linux-kernel - Re: [PATCH 2/2] zstd: Backport Huffman speed improvement from upstream

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Date:   Tue, 21 Nov 2023 12:54:57 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Nick Terrell <terrelln@...a.com>
Cc:     Nick Terrell <nickrterrell@...il.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Yann Collet <cyan@...a.com>,
        Kernel Team <kernel-team@...a.com>,
        Giovanni Cabiddu <giovanni.cabiddu@...el.com>
Subject: Re: [PATCH 2/2] zstd: Backport Huffman speed improvement from upstream

On Tue, 21 Nov 2023 at 12:35, Nick Terrell <terrelln@...a.com> wrote:
> >
> > Honestly, any coding rule that includes "don't use the do-while-zero
> > construct" is actively broken shit.
> >
> > Please just fix your upstream rules. Because they are incredible garbage.
>
> Yeah, that’s the plan. Visual Studios fixed that compiler bug in VS2015 [0],
> so we should be safe to migrate to safer macros.

I don't even use MSVS, but a minute of googling shows that you should
never have done that silly "avoid sane C", and you should always just
have done

  #pragma warning (disable: 4127)

for MSVC.

Honestly, the fact that the result was instead to disable that
standard - and required - construct in the project makes me worry
about the whole zstd thing. WTF?

The do-while-zero construct is _so_ important that there are (sane)
projects that literally *require* the use of it. See for example MISRA
code safety rules.

The kernel rules aren't quite that strict, but yes, do-while-zero is
very much "you should *absolutely* do this" along with all the usual
"make sure you have parentheses around macro arguments" rules.

We had some RFC patches for this area:

   https://lore.kernel.org/all/20230511152951.1970870-1-mathieu.desnoyers@efficios.com/

And on that note, when I googled for the solution to the MSVC brain
damage, I was distressed by how many hits I saw where people thought
the do-while-zero pattern was some "legacy pattern".

That just shows that there are lots of incompetent people simply do
not understand why it's actually *required* for reliable parsing of
macros.  This is not some "historical stylistic" issue, it's literally
a correctness issue for generic macro usage.

           Linus