Re: [PATCH 0/3] add %pX specifier

[Tobin C. Harding]

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.

Tobin.