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Inline assembly

Overview

Inline assembly in rustc mostly revolves around taking an asm! macro invocation and plumbing it through all of the compiler layers down to LLVM codegen. Throughout the various stages, an InlineAsm generally consists of 3 components:

  • The template string, which is stored as an array of InlineAsmTemplatePiece. Each piece represents either a literal or a placeholder for an operand (just like format strings).

    #![allow(unused)]
fn main() {
pub enum InlineAsmTemplatePiece {
    String(String),
    Placeholder { operand_idx: usize, modifier: Option<char>, span: Span },
}
}

  • The list of operands to the asm! (in, [late]out, in[late]out, sym, const). These are represented differently at each stage of lowering, but follow a common pattern:

    • in, out and inout all have an associated register class (reg) or explicit register ("eax").
    • inout 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.
    • out and inout have a late flag (lateout / inlateout) to indicate that the register allocator is allowed to reuse an input register for this output.
    • out and the split variant of inout allow _ to be specified for an output, which means that the output is discarded. This is used to allocate scratch registers for assembly code.
    • const refers to an anonymous constants and generally works like an inline const.
    • sym is a bit special since it only accepts a path expression, which must point to a static or a fn.

  • The options set at the end of the asm! macro. The only ones that are of particular interest to rustc are NORETURN which makes asm! return ! instead of (), and RAW which disables format string parsing. The remaining options are mostly passed through to LLVM with little processing.

    #![allow(unused)]
fn main() {
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;
    }
}
}

AST

InlineAsm is represented as an expression in the AST with the ast::InlineAsm type.

The asm! macro is implemented in rustc_builtin_macros and outputs an InlineAsm AST node. The template string is parsed using fmt_macros, 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.

HIR

InlineAsm is represented as an expression in the HIR with the hir::InlineAsm type.

AST lowering is where InlineAsmRegOrRegClass is converted from Symbols 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).

Type checking

Each register class has a whitelist of types that it may be used with. After the types of all operands have been determined, the intrinsicck pass will check that these types are in the whitelist. It also checks that split inout operands have compatible types and that const 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.

THIR

InlineAsm is represented as an expression in the THIR with the InlineAsmExpr type.

The only significant change compared to HIR is that Sym has been lowered to either a SymFn whose expr is a Literal ZST of the fn, or a SymStatic which points to the DefId of a static.

MIR

InlineAsm is represented as a Terminator in the MIR with the TerminatorKind::InlineAsm variant

As part of THIR lowering, InOut and SplitInOut operands are lowered to a split form with a separate in_value and out_place.

Semantically, the InlineAsm terminator is similar to the Call terminator except that it has multiple output places where a Call only has a single return place output.

Codegen

Operands are lowered one more time before being passed to LLVM codegen, this is represented by the InlineAsmOperandRef type from rustc_codegen_ssa.

The operands are lowered to LLVM operands and constraint codes as follows:

  • out and the output part of inout operands are added first, as required by LLVM. Late output operands have a = prefix added to their constraint code, non-late output operands have a =& prefix added to their constraint code.
  • in operands are added normally.
  • inout operands are tied to the matching output operand.
  • sym operands are passed as function pointers or pointers, using the "s" constraint.
  • const operands are formatted to a string and directly inserted in the template string.

The template string is converted to LLVM form:

  • $ characters are escaped as $$.
  • const operands are converted to strings and inserted directly.
  • Placeholders are formatted as ${X:M} where X is the operand index and M is the modifier character. Modifiers are converted from the Rust form to the LLVM form.

The various options are converted to clobber constraints or LLVM attributes, refer to the RFC for more details.

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.

Adding support for new architectures

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 compiler/rustc_target/asm/.

Additionally you will need to implement lowering of these register classes to LLVM constraint codes in compiler/rustc_codegen_llvm/asm.rs.

When adding a new architecture, make sure to cross-reference with the LLVM source code:

  • LLVM has restrictions on which types can be used with a particular constraint code. Refer to the getRegForInlineAsmConstraint function in lib/Target/${ARCH}/${ARCH}ISelLowering.cpp.
  • 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 getReservedRegs function in lib/Target/${ARCH}/${ARCH}RegisterInfo.cpp. 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.

Tests

Various tests for inline assembly are available:

  • tests/assembly-llvm/asm
  • tests/ui/asm
  • tests/codegen-llvm/asm-*

Every architecture supported by inline assembly must have exhaustive tests in tests/assembly-llvm/asm which test all combinations of register classes and types.