linux-kernel - Re: detecting integer constant expressions in macros

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Message-ID: <1521587247.32102.1.camel@med.uni-goettingen.de>
Date:   Tue, 20 Mar 2018 23:07:28 +0000
From:   "Uecker, Martin" <Martin.Uecker@....uni-goettingen.de>
To:     "torvalds@...ux-foundation.org" <torvalds@...ux-foundation.org>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: detecting integer constant expressions in macros



talking of crazy ideas, here is another way to preserve
integer const expressions in macros by storing it a
VLA type (only for positive integers I guess):


#define MAX(a, b) sizeof(*({    \
        typedef char _Ta[a];    \
        typedef char _Tb[b];    \
        (char(*)[sizeof(_Ta) > sizeof(_Tb) ? sizeof(_Ta) :
sizeof(_Tb)])0; }))

Am Dienstag, den 20.03.2018, 23:13 +0100 schrieb Martin Uecker:
> Hi Linus,
> 
> here is an idea:
> 
> a test for integer constant expressions which returns an
> integer constant expression itself which should be suitable
> for passing to __builtin_choose_expr might be:
> 
> #define ICE_P(x) (sizeof(int) == sizeof(*(1 ? ((void*)((x) * 0l)) :
> (int*)1)))
> 
> This also does not evaluate x itself on gcc although this is
> not guaranteed by the standard. (And I haven't tried any older
> gcc.)
> 
> Best,
> Martin