Subject: Re: bug#48264: [PATCH v3 15/15] Add and use BVAR_FIELD macros

From: Eli Zaretskii <[email-masked]>
Date: Fri, 07 May 2021 14:03:28 +0300

> From: Spencer Baugh <[email-masked]>
> Date: Thu,  6 May 2021 17:33:46 -0400
> Cc: Spencer Baugh <[email-masked]>
> 
> diff --git a/src/buffer.c b/src/buffer.c
> index abf112a898..e048e7559f 100644
> --- a/src/buffer.c
> +++ b/src/buffer.c
> @@ -122,17 +122,17 @@ fix_position (Lisp_Object pos)
>  static void
>  bset_abbrev_mode (struct buffer *b, Lisp_Object val)
>  {
> -  b->abbrev_mode_ = val;
> +  b->BVAR_DEFAULTED_FIELD(abbrev_mode) = val;

Yuck!  Can we avoid having a macro in the struct field names?  I'd
prefer that the BVAR_DEFAULTED_FIELD macro accepted the buffer as
argument instead (if we need a macro at all; see below).

> --- a/src/buffer.h
> +++ b/src/buffer.h
> @@ -280,13 +280,18 @@ struct buffer_text
>      bool_bf redisplay : 1;
>    };
>  
> +#define BVAR_FIELD(field) field ## _
> +#define BVAR_DEFAULTED_FIELD(field) field ## _defaulted_

I'm not sure these changes are for the better.  Why not use _ and
_defaulted_ literally?  The macros don't make code easier to write,
and at least for me make it harder to understand (because I need to
look up the macro definition).

> @@ -714,12 +719,12 @@ XBUFFER (Lisp_Object a)
>  INLINE void
>  bset_bidi_paragraph_direction (struct buffer *b, Lisp_Object val)
>  {
> -  b->bidi_paragraph_direction_ = val;
> +  b->BVAR_DEFAULTED_FIELD(bidi_paragraph_direction) = val;
>  }
>  INLINE void
>  bset_cache_long_scans (struct buffer *b, Lisp_Object val)
>  {
> -  b->cache_long_scans_ = val;
> +  b->BVAR_DEFAULTED_FIELD(cache_long_scans) = val;
>  }
>  INLINE void
>  bset_case_canon_table (struct buffer *b, Lisp_Object val)
> @@ -744,12 +749,12 @@ bset_display_count (struct buffer *b, Lisp_Object val)
>  INLINE void
>  bset_left_margin_cols (struct buffer *b, Lisp_Object val)
>  {
> -  b->left_margin_cols_ = val;
> +  b->BVAR_DEFAULTED_FIELD(left_margin_cols) = val;
>  }

Hmm... I'm not sure I understand the effect of these.  Does it mean C
code can no longer set the buffer-local value of these variables, only
the default value?

> +#define PER_BUFFER_VAR_DEFAULTED_OFFSET(VAR) \
> +  offsetof (struct buffer, BVAR_DEFAULTED_FIELD(VAR))

Likewise here: I don't see how such macros make the code more
readable.  I think they make it less readable.