From: Andrew Morton <akpm@linux-foundation.org>
To: Nathan Chancellor <nathan@kernel.org>
Cc: Nick Desaulniers <ndesaulniers@google.com>,
linux-kernel@vger.kernel.org, clang-built-linux@googlegroups.com,
terrelln@fb.com
Subject: Re: [PATCH] lib/zstd: Fix bitwise vs logical operators
Date: Mon, 16 Aug 2021 18:05:32 -0700 [thread overview]
Message-ID: <20210816180532.6e85c93f84d62dc720d21178@linux-foundation.org> (raw)
In-Reply-To: <20210815004154.1781834-1-nathan@kernel.org>
On Sat, 14 Aug 2021 17:41:54 -0700 Nathan Chancellor <nathan@kernel.org> wrote:
> clang warns several times along the lines of:
>
> lib/zstd/compress.c:1043:7: warning: bitwise and of boolean expressions; did you mean logical and? [-Wbool-operation-and]
> if ((offset_1 > 0) & (ZSTD_read32(ip + 1 - offset_1) == ZSTD_read32(ip + 1))) {
> ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> &&
>
> Bitwise ANDs do not short circuit, meaning that the ZSTD_read32 calls
> will be evaluated even if the first condition is not true. This is not
> always a problem but it is not a standard way to do conditionals so
> replace the bitwise ANDs with logical ones to fix the warning and make
> the code clearer.
>
> ...
>
> --- a/lib/zstd/compress.c
> +++ b/lib/zstd/compress.c
> @@ -1040,7 +1040,7 @@ void ZSTD_compressBlock_fast_generic(ZSTD_CCtx *cctx, const void *src, size_t sr
> const BYTE *match = base + matchIndex;
> hashTable[h] = curr; /* update hash table */
>
> - if ((offset_1 > 0) & (ZSTD_read32(ip + 1 - offset_1) == ZSTD_read32(ip + 1))) {
> + if ((offset_1 > 0) && (ZSTD_read32(ip + 1 - offset_1) == ZSTD_read32(ip + 1))) {
yeah, this is a late night party trick which is sometimes used to
attempt to speed things up by avoiding a branch. It is perhaps
beneficial if the LHS is almost always true. I guess.
I'd prefer that the code not do this - it's silly, looks wrong and I
bet it's unmeasurable.
But I think this code is supposed to be kept in sync with an
out-of-tree upstream version so this change might be problematic.
Dunno, let's see what Nick thinks.
next prev parent reply other threads:[~2021-08-17 1:05 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-08-15 0:41 [PATCH] lib/zstd: Fix bitwise vs logical operators Nathan Chancellor
2021-08-16 15:14 ` David Sterba
2021-08-16 16:53 ` Nathan Chancellor
2021-08-16 21:17 ` David Sterba
2021-08-17 1:05 ` Andrew Morton [this message]
2021-08-19 21:22 ` David Laight
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