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

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Date:   Tue, 12 Mar 2019 12:50:17 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Alexey Dobriyan <adobriyan@...il.com>
Cc:     linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Drop -Wdeclaration-after-statement

On Tue, 12 Mar 2019 20:24:47 +0300 Alexey Dobriyan <adobriyan@...il.com> wrote:

> 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.

Sure.

> > 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.

Why?

> > 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.

I think you mean cases where static_assert() is used outside functions?

We could have two versions of it, one for use inside functions, one for
use outside functions?