On Wed, 2017-10-11 at 10:32 +1100, Tobin C. Harding wrote: > On Tue, Oct 10, 2017 at 04:15:01PM -0700, Linus Torvalds wrote: > > On Tue, Oct 10, 2017 at 4:09 PM, Tobin C. Harding <me@xxxxxxxx> wrote: > > > > > > I did not understand the code (specifically why the right shift of 16 twice?) > > > > It's a traditional trick to get the upper 32 bits. > > > > So it basically splits the (possibly 64-bit) pointer into the lower 32 > > bits and the upper 32 bits for a hash such as "jhash()" that takes > > data that is "unsigned int". > > > > (NOTE! Using jhash here is not acceptable, since it's not > > cryptographically safe, but think of it as an example of a hash that > > takes 32-bit input). > > > > Doing ">> 32" is undefined on 32-bit architectures, and wouldn't work. > > > > But doing >> 16 >> 16 is a fine way to say "shift right by 32 on a > > 64-bit architecture" while also being well-defined on a 32-bit one. > > > > Linus > > Awesome, thanks. Another way is using the upper_32_bits() macro. It's perhaps a bit more readable.
