Re: [PATCH v2 07/13] btrfs-progs: Fix Wmaybe-uninitialized warning

From: David Sterba <dsterba@suse.cz>
To: Qu Wenruo <wqu@suse.com>
Cc: linux-btrfs@vger.kernel.org
Subject: Re: [PATCH v2 07/13] btrfs-progs: Fix Wmaybe-uninitialized warning
Date: Wed, 5 Dec 2018 14:40:05 +0100	[thread overview]
Message-ID: <20181205134005.GB3905@twin.jikos.cz> (raw)
In-Reply-To: <20181205064018.27755-8-wqu@suse.com>

On Wed, Dec 05, 2018 at 02:40:12PM +0800, Qu Wenruo wrote:
> GCC 8.2.1 will report the following warning with "make W=1":
> 
>   ctree.c: In function 'btrfs_next_sibling_tree_block':
>   ctree.c:2990:21: warning: 'slot' may be used uninitialized in this function [-Wmaybe-uninitialized]
>     path->slots[level] = slot;
>     ~~~~~~~~~~~~~~~~~~~^~~~~~
> 
> The culprit is the following code:
> 
> 	int slot;		<< Not initialized
> 	int level = path->lowest_level + 1;
>  	BUG_ON(path->lowest_level + 1 >= BTRFS_MAX_LEVEL);
>  	while(level < BTRFS_MAX_LEVEL) {
> 		slot = path->slots[level] + 1;
> 		^^^^^^ but we initialize @slot here.
> 		...
> 	}
> 	path->slots[level] = slot;
> 
> It's possible that compiler doesn't get enough hint for BUG_ON() on
> lowest_level + 1 >= BTRFS_MAX_LEVEL case.
> 
> Fix it by using a do {} while() loop other than while() {} loop, to
> ensure we will run the loop for at least once.

I was hoping that we can actually add the hint to BUG_ON so the code
does not continue if the condition is true.