linux-kernel - RE: [PATCH 3/7] compiler.h: Add __if_constexpr(expr, if_const, if_not

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Message-ID: <3d6217d937994620a5de0d8967f518c0@AcuMS.aculab.com>
Date: Thu, 25 Jul 2024 09:12:57 +0000
From: David Laight <David.Laight@...LAB.COM>
To: 'Arnd Bergmann' <arnd@...nel.org>, "'linux-kernel@...r.kernel.org'"
	<linux-kernel@...r.kernel.org>, Linus Torvalds <torvalds@...uxfoundation.org>
CC: Matthew Wilcox <willy@...radead.org>, Christoph Hellwig
	<hch@...radead.org>, Andrew Morton <akpm@...ux-foundation.org>, "Andy
 Shevchenko" <andriy.shevchenko@...ux.intel.com>, Dan Carpenter
	<dan.carpenter@...aro.org>, "Jason A . Donenfeld" <Jason@...c4.com>,
	"'pedro.falcato@...il.com'" <pedro.falcato@...il.com>, Mateusz Guzik
	<mjguzik@...il.com>, "'linux-mm@...ck.org'" <linux-mm@...ck.org>
Subject: RE: [PATCH 3/7] compiler.h: Add __if_constexpr(expr, if_const,
 if_not_const)

From: Arnd Bergmann
> Sent: 24 July 2024 18:32
> 
> On Wed, Jul 24, 2024, at 16:29, David Laight wrote:
> 
> > +#define __if_constexpr(expr, if_const, if_not_const)		\
> > +	_Generic(0 ? ((void *)((long)(expr) * 0l)) : (char *)0,	\
> > +		char *: (if_const),				\
> > +		void *: (if_not_const))
> > +
> > -#define __is_constexpr(x) \
> > -	(sizeof(int) == sizeof(*(8 ? ((void *)((long)(x) * 0l)) : (int *)8)))
> > +#define __is_constexpr(expr) __if_constexpr((expr), 1, 0)
> 
> I don't immediately see anything wrong with this, but I'm
> still scared of any change to it, especially if this is
> meant to go straight into mainline.

Well it would be -rc1 (or maybe -rc2).

> Would it be possible to do patch 4/7 without the new
> __if_constexpr() and instead still using __builtin_choose_expr()?

The safer option would be to add __if_constexpr() but leave the
change to __is_constexpr() for 'next'.

	David

> 
>      Arnd

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