linux-kernel - Re: [PATCH: 1/1] sh4: avoid spurious gcc warning

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Date:   Sun, 22 Jan 2023 12:33:41 +0100
From:   Michael Karcher <kernel@...rcher.dialup.fu-berlin.de>
To:     Randy Dunlap <rdunlap@...radead.org>, linux-kernel@...r.kernel.org,
        linux-sh@...r.kernel.org,
        Segher Boessenkool <segher@...nel.crashing.org>,
        Rich Felker <dalias@...c.org>,
        Yoshinori Sato <ysato@...rs.sourceforge.jp>,
        John Paul Adrian Glaubitz <glaubitz@...sik.fu-berlin.de>
Cc:     jakub@....gnu.org
Subject: Re: [PATCH: 1/1] sh4: avoid spurious gcc warning

Am 22.01.2023 um 08:00 schrieb Randy Dunlap:
>> -#define _INTC_ARRAY(a) a, __same_type(a, NULL) ? 0 : sizeof(a)/sizeof(*a)
>> +#define _INTC_ARRAY(a) a, sizeof(a)/(_Generic((a), typeof(NULL): 0xFFFFFFFFU, default: sizeof(*a)))
> s/: / : / in 2 places.
>
> Tested-by: Randy Dunlap <rdunlap@...radead.org> # build-tested

Thanks for your confirmation! Are you sure about the space before the 
colon? The colon in this case terminates a case descriptor for the 
type-level switch construction using "_Generic". It says: "In case 'a' 
has the 'type of NULL', divide by 0xFFFFFFFFU, in all other cases, 
divide by the size of a single array element". It's not a colon of the 
ternary ?: operator, in which case I would agree with the space before it.

If you confirm that you want a space before the colon in this case as 
well, I'm going to add it, though.

> How far back in gcc versions does this work?

I tested the support of _Generic on Compiler Explorer at godbolt.org. 
This construction is rejected by gcc 4.8, but accepted by gcc 4.9.

Kind regards,
   Michael Karcher