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* ADR-052 slice assignment: E0858-E0861.
*
* #1322. This replaces TWELVE throws in `ArrayHandlers`, every one of which
* already knew its line and spent it on a message prefix:
*
* throw new Error(`${line}:0 Error: Slice assignment out of bounds: ...`)
*
* The `${line}:0` was read back out of the message by `parseErrorLocation`, so
* the diagnostic reached the user with a real line and a hard-coded column 0.
* That is what makes this tier A: the position is not missing, it is being
* smuggled through the one channel a throw has. Relocating removes the hack
* rather than adding a position.
*
* ## What 2.1 has to recompute, and why that is not a duplicate decision
*
* These checks sit inside the code that EMITS the unrolled copy, and they share
* its arithmetic: the element stride, the element count, the buffer capacity.
* Moving the checks means computing those here as well.
*
* That is a shared FACT, not a duplicated decision. 2.1 decides "reject"; 2.3
* decides "emit these writes". What matters is that they cannot disagree
* silently, so every check relocated here leaves an assertion behind at the
* emission site: if a slice reaches codegen that this pass would have rejected,
* it fails loudly as an internal invariant instead of emitting wrong C.
*
* ## Compile-time constants in pass 2.1
*
* The offset and length must fold at compile time. They fold with the const
* values visible where they are written, which 1.4 Resolve settles before any
* pass reads them (`ConstantExpression.valueAt`). A codegen-time map filled
* during generation saw the PREVIOUS file's consts (#1399); render no longer
* keeps one (#1664 box 7).
*/
import { ParseTreeWalker } from "antlr4ng";
import * as Parser from "../../PARSE/2-Parse/grammar/CNextParser";
import TYPE_WIDTH from "../../types/TYPE_WIDTH";
import ParserUtils from "../../utils/ParserUtils";
import OperandTyper from "../../utils/OperandTyper";
import AssignmentSiteListener from "./AssignmentSiteListener";
import ISliceAssignmentError from "./types/ISliceAssignmentError";
import ConstantExpression from "./helpers/ConstantExpression";
import type IAnalysisContext from "./types/IAnalysisContext";
import type IOperandType from "../../types/IOperandType";
import type TAssignmentSite from "./types/TAssignmentSite";
/** `string<N>` holds N characters plus the terminator. */
const STRING_TERMINATOR_BYTES = 1;
/** What a slice destination writes, one element at a time. */
interface IDestination {
readonly elementBytes: number;
readonly capacity: number;
}
class SliceAssignmentListener {
private readonly found: ISliceAssignmentError[] = [];
public constructor(private readonly context: IAnalysisContext) {}
public errors(): ISliceAssignmentError[] {
return this.found;
}
/** `buf[offset, length] <- value`, in a statement or a `for` header */
public checkSite(site: TAssignmentSite): void {
// A compound operator on any two-subscript target is E0857's, reported
// before this step runs. Checking the slice as well would give one mistake
// two diagnostics.
if (!site.assignmentOperator().ASSIGN()) return;
const target = site.assignmentTarget();
const ops = target.postfixTargetOp();
if (ops.length !== 1) return;
const subscripts = ops[0].expression();
if (subscripts.length !== 2) return;
const name = target.IDENTIFIER()?.getText();
// The buffer as its declaration states it, bound by the one binder --
// this file's locals first, then scope members, globals and includes
// (#1668), so a buffer declared in an included file is found
const declared = OperandTyper.chainOf(target, this.context).steps[0]
?.before;
// Not established, or a SCALAR base -- on a scalar, two subscripts are a
// bit RANGE (ADR-007), which is a different construct with its own rules.
Iif (name === undefined || !declared) return;
if (declared.dimensions.length === 0 && declared.stringCapacity === null) {
return;
}
this.check(site, name, declared, subscripts);
}
private check(
site: TAssignmentSite,
name: string,
declared: IOperandType,
subscripts: readonly Parser.ExpressionContext[],
): void {
const destination = this.destinationOf(name, declared, subscripts[0]);
if (destination === null) return;
const offset = this.constantOf(subscripts[0]);
if (offset === undefined) {
this.report(
subscripts[0],
"E0859",
`Slice assignment offset must be a compile-time constant for '${name}'`,
"A runtime offset cannot be bounds-checked at compile time. Use a literal or a `const`.",
);
return;
}
const length = this.constantOf(subscripts[1]);
if (length === undefined) {
this.report(
subscripts[1],
"E0859",
`Slice assignment length must be a compile-time constant for '${name}'`,
"A runtime length cannot be bounds-checked at compile time. Use a literal or a `const`.",
);
return;
}
if (offset < 0) {
this.report(
subscripts[0],
"E0860",
`Slice assignment offset cannot be negative: ${offset}`,
"An offset is an element index into the buffer, counted from zero.",
);
return;
}
if (length <= 0) {
this.report(
subscripts[1],
"E0860",
`Slice assignment length must be positive: ${length}`,
"A slice writes at least one byte; a zero-length slice writes nothing and is almost certainly a mistake.",
);
return;
}
if (length % destination.elementBytes !== 0) {
this.report(
subscripts[1],
"E0860",
`Slice assignment length (${length}) must be a multiple of the element size (${destination.elementBytes} bytes) for '${name}'`,
"The length is a BYTE count and the copy writes whole elements, so it has to divide evenly.",
);
return;
}
const elementCount = length / destination.elementBytes;
if (offset + elementCount > destination.capacity) {
this.report(
subscripts[0],
"E0860",
`Slice assignment out of bounds: offset(${offset}) + ${elementCount} element(s) = ${offset + elementCount} exceeds buffer capacity(${destination.capacity}) for '${name}'`,
"The offset is an element index and the capacity an element count, so the span is compared in elements, not bytes.",
);
return;
}
this.checkSource(site, name, length);
}
/** The destination's element stride and capacity, or null if unestablished. */
private destinationOf(
name: string,
declared: IOperandType,
at: Parser.ExpressionContext,
): IDestination | null {
if (declared.stringCapacity !== null) {
// A string buffer is `char[N + 1]`; the terminator is part of it.
