linux-kernel - Re: [PATCH V3 2/4] cpufreq: cppc: Pass structure instance by reference

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Message-ID: <20210624022252.zrxsftrvcd43eqra@vireshk-i7>
Date:   Thu, 24 Jun 2021 07:52:52 +0530
From:   Viresh Kumar <viresh.kumar@...aro.org>
To:     Ionela Voinescu <ionela.voinescu@....com>
Cc:     Rafael Wysocki <rjw@...ysocki.net>, linux-pm@...r.kernel.org,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Qian Cai <quic_qiancai@...cinc.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH V3 2/4] cpufreq: cppc: Pass structure instance by
 reference

On 23-06-21, 14:45, Ionela Voinescu wrote:
> On Monday 21 Jun 2021 at 14:49:35 (+0530), Viresh Kumar wrote:
> > Don't pass structure instance by value, pass it by reference instead.
> >
> 
> Might be best to justify the change a bit :)

I had it and removed later as I thought it would be obvious :)

> For me this is a judgement call, and I don't really see the reasons for
> changing it: we don't care if the structure is modified or not, as we're
> not reusing the data after the call to cppc_get_rate_from_fbctrs().
> More so, in this scenario we might not even want for the called function
> to modify the counter values. Also there is no further call to a function
> in cppc_get_rate_from_fbctrs(), that might require references to the
> fb_ctrs.
> 
> So what is the reason behind this change?

How about this commit log then:

Theoretically speaking, call by reference is cheaper/faster than call by value
for structures as the later requires the compiler to make a new copy of the
whole structure (which has four u64 values here), to be used by the called
function, while with call by reference we just need to pass a single pointer
(u64 on 64-bit architectures) to the existing structure.

Yes, on modern architectures, the compilers will likely end up using the
processor registers for passing this structure as it isn't doesn't have lot of
fields and it shouldn't be bad eventually, but nevertheless the code should do
the right thing without assuming about the compiler's or architecture's
optimizations.

Don't pass structure instance by value, pass it by reference instead.

-- 
viresh