Re: [PATCH] revision: mark blobs needed for resolve-undo as reachable

[Taylor Blau <me@ttaylorr.com>]

On Tue, Jun 14, 2022 at 11:48:20PM -0400, Jeff King wrote:
> On Tue, Jun 14, 2022 at 10:02:32PM -0400, Taylor Blau wrote:
>
> > --- >8 ---
> >
> > diff --git a/string-list.h b/string-list.h
> > index d5a744e143..425abc55f4 100644
> > --- a/string-list.h
> > +++ b/string-list.h
> > @@ -143,7 +143,7 @@ int for_each_string_list(struct string_list *list,
> >
> >  /** Iterate over each item, as a macro. */
> >  #define for_each_string_list_item(item,list)            \
> > -	for (item = (list)->items;                      \
> > +	for (item = (list) ? (list)->items : NULL;      \
> >  	     item && item < (list)->items + (list)->nr; \
> >  	     ++item)
> >
> > --- 8< ---
> >
> > > but even with your suggestion, I get this compiler error:
> >
> > ...so did I. Though I'm not sure I understand the compiler's warning
> > here. Surely the thing being passed as list in the macro expansion
> > _won't_ always evaluate to non-NULL, will it?
>
> In the general case, no, but in this specific expansion of the macro, it
> is passing the address of a local variable (&cpath), which will never be
> NULL. The compiler is overeager here; the check is indeed pointless in
> this expansion, but warning on useless macro-expanded code isn't
> helpful, since other macro users need it.

Ah, that makes sense. The compiler is warning us that the macro-expanded
version of for_each_string_list_item() has a ternary expression that
will never evaluate its right-hand side in cases where it can prove the
second argument to the macro is non-NULL.

> Hiding it in a function seems to work, even with -O2 inlining, like:
>
> diff --git a/string-list.h b/string-list.h
> index d5a744e143..b28b135e11 100644
> --- a/string-list.h
> +++ b/string-list.h
> @@ -141,9 +141,14 @@ void string_list_clear_func(struct string_list *list, string_list_clear_func_t c
>  int for_each_string_list(struct string_list *list,
>  			 string_list_each_func_t func, void *cb_data);
>
> +static inline struct string_list_item *string_list_first_item(const struct string_list *list)
> +{
> +	return list ? list->items : NULL;
> +}
> +
>  /** Iterate over each item, as a macro. */
>  #define for_each_string_list_item(item,list)            \
> -	for (item = (list)->items;                      \
> +	for (item = string_list_first_item(list);       \
>  	     item && item < (list)->items + (list)->nr; \
>  	     ++item)

That works, nice. I don't really want to mess up the tree too much this
close to a release, but this sort of clean-up seems good to do. I know
Stolee identified a handful of spots that would benefit from it. Some
good #leftoverbits, I guess :-).

Thanks,
Taylor