Re: [PATCH] builtin/mv.c: use correct type to compute size of an array element

From: Jeff King <peff@peff.net>
Date: Jul 7, 2022, 18:42 UTC
Message-ID: <YscpDTgwxUko/aZM@coredump.intra.peff.net>
In-Reply-To: <xmqq8rp54r4l.fsf@gitster.g>

On Wed, Jul 06, 2022 at 07:02:18PM -0700, Junio C Hamano wrote:

Show 8 quoted lines

>    * Should we in general use sizeof(TYPE) in these cases, instead
>      of the size of the zeroth element, e.g.
> 
>  		memmove(source + i, source + i + 1,
> 			n * sizeof(source[i]));
>     
>      It would have been caught by the above Coccinelle rule (we are
>      taking the size of *dst).

I'm not sure I understand this. As you noted in a later email, using sizeof(TYPE) is less maintainable if the type of "source" changes. But later you mention using "*source" instead of "source[i]". I don't think there is a particular reason to prefer one over the other from the compiler perspective. I find "*source" more idiomatic (but better still of course is MOVE_ARRAY, which removes the choice entirely).

Show 13 quoted lines

>    * Shouldn't we have an array of struct with four members, instead
>      of four parallel arrays, e.g.
> 
> 		struct {
> 			const char *source;
> 			const char *destination;
> 			enum update_mode mode;
> 			const char *submodule_gitfile;
> 		} *mv_file;
> 
>    The latter question is important to answer before we accept
>    Coccinelle-suggested rewrite to use four MOVE_ARRAY() on these
>    four parallel arrays on top of this fix.

I think that would make the code a lot cleaner. But it looks like "source" and "destination" come from separate calls to internal_prefix_pathspec(). So you'd have to reconcile that. And there's some extra trickiness that sometimes "destination" comes from "dest_path", which _isn't_ always the same size as "source".

So I suspect the code which uses these arrays would be cleaner with a struct, but the setup may get worse. :)

-Peff