Messages in this thread | | | Date | Mon, 16 Aug 2021 18:05:32 -0700 | From | Andrew Morton <> | Subject | Re: [PATCH] lib/zstd: Fix bitwise vs logical operators |
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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.
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