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From: David Laight <David.Laight@ACULAB.COM>
To: 'Kees Cook' <keescook@chromium.org>, Rasmus Villemoes
	<linux@rasmusvillemoes.dk>
CC: "Gustavo A . R . Silva" <gustavoars@kernel.org>, Miguel Ojeda
	<miguel.ojeda.sandonis@gmail.com>, Jani Nikula <jani.nikula@linux.intel.com>,
	Nick Desaulniers <ndesaulniers@google.com>, Martin Uecker
	<Martin.Uecker@med.uni-goettingen.de>, Jonathan Corbet <corbet@lwn.net>,
	"linux-doc@vger.kernel.org" <linux-doc@vger.kernel.org>, Miguel Ojeda
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Subject: RE: [PATCH] compiler.h: Explain how __is_constexpr() works
Thread-Topic: [PATCH] compiler.h: Explain how __is_constexpr() works
Thread-Index: AQHaa5MvghvgXFQBskOAdfSAj6aDybEilNag
Date: Fri, 1 Mar 2024 09:32:16 +0000
Message-ID: <af0eff12e6bc41039614add550406c11@AcuMS.aculab.com>
References: <20240301044428.work.411-kees@kernel.org>
In-Reply-To: <20240301044428.work.411-kees@kernel.org>
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From: Kees Cook
> Sent: 01 March 2024 04:45
> To: Rasmus Villemoes <linux@rasmusvillemoes.dk>
>=20
> The __is_constexpr() macro is dark magic. Shed some light on it with
> a comment to explain how and why it works.

All the 8s don't help...

I don't think you need that much explanation.

Perhaps just saying that the type of ?: depends on the types
of the values and is independent of the condition.
The type of (0 ? (void *)p : (foo *)q) is normally 'void *'
(so that both values can be assigned to it).
But if 'p' is 'an integer constant expression with value 0'
then (void *)p is NULL and the type is 'foo *'.

The type can then be checked to find out it 'p' is constant 0.
A non-zero constant 'p' can be multiples by 0.

I need to replace the definition with (the more portable):
#define __if_constexpr(cond, if_const, if_not_const) \
=09_Generic(0 ? (void *)((long)(cond) * 0) : (char *)0, \
=09=09char *: (if_const), \
=09=09void *: (if_not_const))
which is arguably less cryptic.

#define __is_constexpr(cond) __if_constexpr(cond, 1, 0)

So that I can write:
#define is_non_neg_const(x) (__if_constexpr(x, x , -1) >=3D 0)
and avoid the compiler bleating about some comparisons
in unreachable code.

=09David

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