builtin-functions - SKILL.md Agent Skill

name: Builtin functions description: Instructions for registering, mapping, constant evaluating, and lowering builtin functions in the Carbon toolchain.

Builtin Functions in the Carbon Toolchain

Builtin functions are compiler-recognized primitives mapping directly from Carbon code expressions (via standard prelude bindings) to optimized backend execution. This document defines the complete structural workflow, C++ patterns, constant evaluation logic, machine lowering mechanics, library bindings, and validation strategies required to implement builtin functions in the Carbon compiler.

Technical Flow & Lifecycle

graph TD
  Src[Carbon Source Code] -->|Prelude Map| Sem[Semantic Analysis / SemIR]
  Sem -->|Signature Constraint| Sig[builtin_function_kind.cpp]
  Sem -->|Phase Evaluation| Eval[eval.cpp Constant Interpreter]
  Sem -->|Machine Codegen| Lower[handle_call.cpp LLVM Lowering]
  Eval -->|Diagnostics| Diag[diagnostics/kind.def]
  Lower -->|Native Instructions| LLVM[LLVM IR Generation]

Adding a builtin function involves a 5-step integration:

Define the Builtin Kind: Register the enum in builtin_function_kind.def.
Signature & Compile-Time Registry: Declare the mapping name, parameter constraints, and compile-time evaluation residency in builtin_function_kind.cpp.
Compile-Time Interpreter Support: Wire constant evaluation hooks and bounds/exception diagnostics in eval.cpp.
LLVM IR Lowering Support: Connect target machine generation in handle_call.cpp.
Prelude Library Mapping: Bind primitive interfaces to named builtins under core/prelude/.

Detailed Step-by-Step Implementation Guide

Step 1: Kind Definition & Registration

// toolchain/sem_ir/builtin_function_kind.def

// Converts an integer type to a floating-point type.
CARBON_SEM_IR_BUILTIN_FUNCTION_KIND(IntConvertFloat)

Step 2: Signature Validation & Compile-Time Residence

Inside builtin_function_kind.cpp:

Define Parameter Constraints: If the parameter requires novel constraints (e.g. "must be a float type"), define a template constraint struct checking the matching SemIR type instruction (such as FloatType or FloatLiteralType). Use pre-established semantic helpers:
- TypeParam<I, T>: Ensures different parameters resolve to identical type structures (e.g., generic constraint matching).
- AnyInt, AnyFloat, AnySizedInt, AnySizedFloat, CharCompatible, StdInitializerList, NoReturn.

Map Literal Name & Register Constraint Signature: Declare a BuiltinInfo constant inside namespace BuiltinFunctionInfo matching the macro-defined name:

// toolchain/sem_ir/builtin_function_kind.cpp

constexpr BuiltinInfo IntConvertFloat = {
    "int.convert_float", ValidateSignature<auto(AnyInt)->AnyFloat>};

Establish Compile-Time Residency Status: Update BuiltinFunctionKind::IsCompTimeOnly to determine if a call requires compile-time evaluation:
- Checked/Diagnostics Primitives: Return true immediately. Runtime lowering of these is illegal (e.g. IntConvertFloatChecked).
- Runtime Primitives: Return AnyLiteralTypes(sem_ir, arg_ids, return_type_id) to enforce that expressions involving unsized literal values (like IntLiteral or FloatLiteral) are evaluated exclusively at compile-time (as they lack runtime representation).

Step 3: Constant Evaluation Support

Wire the interpreter inside eval.cpp to execute compile-time computations:

Implement Constant Evaluation Logic:
- Handle the builtin case inside MakeConstantForBuiltinCall (which processes the compile-time execution of the call).
- Confirm type validation phase is Phase::Concrete to reject incomplete bindings:
```
case SemIR::BuiltinFunctionKind::IntConvertFloat: {
  if (phase != Phase::Concrete) {
    return MakeConstantResult(context, call, phase);
  }
  return PerformIntToFloatConvert(context, loc_id, arg_ids[0], call.type_id,
                                  /*require_exact=*/false);
}
```
- Extract inputs safely from local value stores (e.g. context.ints().Get(arg.int_id) or context.floats().Get(arg.float_id)).
- Leverage high-precision LLVM mathematical structures (llvm::APInt, llvm::APFloat, llvm::APSInt) to handle custom bits and signedness safely.

