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* The one rule for what a constant expression is worth (#1175, #1669).
*
* ADR-044 "Values fixed at compile time": a value the program needs while it
* compiles -- an array dimension, an enum member's value, a const's initializer
* -- is the value the same expression has when the program runs. So an
* operation happens at its operands' width, an untyped literal takes the type
* of the operand beside it, or `i32` when nothing gives it one, and where that
* arithmetic would clamp or wrap the expression has no value: the program would
* compute something other than the exact result.
*
* Exact arithmetic is `bigint`: a u64 is past what a `number` holds exactly.
*
* Until this, four things decided a constant's value: a regex table for array
* dimensions, a prefix parser for enum values (`1 + 2` was 1), render's fold
* over generated C operands, and a literal evaluator nothing called. They
* disagreed -- `u8[1 - -1]` was 2 in the .c and `1--1` in the .h.
*/
import TYPE_WIDTH from "../types/TYPE_WIDTH";
import TypeCheckUtils from "./TypeCheckUtils";
import type TConstExpr from "../types/TConstExpr";
import type TConstResult from "../types/TConstResult";
import type IConstantEnvironment from "./types/IConstantEnvironment";
type TBinary = Extract<TConstExpr, { kind: "binary" }>;
type TValue = Extract<TConstResult, { kind: "value" }>;
type TComparison = "<" | ">" | "<=" | ">=" | "=" | "!=";
type TArithmetic = Exclude<TBinary["op"], TComparison | "&&" | "||">;
/** ADR-044: a literal nothing else gives a type is an i32 */
const DEFAULT_TYPE = "i32";
const BOOL = "bool";
const COMPARISONS: ReadonlySet<string> = new Set<TComparison>([
"<",
">",
"<=",
">=",
"=",
"!=",
]);
class ConstantEvaluator {
/**
* `context` is the type the expression is written into, when it has one: a
* declaration's or a parameter's type, a const's declared type. ADR-044
* "Integer Literals" (owner ruling, 2026-10-03): a literal takes the
* smallest type that fits its context at compile time, or `i32` when there
* is none -- so an operation between untyped literals happens at it.
*/
static evaluate(
expr: TConstExpr,
env: IConstantEnvironment,
context: string | null = null,
): TConstResult {
switch (expr.kind) {
case "literal":
return ConstantEvaluator.literal(expr.digits, expr.typeName);
case "name":
return env.valueOf(expr);
case "sizeof":
return ConstantEvaluator.sizeOf(expr.typeName);
case "cast":
// A cast is the context of what it encloses
return ConstantEvaluator.cast(
expr.typeName,
ConstantEvaluator.evaluate(expr.operand, env, expr.typeName),
);
case "unary":
return ConstantEvaluator.unary(
expr.op,
ConstantEvaluator.evaluate(expr.operand, env, context),
context,
);
case "binary":
return ConstantEvaluator.binary(expr, env, context);
case "ternary":
return ConstantEvaluator.ternary(expr, env, context);
case "other":
return {
kind: "notConstant",
reason: expr.what,
spelling: expr.spelling,
at: expr.at,
};
}
}
/** A value as a `number`, when a `number` holds it exactly */
static toNumber(value: bigint): number | undefined {
return value >= BigInt(Number.MIN_SAFE_INTEGER) &&
value <= BigInt(Number.MAX_SAFE_INTEGER)
? Number(value)
: undefined;
}
/** A suffixed literal is its type, and must fit it: `300u8` does not */
private static literal(
digits: string,
typeName: string | null,
): TConstResult {
const value = BigInt(digits);
const integer = ConstantEvaluator.integerType(typeName);
return integer === null
? { kind: "value", value, typeName }
: ConstantEvaluator.held(value, integer, integer);
}
/** A primitive's size in bytes; any other type's is the target's to decide */
private static sizeOf(typeName: string): TConstResult {
const width = TYPE_WIDTH[typeName];
return width === undefined
? { kind: "foreign", spelling: `sizeof(${typeName})`, why: "targetSize" }
: { kind: "value", value: BigInt(width / 8), typeName: null };
}
private static cast(typeName: string, operand: TConstResult): TConstResult {
if (operand.kind !== "value") return operand;
if (typeName === BOOL) {
return { kind: "value", value: operand.value === 0n ? 0n : 1n, typeName };
}
Iif (TypeCheckUtils.isFloat(typeName)) {
return {
kind: "notConstant",
reason: "float",
spelling: `(${typeName})`,
at: null,
};
}
return ConstantEvaluator.held(operand.value, typeName, typeName);
