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@xxxxxxxxx> wrote: > 2010/1/27 Tony Bernardin <sarusama@xxxxxxxxx>: >> >> 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 >
