Am 10.02.2012 10:51, schrieb Jay Foad:
> 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 :-)
Are these just theoretical issues or does anything in particular break
for you? These functions were introduced to tackle int64_t overflow
issues in the Memory API, not as an arbitrary API AFAIU.
If nothing's broken in practice it would be best if you could just send
a patch with your proposed fix rather than describing it in words. :)
Andreas
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