On Thu, 9 May 2024 at 10:54, Al Viro <v...@zeniv.linux.org.uk> wrote:
>
> On Thu, May 09, 2024 at 08:38:28AM -0700, Linus Torvalds wrote:
>
> > Going the other way is similar:
> >
> > all_bits = low_bits + ((u64) high_bits << 16) << 16);
> >
> > and again, the compiler will recognize this idiom and do the right
> > thing (and if 'all_bits' is only 32-bit, the compiler will optimize
> > the high bit noise away).
>
> Umm... That would make sense if it was
> all_bits = low_bits + ((T) high_bits << 16) << 16);
> with possibly 32bit T. But the way you wrote that (with u64) it's
> pointless - u64 _can_ be shifted by 32 just fine.
Casting to 'T' is probably a clearer option but doesn't work very well
in a helper functions that may not know what the final type is.
Any half-way decent compiler will end up optimizing away the shifts
and adds for the high bits because they see the assignment to
'all_bits'. There's no point in generating high bits that just get
thrown away.
Linus
