linux-kernel - Re: [RFC PATCH 1/1] checkpatch: don't suggest else is not useful with chained

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Date:   Mon, 21 Mar 2022 07:24:53 -0700
From:   Joe Perches <joe@...ches.com>
To:     Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>,
        Andy Whitcroft <apw@...onical.com>,
        Dwaipayan Ray <dwaipayanray1@...il.com>,
        Lukas Bulwahn <lukas.bulwahn@...il.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH 1/1] checkpatch: don't suggest else is not useful
 with chained

On Mon, 2022-03-21 at 11:18 +0200, Ilpo Järvinen wrote:
> When if blocks are chained with "else if" like this:
> 
> 	if (a)
> 		c = 0;
> 	else if (b)
> 		break;
> 	else
> 		c = 1;
> 
> checkpatch recommends removing else:
> 
> WARNING: else is not generally useful after a break or return
> 
> Removing else will easily introduce logic errors in this situation
> so it's better to check if the preceding line has "else if" before
> issuing that warning.
> 
> Fixes: 032a4c0f9a77 ("checkpatch: warn on unnecessary else after return or break")
> Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>
> ---
> 
> I was only able to solve this case for single line if blocks but
> it would be useful to address also multi-line if blocks but it is
> beyond my understanding about checkpatch internals.
> 
>  scripts/checkpatch.pl | 13 ++++++++-----
>  1 file changed, 8 insertions(+), 5 deletions(-)
> 
> diff --git a/scripts/checkpatch.pl b/scripts/checkpatch.pl
[]
> @@ -4037,14 +4037,17 @@ sub process {
>  # check indentation of any line with a bare else
>  # (but not if it is a multiple line "if (foo) return bar; else return baz;")
>  # if the previous line is a break or return and is indented 1 tab more...
> +# but don't warn when there is "else if" on before that line to avoid logic errors
>  		if ($sline =~ /^\+([\t]+)(?:}[ \t]*)?else(?:[ \t]*{)?\s*$/) {
>  			my $tabs = length($1) + 1;
> -			if ($prevline =~ /^\+\t{$tabs,$tabs}break\b/ ||
> -			    ($prevline =~ /^\+\t{$tabs,$tabs}return\b/ &&
> -			     defined $lines[$linenr] &&
> -			     $lines[$linenr] !~ /^[ \+]\t{$tabs,$tabs}return/)) {
> +			if ($linenr >= 3 &&
> +			    $lines[$linenr - 3] !~ /[} \t]else\s+if\s*\(/ &&
> +			    ($prevline =~ /^\+\t{$tabs,$tabs}break\b/ ||
> +			     ($prevline =~ /^\+\t{$tabs,$tabs}return\b/ &&
> +			      defined $lines[$linenr] &&
> +			      $lines[$linenr] !~ /^[ \+]\t{$tabs,$tabs}return/))) {
>  				WARN("UNNECESSARY_ELSE",
> -				     "else is not generally useful after a break or return\n" . $hereprev);
> +				     "else is not generally useful after a break or return\n" . $hereprev . "\n");

I believe this wouldn't work on patches if the lines above
are deletions.