linux-kernel - Re: [PATCH] lib/zstd: Fix bitwise vs logical operators

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Date:   Mon, 16 Aug 2021 18:05:32 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Nathan Chancellor <nathan@...nel.org>
Cc:     Nick Desaulniers <ndesaulniers@...gle.com>,
        linux-kernel@...r.kernel.org, clang-built-linux@...glegroups.com,
        terrelln@...com
Subject: Re: [PATCH] lib/zstd: Fix bitwise vs logical operators

On Sat, 14 Aug 2021 17:41:54 -0700 Nathan Chancellor <nathan@...nel.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.