h2 id="codegen"><a class="header" href="#codegen">Codegen</a></h2> |
| <p>Operands are lowered one more time before being passed to LLVM codegen, this is represented by the <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_codegen_ssa/traits/enum.InlineAsmOperandRef.html"><code>InlineAsmOperandRef</code> type</a> from <code>rustc_codegen_ssa</code>.</p> |
| <p>The operands are lowered to LLVM operands and constraint codes as follows:</p> |
| <ul> |
| <li><code>out</code> and the output part of <code>inout</code> operands are added first, as required by LLVM. Late output |
| operands have a <code>=</code> prefix added to their constraint code, non-late output operands have a <code>=&</code> |
| prefix added to their constraint code.</li> |
| <li><code>in</code> operands are added normally.</li> |
| <li><code>inout</code> operands are tied to the matching output operand.</li> |
| <li><code>sym</code> operands are passed as function pointers or pointers, using the <code>"s"</code> constraint.</li> |
| <li><code>const</code> operands are formatted to a string and directly inserted in the template string.</li> |
| </ul> |
| <p>The template string is converted to LLVM form:</p> |
| <ul> |
| <li><code>$</code> characters are escaped as <code>$$</code>.</li> |
| <li><code>const</code> operands are converted to strings and inserted directly.</li> |
| <li>Placeholders are formatted as <code>${X:M}</code> where <code>X</code> is the operand index and <code>M</code> is the modifier |
| character. Modifiers are converted from the Rust form to the LLVM form.</li> |
| </ul> |
| <p>The various options are converted to clobber constraints or LLVM attributes, refer to the |
| <a href="https://github.com/Amanieu/rfcs/blob/inline-asm/text/0000-inline-asm.md#mapping-to-llvm-ir">RFC</a> |
| for more details.</p> |
| <p>Note that LLVM is sometimes rather picky about what types it accepts for certain constraint codes |
| so we sometimes need to insert conversions to/from a supported type. See the target-specific |
| ISelLowering.cpp files in LLVM for details of what types are supported for each register class.</p> |
| <h2 id="adding-support-for-new-architectures"><a class="header" href="#adding-support-for-new-architectures">Adding support for new architectures</a></h2> |
| <p>Adding inline assembly support to an architecture is mostly a matter of defining the registers and |
| register classes for that architecture. All the definitions for register classes are located in |
| <code>compiler/rustc_target/asm/</code>.</p> |
| <p>Additionally you will need to implement lowering of these register classes to LLVM constraint codes |
| in <code>compiler/rustc_codegen_llvm/asm.rs</code>.</p> |
| <p>When adding a new architecture, make sure to cross-reference with the LLVM source code:</p> |
| <ul> |
| <li>LLVM has restrictions on which types can be used with a particular constraint code. Refer to the |
| <code>getRegForInlineAsmConstraint</code> function in <code>lib/Target/${ARCH}/${ARCH}ISelLowering.cpp</code>.</li> |
| <li>LLVM reserves certain registers for its internal use, which causes them to not be saved/restored |
| properly around inline assembly blocks. These registers are listed in the <code>getReservedRegs</code> |
| function in <code>lib/Target/${ARCH}/${ARCH}RegisterInfo.cpp</code>. Any “conditionally” reserved register |
| such as the frame/base pointer must always be treated as reserved for Rust purposes because we |
| can’t know ahead of time whether a function will require a frame/base pointer.</li> |
| </ul> |
| <h2 id="tests"><a class="header" href="#tests">Tests</a></h2> |
| <p>Various tests for inline assembly are available:</p> |
| <ul> |
| <li><code>tests/assembly-llvm/asm</code></li> |
| <li><code>tests/ui/asm</code></li> |
| <li><code>tests/codegen-llvm/asm-*</code></li> |
| </ul> |
| <p>Every architecture supported by inline assembly must have exhaustive tests in |
| <code>tests/assembly-llvm/asm</code> which test all combinations of register classes and types.</p> |
| |
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