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Re: Strange conversion to int64
To ensure you get what you'd expect, it's a good idea to explicitly
cast your operands in these mixed sign cases.
Brian
On Wed, Jan 27, 2010 at 7:53 PM, me22 <me22.ca@gmail.com> wrote:
> 2010/1/27 Tony Bernardin <sarusama@gmail.com>:
>>
>> I was expecting to get -64 in every case. I don't understand why
>> (int64) = -1 (int32) * 64 (uint32)
>> is different from
>> (int64) = -1 (int32) * 64 (uint64)
>>
>> Also why is
>> (int64) = -1 (int32) * 64 (uint32)
>> is different from
>> (int32) = -1 (int32) * 64 (uint32)
>>
>
> The difference is in whether the multiplicand or the product is being
> promoted, and thus whether things are being sign- or zero-extended.
>
> Here's why (and how) you get 4294967232:
>
> ? ?(int32)-1 * (uint32)64
>
> Then promote both multiplicands to unsigned
>
> ? ?(uint32)4294967295 * (uint32)64
>
> Perform the unsigned 32-bit multiplication
>
> ? ?(uint32)4294967232
>
> Promote to 64-bits, using zero-extension since it's unsigned
>
> ? ?(uint64)4294967232
>
> Then coerce it to signed
>
> ? ?(int64)4294967232
>
> HTH,
> ~ Scott
>