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

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Message-ID: <2c664a6701b44050a0525b541292ce21@AcuMS.aculab.com>
Date:   Tue, 26 Oct 2021 21:04:55 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Nathan Chancellor' <nathan@...nel.org>
CC:     'Nick Terrell' <terrelln@...com>,
        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

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.

	David

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