radiance · commit

lang: Preserve folded shift count signs

168352d2407d94d280a90b7fbbb7e6cfcc16dfb85f7ef6aa88a946d2f1025ee5

The constant folder masked a shift count from its absolute
magnitude. Negative counts therefore selected a different
hardware-masked shift than the equivalent runtime expression.

Derive folded shift counts from their two’s-complement bit patterns
before masking them to the operand width.

Assisted-by: Codex:gpt-5.6-sol

Alexis Sellier committed 1 month ago 1 parent 30590736

lib/std/lang/resolver.rad +8 -3

        return -(c.magnitude as i64);
    }
    return c.magnitude as i64;
}

/// Convert a [`ConstInt`] to its two's-complement bit pattern.
fn constIntToBits(c: ConstInt) -> u64 {
    return (0 - c.magnitude) if c.negative else c.magnitude;
}

/// Build a [`ConstInt`] from a signed result, preserving bit width and signedness.
fn constIntFromSigned(value: i64, bits: u8, signed: bool) -> ConstInt {
    if value < 0 {
        // Compute magnitude without signed overflow.
        let uval = value as u64;


    match op {
        // Shift counts are masked to the left operand's width, matching
        // the runtime word instructions.
        case ast::BinaryOp::Shl => {
            let raw = (0 - left.magnitude) if left.negative else left.magnitude;
            let shamt = right.magnitude % left.bits as u64;
            let raw = constIntToBits(left);
            let shamt = constIntToBits(right) % left.bits as u64;
            return ConstValue::Int(constIntFromBits(raw << shamt, left.bits, left.signed));
        },
        case ast::BinaryOp::Shr => {
            let shamt = right.magnitude % left.bits as u64;
            let shamt = constIntToBits(right) % left.bits as u64;
            if left.signed {
                let shifted = constIntToSigned(left) >> shamt as i64;
                return ConstValue::Int(
                    constIntFromBits(shifted as u64, left.bits, true)
                );

test/tests/const.shift.negative.count.rad added +16 -0

1	+	//! returns: 0
2	+	//! Folded and runtime shifts must interpret negative counts identically.
3	+
4	+	constant ONE: u32 = 1;
5	+	constant FOLDED: u32 = ONE << -1;
6	+
7	+	fn runtime(value: u32) -> u32 {
8	+	return value << -1;
9	+	}
10	+
11	+	@default fn main() -> i32 {
12	+	if FOLDED == runtime(1) {
13	+	return 0;
14	+	}
15	+	return 1;
16	+	}

6285	6285		return -(c.magnitude as i64);
6286	6286		}
6287	6287		return c.magnitude as i64;
6288	6288		}
6289	6289
	6290	+	/// Convert a [`ConstInt`] to its two's-complement bit pattern.
	6291	+	fn constIntToBits(c: ConstInt) -> u64 {
	6292	+	return (0 - c.magnitude) if c.negative else c.magnitude;
	6293	+	}
	6294	+
6290	6295		/// Build a [`ConstInt`] from a signed result, preserving bit width and signedness.
6291	6296		fn constIntFromSigned(value: i64, bits: u8, signed: bool) -> ConstInt {
6292	6297		if value < 0 {
6293	6298		// Compute magnitude without signed overflow.
6294	6299		let uval = value as u64;

6340	6345
6341	6346		match op {
6342	6347		// Shift counts are masked to the left operand's width, matching
6343	6348		// the runtime word instructions.
6344	6349		case ast::BinaryOp::Shl => {
6345		-	let raw = (0 - left.magnitude) if left.negative else left.magnitude;
6346		-	let shamt = right.magnitude % left.bits as u64;
	6350	+	let raw = constIntToBits(left);
	6351	+	let shamt = constIntToBits(right) % left.bits as u64;
6347	6352		return ConstValue::Int(constIntFromBits(raw << shamt, left.bits, left.signed));
6348	6353		},
6349	6354		case ast::BinaryOp::Shr => {
6350		-	let shamt = right.magnitude % left.bits as u64;
	6355	+	let shamt = constIntToBits(right) % left.bits as u64;
6351	6356		if left.signed {
6352	6357		let shifted = constIntToSigned(left) >> shamt as i64;
6353	6358		return ConstValue::Int(
6354	6359		constIntFromBits(shifted as u64, left.bits, true)
6355	6360		);