On Wed, Mar 5, 2025 at 10:40 AM Jeff Davis <pgsql@j-davis.com> wrote:
>
> On Thu, 2025-03-06 at 01:07 +1300, David Rowley wrote:
> > I've attached the results. The 3990x with clang looks good, but the
> > rest are mostly slower.
>
> I am still curious why.
>
> If it's due to compiler misoptimization, is that kind of thing often
> misoptimized, or is there something we're doing in particular?
>
> Even if we don't have answers, it might be worth adding a brief comment
> that we empirically determined that booleans are faster than bitfields
> or flags. In the future, maybe compilers mostly get this right, and we
> want to change to bitfields.
I haven't run this sort of experiment in years, and CPUs continue to
improve -- but with a Boolean, the CPU can read from or write to that
Boolean directly, one instruction. A Boolean tends to be a byte, and
memory is byte-addressable.
For a bitfield, however, the CPU has to read from or write to the byte
that contains the bit, but then it also has to mask out the *other*
bits in that bitfield. This is a data dependency, so it stalls the CPU
pipeline.
So Booleans tend to be faster than bitfields, because they avoid a
pipeline stall.
James Hunter
