On 30 Oct 2011, Avi Kivity wrote:
> The memory API supports 64-bit buses (e.g. PCI). A size on such a bus cannot
> be represented with a 64-bit data type, if both 0 and the entire address
> space size are to be represented. Futhermore, any address arithemetic may
> overflow and return unexpected results.
>
> Introduce a 128-bit signed integer type for use in such cases. Addition,
> subtraction, and comparison are the only operations supported.
>
> Signed-off-by: Avi Kivity <a...@redhat.com>
> ---
> int128.h | 116
> ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
> 1 files changed, 116 insertions(+), 0 deletions(-)
> create mode 100644 int128.h
>
> diff --git a/int128.h b/int128.h
> new file mode 100644
> index 0000000..b3864b6
> --- /dev/null
> +++ b/int128.h
> @@ -0,0 +1,116 @@
> +#ifndef INT128_H
> +#define INT128_H
> +
> +typedef struct Int128 Int128;
> +
> +struct Int128 {
> + uint64_t lo;
> + int64_t hi;
> +};
> +static inline Int128 int128_add(Int128 a, Int128 b)
> +{
> + Int128 r = { a.lo + b.lo, a.hi + b.hi };
> + r.hi += (r.lo < a.lo) || (r.lo < b.lo);
> + return r;
> +}
This is a bit redundant. You only need either:
r.hi += r.lo < a.lo;
or:
r.hi += r.lo < b.lo;
because the way that two's complement addition works means that r.lo
will always be less than both a.lo and b.lo, or
greater-than-or-equal-to both of them.
> +static inline bool int128_ge(Int128 a, Int128 b)
> +{
> + return int128_nonneg(int128_sub(a, b));
> +}
This is wrong if you get signed overflow in int128_sub(a, b).
> Regardless, the need for careful coding means subtle bugs,
Indeed :-)
Jay.
