linux-kernel - Re: [PATCH] lib/lz4: smatch warning in LZ4_decompress

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Date:   Sun, 07 Jun 2020 18:02:44 -0700
From:   Joe Perches <joe@...ches.com>
To:     Yann Collet <cyan@...com>, Vasily Averin <vvs@...tuozzo.com>,
        Gao Xiang <hsiangkao@....com>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Gao Xiang <xiang@...nel.org>
Subject: Re: [PATCH] lib/lz4: smatch warning in LZ4_decompress_generic()

On Mon, 2020-06-08 at 00:40 +0000, Yann Collet wrote:
> Hi Vasily
> 
> 
> If I do understand the discussion, the question is about usage of `&` instead of `&&`,
> and the speculation that it might be an error.
> 
> It's not an error. Unfortunately, explaining the reasoning behind this decision is a bit long.

Likely better to add a comment around the use so that
another patch like this doesn't get submitted again.

Perhaps something like:
---
 lib/lz4/lz4_decompress.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/lib/lz4/lz4_decompress.c b/lib/lz4/lz4_decompress.c
index 0c9d3ad17e0f..5371dab6b481 100644
--- a/lib/lz4/lz4_decompress.c
+++ b/lib/lz4/lz4_decompress.c
@@ -141,6 +141,9 @@ static FORCE_INLINE int LZ4_decompress_generic(
 		 * space in the output for those 18 bytes earlier, upon
 		 * entering the shortcut (in other words, there is a
 		 * combined check for both stages).
+		 *
+		 * The & in the likely() below is intentionally not && so that
+		 * some compilers can produce better parallelized runtime code
 		 */
 		if ((endOnInput ? length != RUN_MASK : length <= 8)
 		   /*