Re: [PATCH 1/2] Add a parameter for the builtin function of prefetch to align with LLVM

[Andrew Pinski <pinskia@gmail.com>]

On Thu, Oct 20, 2022 at 10:28 AM Segher Boessenkool
<segher@kernel.crashing.org> wrote:
>
> On Thu, Oct 20, 2022 at 01:44:15AM +0000, Jiang, Haochen wrote:
> > Maybe the testcase change cause some misunderstanding and concern.
> >
> > Actually, the patch did not disrupt the previous builtins, as the builtin_prefetch
> > uses vargs. I set the default value of the new parameter as data prefetch, which
> > means that if we are not using the fourth parameter, just like how we use
> > prefetch previously, it is still what it is.
>
> I still think it is a mistake to have one builtin do two very distinct
> operations, only very superficially related.  Instruction fetch and data
> demand loads are almosty entirely unrelated, and so is the prefetch
> machinery for them, on all machines I am familiar with.

On aarch64 (armv8), it is actually the same instruction: PRFM. It
might be the only one which is that way though.
It even allows to specify the level for the instruction prefetch too
(which is actually useful for say OcteonTX2 which has an interesting
cache hierarchy).

Though I agree it is a mistake to have one builtin which handles both
data and instruction prefetch.

Thanks,
Andrew


> Which makes
> sense anyway, since instruction prefetch and data prefetch have
> completely different performance characteristics and considerations.
> Maybe if you start with the mistake of having unified L1 caches it
> seems natural, but thankfully most machines do not do that.
>
>
> Segher