Re: [PATCH 1/4] -Wmisleading-indentation: don't warn in presence of entirely blank lines

[Patrick Palka <patrick at parcs dot ath dot cx>]

On Thu, Oct 29, 2015 at 12:49 PM, David Malcolm <dmalcolm@redhat.com> wrote:
> Attempting to bootstrap gcc with -Wmisleading-indentation enabled I ran
> into a few failures where the indentation, although bad, was arguably
> not misleading.
>
> In regrename.c:scan_rtx_address:
>
>   1308      case PRE_MODIFY:
>   1309        /* If the target doesn't claim to handle autoinc, this must be
>   1310           something special, like a stack push.  Kill this chain.  */
>   1311      if (!AUTO_INC_DEC)
>   1312        action = mark_all_read;
>   1313
>   1314        break;
>               ^ this is indented at the same level as the "action =" code,
>               but clearly isn't guarded by the if () at line 1311.
>
> In gcc/fortran/io.c:gfc_match_open:
>
>   1997          {
>   1998            static const char *delim[] = { "APOSTROPHE", "QUOTE", "NONE", NULL };
>   1999
>   2000          if (!is_char_type ("DELIM", open->delim))
>   2001            goto cleanup;
>   2002
>   2003            if (!compare_to_allowed_values ("DELIM", delim, NULL, NULL,
>   2004                                            open->delim->value.character.string,
>   2005                                            "OPEN", warn))
>                   ^ this is indented with the "goto cleanup;" due to
>                     lines 2000-2001 not being indented enough, but
>                     line 2003 clearly isn't guarded by the
>                     "if (!is_char_type" conditional.
>
> In gcc/function.c:locate_and_pad_parm:
>
>   4118        locate->slot_offset.constant = -initial_offset_ptr->constant;
>   4119        if (initial_offset_ptr->var)
>   4120          locate->slot_offset.var = size_binop (MINUS_EXPR, ssize_int (0),
>   4121                                                initial_offset_ptr->var);
>   4122
>   4123          {
>   4124            tree s2 = sizetree;
>   4125            if (where_pad != none
>   4126                && (!tree_fits_uhwi_p (sizetree)
>   4127                    || (tree_to_uhwi (sizetree) * BITS_PER_UNIT) % round_boundary))
>   4128              s2 = round_up (s2, round_boundary / BITS_PER_UNIT);
>   4129            SUB_PARM_SIZE (locate->slot_offset, s2);
>   4130          }
>                 ^ this block is not guarded by the
>                   "if (initial_offset_ptr->var)"
>                   and the whitespace line (4122) is likely to make a
>                   human reader of the code read it that way also.
>
> In each case, a blank line separated the guarded code from followup code
> that wasn't guarded, and to my eyes, the blank line makes the meaning of
> the badly-indented code sufficiently clear that it seems unjustified to
> issue a -Wmisleading-indentation warning.

This makes sense to me.

Though I've been thinking about proposing a simpler and more relaxed heuristic:

    if (next_stmt_exploc.line - body_exploc.line > 1)
      return false;

That is, don't warn if there are any lines between the (start of the)
body statement and the next statement.

This would catch the presence of blank lines as well as code like:

    if (foo)
      bar (an_argument_1,
           an_argument_2);
      baz ();

and

    if (foo)
      bar ();
      /* Some comment.  */
      baz ();

Though I am not confident that we should not warn in such cases. At
this point whether some code is misleadingly indented or not becomes
pretty subjective (so it may be better to not warn?)

>
> The attached patch suppresses the warning for such a case.
>
> OK for trunk?
>
> gcc/c-family/ChangeLog:
>         * c-indentation.c (line_is_blank_p): New function.
>         (separated_by_blank_lines_p): New function.
>         (should_warn_for_misleading_indentation): Don't warn if the
>         guarded and unguarded code are separated by one or more entirely
>         blank lines.
>
> gcc/testsuite/ChangeLog:
>         * c-c++-common/Wmisleading-indentation.c (fn_40): New function.
> ---
>  gcc/c-family/c-indentation.c                       | 58 ++++++++++++++++++++++
>  .../c-c++-common/Wmisleading-indentation.c         | 32 ++++++++++++
>  2 files changed, 90 insertions(+)
>
> diff --git a/gcc/c-family/c-indentation.c b/gcc/c-family/c-indentation.c
> index 5b119f7..d9d4380 100644
> --- a/gcc/c-family/c-indentation.c
> +++ b/gcc/c-family/c-indentation.c
> @@ -77,6 +77,42 @@ get_visual_column (expanded_location exploc,
>    return true;
>  }
>
> +/* Determine if the given source line of length LINE_LEN is entirely blank,
> +   or contains one or more non-whitespace characters.  */
> +
> +static bool
> +line_is_blank_p (const char *line, int line_len)
> +{
> +  for (int i = 0; i < line_len; i++)
> +    if (!ISSPACE (line[i]))
> +      return false;
> +
> +  return true;
> +}
> +
> +/* Helper function for should_warn_for_misleading_indentation.
> +   Determines if there are one or more blank lines between the
> +   given source lines.  */
> +
> +static bool
> +separated_by_blank_lines_p (const char *file,
> +                           int start_line, int end_line)
> +{
> +  gcc_assert (start_line < end_line);
> +  for (int line_num = start_line; line_num < end_line; line_num++)

Shouldn't this loop begin at start_line + 1?