transPtrDiffExpr

fn transPtrDiffExpr(t: *Translator, scope: *Scope, bin: Node.Binary) TransError!ZigNode

File

Code

fn transPtrDiffExpr(t: *Translator, scope: *Scope, bin: Node.Binary) TransError!ZigNode {
    const lhs_uncasted = try t.transExpr(scope, bin.lhs, .used);
    const rhs_uncasted = try t.transExpr(scope, bin.rhs, .used);

    const lhs = try ZigTag.int_from_ptr.create(t.arena, lhs_uncasted);
    const rhs = try ZigTag.int_from_ptr.create(t.arena, rhs_uncasted);

    const sub_res = try t.createBinOpNode(.sub_wrap, lhs, rhs);

    // @divExact(@as(<platform-ptrdiff_t>, @bitCast(@intFromPtr(lhs)) -% @intFromPtr(rhs)), @sizeOf(<lhs target type>))
    const ptrdiff_type = try t.transTypeIntWidthOf(bin.qt, true);

    const bitcast = try ZigTag.as.create(t.arena, .{
        .lhs = ptrdiff_type,
        .rhs = try ZigTag.bit_cast.create(t.arena, sub_res),
    });

    // C standard requires that pointer subtraction operands are of the same type,
    // otherwise it is undefined behavior. So we can assume the left and right
    // sides are the same Type and arbitrarily choose left.
    const lhs_ty = try t.transType(scope, bin.lhs.qt(t.tree), bin.lhs.tok(t.tree));
    const c_pointer = t.getContainer(lhs_ty).?;

    if (c_pointer.castTag(.c_pointer)) |c_pointer_payload| {
        const sizeof = try ZigTag.sizeof.create(t.arena, c_pointer_payload.data.elem_type);
        return ZigTag.div_exact.create(t.arena, .{
            .lhs = bitcast,
            .rhs = sizeof,
        });
    } else {
        // This is an opaque/incomplete type. This subtraction exhibits Undefined Behavior by the C99 spec.
        // However, allowing subtraction on `void *` and function pointers is a commonly used extension.
        // So, just return the value in byte units, mirroring the behavior of this language extension as implemented by GCC and Clang.
        return bitcast;
    }
}