Re: [PATCH] net/ipv4: fib_trie: Avoid cryptic ternary expressions

[Nathan Chancellor <natechancellor@gmail.com>]

On Tue, Jun 18, 2019 at 02:14:40PM -0700, Matthias Kaehlcke wrote:
> empty_child_inc/dec() use the ternary operator for conditional
> operations. The conditions involve the post/pre in/decrement
> operator and the operation is only performed when the condition
> is *not* true. This is hard to parse for humans, use a regular
> 'if' construct instead and perform the in/decrement separately.
> 
> This also fixes two warnings that are emitted about the value
> of the ternary expression being unused, when building the kernel
> with clang + "kbuild: Remove unnecessary -Wno-unused-value"
> (https://lore.kernel.org/patchwork/patch/1089869/):
> 
> CC      net/ipv4/fib_trie.o
> net/ipv4/fib_trie.c:351:2: error: expression result unused [-Werror,-Wunused-value]
>         ++tn_info(n)->empty_children ? : ++tn_info(n)->full_children;
> 

As an FYI, this is also being fixed in clang:

https://bugs.llvm.org/show_bug.cgi?id=42239

https://reviews.llvm.org/D63369

> Signed-off-by: Matthias Kaehlcke <mka@chromium.org>
> ---
> I have no good understanding of the fib_trie code, but the
> disentangled code looks wrong, and it should be equivalent to the
> cryptic version, unless I messed it up. In empty_child_inc()
> 'full_children' is only incremented when 'empty_children' is -1. I
> suspect a bug in the cryptic code, but am surprised why it hasn't
> blown up yet. Or is it intended behavior that is just
> super-counterintuitive?
> 
> For now I'm leaving it at disentangling the cryptic expressions,
> if there is a bug we can discuss what action to take.
> ---
>  net/ipv4/fib_trie.c | 10 ++++++++--
>  1 file changed, 8 insertions(+), 2 deletions(-)
> 
> diff --git a/net/ipv4/fib_trie.c b/net/ipv4/fib_trie.c
> index 868c74771fa9..cf7788e336b7 100644
> --- a/net/ipv4/fib_trie.c
> +++ b/net/ipv4/fib_trie.c
> @@ -338,12 +338,18 @@ static struct tnode *tnode_alloc(int bits)
>  
>  static inline void empty_child_inc(struct key_vector *n)
>  {
> -	++tn_info(n)->empty_children ? : ++tn_info(n)->full_children;
> +	tn_info(n)->empty_children++;
> +
> +	if (!tn_info(n)->empty_children)
> +		tn_info(n)->full_children++;
>  }
>  
>  static inline void empty_child_dec(struct key_vector *n)
>  {
> -	tn_info(n)->empty_children-- ? : tn_info(n)->full_children--;
> +	if (!tn_info(n)->empty_children)
> +		tn_info(n)->full_children--;
> +
> +	tn_info(n)->empty_children--;
>  }
>  
>  static struct key_vector *leaf_new(t_key key, struct fib_alias *fa)
> -- 
> 2.22.0.410.gd8fdbe21b5-goog
>