}
private static unary(
op: Extract<TConstExpr, { kind: "unary" }>["op"],
operand: TConstResult,
context: string | null,
): TConstResult {
Iif (operand.kind !== "value") return operand;
Iif (op === "!") {
return {
kind: "value",
value: operand.value === 0n ? 1n : 0n,
typeName: BOOL,
};
}
// An untyped operand stays untyped, at its context's width (ADR-044)
const at = ConstantEvaluator.integerType(operand.typeName);
const width = at ?? ConstantEvaluator.integerType(context) ?? DEFAULT_TYPE;
if (op === "-") {
return ConstantEvaluator.held(-operand.value, width, at);
}
// `~` flips every bit of the operand's width; an unsigned result is the
// complement within it, a signed one is two's complement
const flipped = ~operand.value;
const value = TypeCheckUtils.isUnsigned(width)
? flipped & ((1n << BigInt(TYPE_WIDTH[width])) - 1n)
: flipped;
return ConstantEvaluator.held(value, width, at);
}
private static ternary(
expr: Extract<TConstExpr, { kind: "ternary" }>,
env: IConstantEnvironment,
context: string | null,
): TConstResult {
const condition = ConstantEvaluator.evaluate(expr.condition, env);
if (condition.kind === "foreign") {
// C evaluates the condition, so either arm may be the size: both must
// have one, or the whole is not a constant C can evaluate either
return ConstantEvaluator.foreignUnless(condition, [
ConstantEvaluator.evaluate(expr.whenTrue, env, context),
ConstantEvaluator.evaluate(expr.whenFalse, env, context),
]);
}
Iif (condition.kind !== "value") return condition;
return ConstantEvaluator.evaluate(
condition.value === 0n ? expr.whenFalse : expr.whenTrue,
env,
context,
);
}
/**
* `foreign`, when every other operand has a value or is C's too; otherwise
* the first that has none. A name only C knows does not make an operand
* beside it constant (a variable arm of `MACRO ? 4 : n` is still a VLA).
*/
private static foreignUnless(
foreign: TConstResult,
others: ReadonlyArray<TConstResult>,
): TConstResult {
return (
others.find((o) => o.kind === "notConstant" || o.kind === "overflow") ??
foreign
);
}
private static binary(
expr: TBinary,
env: IConstantEnvironment,
context: string | null,
): TConstResult {
if (expr.op === "&&" || expr.op === "||") {
return ConstantEvaluator.logical(expr, env);
}
const op = expr.op;
// A comparison's operands are not written into its context (a `bool`)
const comparison = ConstantEvaluator.isComparison(op);
const operandContext = comparison ? null : context;
const left = ConstantEvaluator.evaluate(expr.left, env, operandContext);
const right = ConstantEvaluator.evaluate(expr.right, env, operandContext);
if (left.kind !== "value" || right.kind !== "value") {
return ConstantEvaluator.firstWithoutValue(left, right);
}
if (comparison) return ConstantEvaluator.compare(op, left, right);
return ConstantEvaluator.arithmetic(op, left, right, context);
}
/** `&&` and `||` evaluate their right operand only when C would */
private static logical(
expr: TBinary,
env: IConstantEnvironment,
): TConstResult {
const left = ConstantEvaluator.evaluate(expr.left, env);
if (left.kind === "foreign") {
return ConstantEvaluator.foreignUnless(left, [
ConstantEvaluator.evaluate(expr.right, env),
]);
}
Iif (left.kind !== "value") return left;
const decided = expr.op === "&&" ? left.value === 0n : left.value !== 0n;
if (decided) {
return {
kind: "value",
value: expr.op === "&&" ? 0n : 1n,
typeName: BOOL,
};
}
const right = ConstantEvaluator.evaluate(expr.right, env);
if (right.kind !== "value") return right;
return {
kind: "value",
value: right.value === 0n ? 0n : 1n,
typeName: BOOL,
};
}
/**
* Of two results that are not both values, the one to report: a missing
* value first (it names what to fix), then an overflow, then a name C knows
*/
private static firstWithoutValue(
left: TConstResult,
right: TConstResult,
): TConstResult {
for (const kind of ["notConstant", "overflow", "foreign"] as const) {
if (left.kind === kind) return left;
if (right.kind === kind) return right;
}
return left;
}
private static isComparison(
op: Exclude<TBinary["op"], "&&" | "||">,
): op is TComparison {
return COMPARISONS.has(op);
}
/**
* A comparison happens at its operands' type too: an untyped operand takes
* the other's, and must fit it. `N > -1` with a u32 `N` has no value, as C
* would compare against UINT_MAX there.
