From: Junio C Hamano <gitster@pobox.com>
Date: Mon, 16 Mar 2009 07:12:50 GMT
Subject: Re: [PATCH 2/2] make the ST_{C,M}TIME_NSEC macros more function like
Message-ID: <7vhc1ux7nx.fsf@gitster.siamese.dyndns.org>
In-Reply-To: <86tz5u1m7i.fsf@broadpark.no>

Kjetil Barvik <barvik@broadpark.no> writes:

>     [...] in C or Pascal, calling a function with a large structure as
>     an argument will cause the entire structure to be copied,
>     potentially causing serious performance degradation, and mutations
>     to the structure are invisible to the caller. [...]
>
>   So in my eyes it make more sense to be consistent and take the address
>   of all struct like objects (&st in this case) for all arguments to
>   "function-like" things.

Notice the "mutations to the structure are invisible to the caller" part.
The call site of st_ctime_nsec(st) can be sure that st won't be modified,
without checking the definition of the function.

Which is actually a nice property.  When st_ctime_nsec(st) is implemented as
a macro, you _could_ write it in such a way to mutate what is in st, but
the implementation does not do so, and will be unlikely to in the future,
so I think writing it as if it is a function that receives a structure by
value will help readers of the calling code.

And the readability is what we should optimize for when picking from two
ways to write it, and when the generated code is the same.