linux-kernel - Re: [PATCH] lib: zstd: Add cast to silence clang's -Wbitwise-instead-of-logical

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Message-ID: <F7676163-1EEA-4A7E-A0F7-475411ECD7A8@fb.com>
Date:   Fri, 5 Nov 2021 21:16:07 +0000
From:   Nick Terrell <terrelln@...com>
To:     David Laight <David.Laight@...LAB.COM>
CC:     Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "llvm@...ts.linux.dev" <llvm@...ts.linux.dev>
Subject: Re: [PATCH] lib: zstd: Add cast to silence clang's
 -Wbitwise-instead-of-logical



> On Oct 26, 2021, at 2:04 PM, David Laight <David.Laight@...LAB.COM> wrote:
> 
> From: Nathan Chancellor
>> Sent: 26 October 2021 15:03
> ...
>>> Isn't enabling that warning completely stupid?
>>> The casts required to silence it could easily cause more problems
>>> - by hiding more important bugs. And seriously affect code readability.
>> 
>> Which warning?
>> 
>> -Wbitwise-instead-of-logical is included in clang's -Wall and I do not
>> think it should be disabled; this is the first instance of the warning
>> that has been silenced with a cast.
> 
> I'm not sure about that one.
> I have a feeling it will generate false positives for carefully optimised
> code more often that it finds anything where 'short circuiting' will
> be a real gain.
> Especially for values with are known to be either 0 or 1.
> 
>> -Wshorten-64-to-32 will never be enabled for Linux but zstd is a
>> separate project that can be built for a variety of operating systems so
>> that has to be considered when developing changes for the kernel because
>> the kernel changes need to go upstream eventually if they touch core
>> zstd code, otherwise they will just get blown away on the next import.
>> Specifically, this warning was enabled on iOS:
>> https://github.com/facebook/zstd/pull/2062
> 
> That one...
> If you are going to enable it, then you need a static inline function
> to convert u64 to u32, not a C cast.
> 
> I'm sure that it won't be long before the compiler writes start an
> 'open season' on casts.
> They really are more dangerous than the warnings they are trying to remove.
> 
>>> ...c
>>>>> index 05570ed5f8be..5105e59ac04a 100644
>>>>> --- a/lib/zstd/decompress/huf_decompress.c
>>>>> +++ b/lib/zstd/decompress/huf_decompress.c
>>>>> @@ -886,7 +886,7 @@ HUF_decompress4X2_usingDTable_internal_body(
>>>>>            HUF_DECODE_SYMBOLX2_0(op2, &bitD2);
>>>>>            HUF_DECODE_SYMBOLX2_0(op3, &bitD3);
>>>>>            HUF_DECODE_SYMBOLX2_0(op4, &bitD4);
>>>>> -            endSignal = (U32)LIKELY(
>>>>> +            endSignal = (U32)LIKELY((U32)
>>>>>                        (BIT_reloadDStreamFast(&bitD1) == BIT_DStream_unfinished)
>>>>>                      & (BIT_reloadDStreamFast(&bitD2) == BIT_DStream_unfinished)
>>>>>                      & (BIT_reloadDStreamFast(&bitD3) == BIT_DStream_unfinished)
>>> 
>>> Isn't that the same as:
>>> 	((BIT_reload() & BIT_reload() & BIT_reload()) == BIT_DStream_unfinished)
>>> which will generate much better code.
>>> Especially on cpu without 'seteq' instructions.
>> 
>> I don't think so. Feel free to double check my math.
>> 
>> BIT_reloadDStreamFast() can return either BIT_DStream_unfinished (0) or
>> BIT_DStream_overflow (3)....
> 
> Ah, I'd assumed that BIT_DStream_unfinished was non-zero.
> So you actually want:
> 	endSignal = !(BIT() | BIT() | BIT());
> 
> Just kill the CaMeLs and unnecessary constants.
> Then the code becomes succint, easier to read/check etc.

`BIT_reloadDStreamFast()` has a likely branch which returns `BIT_DStream_unfinished`.
This construction is telling the compiler that it is allowed to re-order each call and collect
the results. I don’t expect that it will translate directly to a set of and instructions, though
I’d have to double check the assembly to be sure.

If you feel the code could be clearer, you’re welcome to submit a PR upstream! However,
since is a hot loop, we generally favor performance over clarity to some extent, so it will
have to be a perf neutral refactoring.

Best,
Nick Terrell

> 	David
> 
> -
> Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
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