Re: [Qemu-devel] [PATCH 1/3] Add support for 128-bit arithmeticRe: [PATCH 1/3] Add support for 128-bit arithmetic

[Jay Foad <jay.foad@gmail.com>]

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 <avi@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.