Subject: Re: [PATCH v2 1/3] merge-base, xdiff: zero out xpparam_t structures

From: Junio C Hamano <[email-masked]>
Date: Mon, 12 Oct 2020 12:52:59 -0700

Michał Kępień <[email-masked]> writes:

> diff --git a/builtin/merge-tree.c b/builtin/merge-tree.c
> index e72714a5a8..de8520778d 100644
> --- a/builtin/merge-tree.c
> +++ b/builtin/merge-tree.c
> @@ -109,6 +109,7 @@ static void show_diff(struct merge_list *entry)
>  	xdemitconf_t xecfg;
>  	xdemitcb_t ecb;
>  
> +	memset(&xpp, 0, sizeof(xpp));
>  	xpp.flags = 0;
>  	memset(&xecfg, 0, sizeof(xecfg));
>  	xecfg.ctxlen = 3;

The existing "xpp.flags=0" can then go away, and as Dscho hinted,

	xpparam_t xpp = {
		.flags = 0,
	};

may also be a valid way to write this.  What it says is that we want
the entirety of xpp to be reasonably initialized, but we do care
that its .flags field to be exactly zero.

> diff --git a/xdiff/xhistogram.c b/xdiff/xhistogram.c
> index c7b35a9667..e694bfd9e3 100644
> --- a/xdiff/xhistogram.c
> +++ b/xdiff/xhistogram.c
> @@ -235,6 +235,8 @@ static int fall_back_to_classic_diff(xpparam_t const *xpp, xdfenv_t *env,
>  		int line1, int count1, int line2, int count2)
>  {
>  	xpparam_t xpparam;
> +
> +	memset(&xpparam, 0, sizeof(xpparam));
>  	xpparam.flags = xpp->flags & ~XDF_DIFF_ALGORITHM_MASK;

Likewise.  Giving an initializer to the local variable def, e.g.

	xpparam_t xpparam = {
		.flags = xpp->flags & ~XDF_DIFF_ALGORITHM_MASK,
	};

would allow us to lose one line.

>  	return xdl_fall_back_diff(env, &xpparam,
> diff --git a/xdiff/xpatience.c b/xdiff/xpatience.c
> index 3c5601b602..20699a6f60 100644
> --- a/xdiff/xpatience.c
> +++ b/xdiff/xpatience.c
> @@ -318,6 +318,8 @@ static int fall_back_to_classic_diff(struct hashmap *map,
>  		int line1, int count1, int line2, int count2)
>  {
>  	xpparam_t xpp;
> +
> +	memset(&xpp, 0, sizeof(xpp));
>  	xpp.flags = map->xpp->flags & ~XDF_DIFF_ALGORITHM_MASK;

Likewise.

>  	return xdl_fall_back_diff(map->env, &xpp,

But the patch is good as-is, given especially the way how xecfg is
cleared the same way in builtin/merge-tree.c::show_diff().

Will queue.  Thanks.