From: David Laight <David.Laight@ACULAB.COM>
To: 'Andrew Morton' <akpm@linux-foundation.org>,
Nathan Chancellor <nathan@kernel.org>
Cc: Nick Desaulniers <ndesaulniers@google.com>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
"clang-built-linux@googlegroups.com"
<clang-built-linux@googlegroups.com>,
"terrelln@fb.com" <terrelln@fb.com>
Subject: RE: [PATCH] lib/zstd: Fix bitwise vs logical operators
Date: Thu, 19 Aug 2021 21:22:54 +0000 [thread overview]
Message-ID: <defb9e5133234835950c21511d776fb9@AcuMS.aculab.com> (raw)
In-Reply-To: <20210816180532.6e85c93f84d62dc720d21178@linux-foundation.org>
From: Andrew Morton
> Sent: 17 August 2021 02:06
>
> 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.
Except that in this case you want the short-circuit.
The RH condition is slow and always true when offset_1 is zero.
OTOH subtracting offset_1 makes the lines look odd (out of context).
David
-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)
prev parent reply other threads:[~2021-08-19 21:23 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
2021-08-19 21:22 ` David Laight [this message]
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