*/
private static compare(
op: TComparison,
leftOperand: TValue,
rightOperand: TValue,
): TConstResult {
const at = ConstantEvaluator.operationType(
leftOperand.typeName,
rightOperand.typeName,
);
if (at !== null) {
for (const operand of [leftOperand, rightOperand]) {
const fits = ConstantEvaluator.held(operand.value, at, at);
if (fits.kind !== "value") return fits;
}
}
const left = leftOperand.value;
const right = rightOperand.value;
const holds = {
"<": left < right,
">": left > right,
"<=": left <= right,
">=": left >= right,
"=": left === right,
"!=": left !== right,
}[op];
return { kind: "value", value: holds ? 1n : 0n, typeName: BOOL };
}
private static arithmetic(
op: TArithmetic,
left: TValue,
right: TValue,
context: string | null,
): TConstResult {
const typeName = ConstantEvaluator.operationType(
left.typeName,
right.typeName,
);
// Untyped operands meet at their context's type, or i32 (ADR-044)
const width =
typeName ?? ConstantEvaluator.integerType(context) ?? DEFAULT_TYPE;
const exact = ConstantEvaluator.apply(op, left.value, right.value, width);
if (exact.kind !== "value") return exact;
return ConstantEvaluator.held(exact.value, width, typeName);
}
/**
* The exact result of `left op right`, or why there is none. `width` is the
* type the operation happens at, which bounds a shift's amount.
*/
private static apply(
op: TArithmetic,
left: bigint,
right: bigint,
width: string,
): TConstResult {
const value = (v: bigint): TConstResult => ({
kind: "value",
value: v,
typeName: null,
});
switch (op) {
case "*":
return value(left * right);
case "+":
return value(left + right);
case "-":
return value(left - right);
case "&":
return value(left & right);
case "|":
return value(left | right);
case "^":
return value(left ^ right);
case "/":
case "%":
return ConstantEvaluator.divide(op, left, right);
case "<<":
case ">>":
return ConstantEvaluator.shift(op, left, right, width);
}
}
/** C truncates toward zero, and `%` takes the dividend's sign, as `bigint` does */
private static divide(
op: "/" | "%",
left: bigint,
right: bigint,
): TConstResult {
if (right === 0n) {
return {
kind: "notConstant",
reason: "divisionByZero",
spelling: "",
at: null,
};
}
return {
kind: "value",
value: op === "/" ? left / right : left % right,
typeName: null,
};
}
private static shift(
op: "<<" | ">>",
left: bigint,
right: bigint,
width: string,
): TConstResult {
if (right < 0n || (op === ">>" && left < 0n)) {
return {
kind: "notConstant",
reason: "negativeShift",
spelling: "",
at: null,
};
}
Iif (right >= BigInt(TYPE_WIDTH[width])) {
return { kind: "overflow", typeName: width };
}
return {
kind: "value",
value: op === "<<" ? left << right : left >> right,
typeName: null,
};
}
/**
* The type an operation happens at: its typed operand's, the wider of two
* (ADR-024 widens the narrower), or none when neither operand has a type
*/
private static operationType(
left: string | null,
right: string | null,
): string | null {
const l = ConstantEvaluator.integerType(left);
const r = ConstantEvaluator.integerType(right);
if (l === null || r === null) return l ?? r;
return TYPE_WIDTH[r] > TYPE_WIDTH[l] ? r : l;
}
private static integerType(typeName: string | null): string | null {
return typeName !== null && TypeCheckUtils.isInteger(typeName)
? typeName
: null;
}
/**
* `value` when `rangeOf` holds it, typed `typeName`; otherwise the overflow
* the program would clamp or wrap at
*/
private static held(
value: bigint,
rangeOf: string,
typeName: string | null,
): TConstResult {
const range = TypeCheckUtils.integerRange(rangeOf);
if (range !== null && (value < range[0] || value > range[1])) {
return { kind: "overflow", typeName: rangeOf };
}
return { kind: "value", value, typeName };
}
}
export default ConstantEvaluator;
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