From: Nathan Chancellor <natechancellor@gmail.com>
To: Matthias Kaehlcke <mka@chromium.org>
Cc: "David S . Miller" <davem@davemloft.net>,
Alexey Kuznetsov <kuznet@ms2.inr.ac.ru>,
Hideaki YOSHIFUJI <yoshfuji@linux-ipv6.org>,
Alexander Duyck <alexander.h.duyck@redhat.com>,
netdev@vger.kernel.org, linux-kernel@vger.kernel.org,
Douglas Anderson <dianders@chromium.org>,
Nick Desaulniers <ndesaulniers@google.com>,
Nathan Huckleberry <nhuck@google.com>,
clang-built-linux@googlegroups.com
Subject: Re: [PATCH] net/ipv4: fib_trie: Avoid cryptic ternary expressions
Date: Tue, 18 Jun 2019 16:04:20 -0700 [thread overview]
Message-ID: <20190618230420.GA84107@archlinux-epyc> (raw)
In-Reply-To: <20190618211440.54179-1-mka@chromium.org>
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
>
next prev parent reply other threads:[~2019-06-18 23:04 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-06-18 21:14 [PATCH] net/ipv4: fib_trie: Avoid cryptic ternary expressions Matthias Kaehlcke
2019-06-18 21:45 ` Doug Anderson
2019-06-18 22:57 ` Matthias Kaehlcke
2019-06-18 23:04 ` Nathan Chancellor [this message]
2019-06-18 23:21 ` Matthias Kaehlcke
2019-06-19 2:00 ` Alexander Duyck
2019-06-18 23:23 ` Nick Desaulniers
2019-06-19 9:36 ` Joe Perches
2019-06-19 17:41 ` Nick Desaulniers
2019-06-19 18:10 ` Joe Perches
2019-06-19 21:29 ` David Miller
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