linux-kernel - Re: [PATCH] checkpatch: prefer = {} initializations to = {0}

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Message-ID: <YRfm2RRYla7Nemsj@unreal>
Date:   Sat, 14 Aug 2021 18:52:57 +0300
From:   Leon Romanovsky <leon@...nel.org>
To:     Al Viro <viro@...iv.linux.org.uk>
Cc:     Christophe JAILLET <christophe.jaillet@...adoo.fr>,
        Dan Carpenter <dan.carpenter@...cle.com>,
        Russell King - ARM Linux admin <linux@...linux.org.uk>,
        Joe Perches <joe@...ches.com>,
        Dwaipayan Ray <dwaipayanray1@...il.com>,
        Andy Whitcroft <apw@...onical.com>,
        Lukas Bulwahn <lukas.bulwahn@...il.com>,
        linux-kernel@...r.kernel.org, kernel-janitors@...r.kernel.org,
        Julia Lawall <julia.lawall@...ia.fr>
Subject: Re: [PATCH] checkpatch: prefer = {} initializations to = {0}

On Sat, Aug 14, 2021 at 02:57:06PM +0000, Al Viro wrote:
> On Sat, Aug 14, 2021 at 05:38:27PM +0300, Leon Romanovsky wrote:
> 
> > There are number of reasons why you didn't notice any difference.
> > 1. {} is GCC extension
> > 2. {} was adopted in latest C standards, so need to check which one GCC 10
> > is using by default.
> > 3. Main difference will be in padding - {0} will set to zero fields but
> > won't touch padding, while {} will zero everything.
> 
> References on (3), please?

I reread gcc/c/c-typeck.c and at lest for GCC 10, I'm wrong about padding.
Sorry about that.

   8630 struct c_expr
   8631 pop_init_level (location_t loc, int implicit,
   8632                 struct obstack *braced_init_obstack,
   8633                 location_t insert_before)
....
   8692   switch (vec_safe_length (constructor_elements))
   8693     {
   8694     case 0:
   8695       /* Initialization with { } counts as zeroinit.  */
   8696       constructor_zeroinit = 1;
   8697       break;
   8698     case 1:
   8699       /* This might be zeroinit as well.  */
   8700       if (integer_zerop ((*constructor_elements)[0].value))
   8701         constructor_zeroinit = 1;
   8702       break;
   8703     default:
   8704       /* If the constructor has more than one element, it can't be { 0 }.  */
   8705       constructor_zeroinit = 0;
   8706       break;
   8707     }
   8708