return {
elementBytes: 1,
capacity: declared.stringCapacity + STRING_TERMINATOR_BYTES,
};
}
if (declared.dimensions.length > 1) {
this.report(
at,
"E0858",
`Slice assignment is only valid on one-dimensional arrays; '${name}' has ${declared.dimensions.length} dimensions`,
`Index the outer dimensions first, e.g. ${name}[index][offset, length].`,
);
return null;
}
const element = declared.typeName;
const bits = element === null ? undefined : TYPE_WIDTH[element];
if (element === null || bits === undefined || element.startsWith("f")) {
// A float or a type this pass cannot size cannot be written as integer
// byte chunks -- that needs type punning, which ADR-052 does not do.
// `bool` is excluded for the same reason even though it has a width.
Iif (element !== null && !TYPE_WIDTH[element]) return null;
this.report(
at,
"E0858",
`Slice assignment is not supported for element type '${element ?? "unknown"}' of '${name}'`,
"Only integer and string buffers can be sliced; a float or bool element would need type punning.",
);
return null;
}
if (element === "bool") {
this.report(
at,
"E0858",
`Slice assignment is not supported for element type 'bool' of '${name}'`,
"Only integer and string buffers can be sliced.",
);
return null;
}
const capacity = declared.dimensions[0];
if (typeof capacity !== "number" || capacity <= 0) {
// A dimension this pass cannot fold -- a C macro, or a size that did not
// settle, which UNRESOLVED_DIMENSION records as 0 (#1175). Reporting a
// bounds error would mean guessing at the bound.
this.report(
at,
"E0858",
`Cannot determine the size of '${name}' at compile time`,
"A sliced buffer needs a dimension that folds at compile time, so the span can be checked.",
);
return null;
}
return { elementBytes: bits / 8, capacity };
}
/** The source value has to fit the slice it is written into. */
private checkSource(
site: TAssignmentSite,
name: string,
length: number,
): void {
const value = site.expression();
const literal = this.constantOf(value);
if (literal !== undefined) {
// ADR-052 types the literal to the slice's byte width, so it has to be
// representable there either unsigned or as two's complement. Guarding
// only the unsigned upper bound let a negative literal of any magnitude
// through, silently truncated (#1085 review).
const truncated = BigInt(Math.trunc(literal));
if (
truncated >= 1n << BigInt(8 * length) ||
truncated < -(1n << BigInt(8 * length - 1))
) {
this.report(
value,
"E0861",
`Slice assignment literal value (${literal}) does not fit in the ${length}-byte slice for '${name}'`,
"Narrow the value, or widen the slice.",
);
}
return;
}
// An integer composite's type is the typer's (`CompositeType`); null
// when it cannot name one (a mixed composite, an unknown-width C integer)
const sourceType =
OperandTyper.typeOf(value, this.context)?.typeName ?? null;
Iif (sourceType === null) return;
const bits = TYPE_WIDTH[sourceType];
if (
bits === undefined ||
sourceType.startsWith("f") ||
sourceType === "bool"
) {
this.report(
value,
"E0861",
`Slice assignment source must be an integer value; the value assigned to '${name}' has type '${sourceType}'`,
"A slice copies the source's bytes, so the source has to be an integer.",
);
return;
}
if (length > bits / 8) {
this.report(
value,
"E0861",
`Slice assignment length (${length} bytes) exceeds the source value width (${bits / 8} bytes) for '${name}'`,
"Copying more bytes than the source holds would shift past its width, which is undefined behavior.",
);
}
}
/**
* A compile-time constant, or undefined.
*
* #1322 review: this asked the flat const map, whose bare key every scope
* declaring that name shares. A slice bound named by a scoped const was
* measured against whichever scope was derived last. `ConstantExpression`
* is the one evaluator now, and it asks from where the bound is written.
*/
private constantOf(expr: Parser.ExpressionContext): number | undefined {
return ConstantExpression.valueAt(expr, this.context) ?? undefined;
}
private report(
at: Parser.ExpressionContext,
code: string,
message: string,
helpText: string,
): void {
const { line, column } = ParserUtils.getPosition(at);
this.found.push({ code, line, column, message, helpText });
}
}
class SliceAssignmentAnalyzer {
/** #1456: handed in rather than read off shared state. */
constructor(private readonly context: IAnalysisContext) {}
public analyze(tree: Parser.ProgramContext): ISliceAssignmentError[] {
const listener = new SliceAssignmentListener(this.context);
ParseTreeWalker.DEFAULT.walk(
new AssignmentSiteListener((site) => listener.checkSite(site)),
tree,
);
return listener.errors();
}
}
export default SliceAssignmentAnalyzer;
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