Diagnose Invalid Parameters or Exceptions:

Define compile-time diagnostics inside kind.def:

// toolchain/diagnostics/kind.def
CARBON_DIAGNOSTIC_KIND(IntTooLargeForFloatType)

Emplace localized diagnostic formatting messages where they are caught in eval.cpp:

CARBON_DIAGNOSTIC(IntTooLargeForFloatType, Error,
                  "integer value {0} too large for floating-point type {1}",
                  llvm::APSInt, SemIR::TypeId);
context.emitter().Emit(loc_id, IntTooLargeForFloatType, val, dest_type_id);

Return SemIR::ErrorInst::ConstantId to gracefully abort invalid constant generation rather than crashing the compiler.

Fast-Path Range Limits:
- Before evaluating expensive math operations on giant exponents (e.g. 1.0e1000000), executing range limits check against dest_width + 64 (sized) or IntStore::MaxIntWidth (unsized) is mandatory to prevent out-of-bounds calculations and compile-time memory exhaustion.

Step 4: Machine Code Generation (LLVM Lowering)

Inside handle_call.cpp:

Map to Native LLVM Instructions: For runtime-eligible builtins, map the call inside HandleBuiltinCall to native LLVM IR builder methods:

case SemIR::BuiltinFunctionKind::IntConvertFloat: {
  auto* operand = context.GetValue(arg_ids[0]);
  auto* dest_type = context.GetTypeOfInst(inst_id);
  bool is_signed = IsSignedInt(context, arg_ids[0]);
  context.SetLocal(
      inst_id, is_signed
                   ? context.builder().CreateSIToFP(operand, dest_type)
                   : context.builder().CreateUIToFP(operand, dest_type));
  return;
}

Assert on Compile-Time-Only Builtins: Throw a hard assertion on lowering-cases for checked validator builtins that should never hit code generation:

case SemIR::BuiltinFunctionKind::IntConvertFloatChecked: {
  CARBON_CHECK(builtin_kind.IsCompTimeOnly(
      context.sem_ir(), arg_ids,
      context.sem_ir().insts().Get(inst_id).type_id()));
  CARBON_FATAL("Missing constant value for call to comptime-only function");
}

Step 5: Standard Library Prelude Integration

Map the standard library primitive interfaces to your newly minted named builtins under core/prelude/:

Primitive Mappings: Bind Carbon methods directly to string-literal builtin equivalents:
```
fn Convert[self: Self]() -> Float(To) = "int.convert_float";
```
Strict Orphan Rule Compliance: Carbon's orphan rules prohibit implementing interfaces where neither the type nor the interface is locally defined in the backing source module.
- Literal Conversions: Literal types (like FloatLiteral, IntLiteral) do not have backing Carbon source files. Therefore, an impl of UnsafeAs (which is defined in as.carbon) between two literal types must reside inside as.carbon itself.
- Sized Conversions: Implementations targeting sized primitives (e.g. Int(N), Float(N)) must reside in their respective type source files (such as int.carbon or float.carbon) where the backing target type resides to prevent duplicate symbols and structural recursion loops.

High-Fidelity Validation & Test Authoring

Follow the Toolchain tests skill with specialized patterns for builtins:

1. Checker Builtin File Splits

Create validation splits under toolchain/check/testdata/builtins/:

Test Naming Convention: All tests under toolchain/check/testdata/builtins/ must be named after the builtin they are testing, replacing . characters in the builtin name with / (directories). For example, a test for the builtin "char_literal.convert" must be located at toolchain/check/testdata/builtins/char_literal/convert.carbon.
Minimal Prelude & Direct Call Isolation: Builtin tests must not test the prelude library or operators. They must use the minimal primitive prelude (// INCLUDE-FILE: toolchain/testing/testdata/min_prelude/primitives.carbon) or a smaller prelude, and explicitly declare and call the builtin functions under test directly (e.g., fn Add(a: f64, b: f64) -> f64 = "float.add";). This isolates the testing of compiler builtins from the library prelude.
Min-Prelude Limitations: Standard operators (like +, -, /, <, etc.) are not available in minimized preludes because the core operators library isn't imported. To write tests with a minimal footprint, call primitive builtins directly (e.g. float.negate, float.div) inside your test code to build expressions.
Canonicalized Float Comparison: In SemIR, real literal representations with identical mathematical values can result in mismatched RealId objects based on spelling variations. Verify compile-time constant conversions using canonicalized comparison functions (e.g. passing converted results through Expect(X as f64)) to completely avoid spelling mismatches in expected outputs.
Locals Bypass: If validating generic implicit conversions, compile-time arguments cannot take local runtime variable parameters. Validate compile-time conversions by passing literal constants directly, and sized variable implicit conversions at runtime.

2. Machine Codegen Lowering Splits

Create testing splits under toolchain/lower/testdata/builtins/:

Emplace a simple carbon binding to the tested builtin.
Confirm matching LLVM metadata target definitions are mapped precisely (e.g., matching sitofp i32 %a to float, fptosi float %a to i32).