linux-kernel - Re: [PATCH] Drop -Wdeclaration-after-statement

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Date:   Tue, 12 Mar 2019 20:24:47 +0300
From:   Alexey Dobriyan <adobriyan@...il.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Drop -Wdeclaration-after-statement

On Mon, Mar 11, 2019 at 05:38:45PM -0700, Andrew Morton wrote:
> On Sun, 10 Mar 2019 16:35:35 +0300 Alexey Dobriyan <adobriyan@...il.com> wrote:
> 
> > Newly added static_assert() is formally a declaration, which will give
> > a warning if used in the middle of the function.
> > 
> > ...
> >
> > --- a/Makefile
> > +++ b/Makefile
> > @@ -792,9 +792,6 @@ endif
> >  # arch Makefile may override CC so keep this after arch Makefile is included
> >  NOSTDINC_FLAGS += -nostdinc -isystem $(shell $(CC) -print-file-name=include)
> >  
> > -# warn about C99 declaration after statement
> > -KBUILD_CFLAGS += -Wdeclaration-after-statement
> > -
> >  # Variable Length Arrays (VLAs) should not be used anywhere in the kernel
> >  KBUILD_CFLAGS += $(call cc-option,-Wvla)
> 
> I do wish your changelogs were more elaborate :(

> So the proposal is to disable -Wdeclaration-after-statement in all
> cases for all time because static_assert() doesn't work correctly?

Yes. I converted 2 cases in /proc to static_assert() and you can't write

	{
		[code]
		static_assert()
	}

without a warning because static_assert() is declaration.
So people would move BUILD_BUG_ON() to where it doesn't belong.

> Surely there's something we can do to squish the static_assert() issue
> while retaining -Wdeclaration-after-statement?

It is not good in my opinion to stick to -Wdeclaration-after-statement.

> Perhaps by making
> static_assert() a nop if -Wdeclaration-after-statement is in use. 
> Perhaps simply by putting { } around the static_assert()?

Making a statement out of it would disable current cases where it is
placed in headers.