linux-kernel - Re: [RFC PATCH v1 5/6] tools/nolibc/stdlib: Implement `malloc()`, `calloc()`, `realloc()` and `free()`

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Date:   Sun, 20 Mar 2022 17:46:53 +0100
From:   Willy Tarreau <w@....eu>
To:     Ammar Faizi <ammarfaizi2@...weeb.org>
Cc:     "Paul E. McKenney" <paulmck@...nel.org>,
        Alviro Iskandar Setiawan <alviro.iskandar@...weeb.org>,
        Nugraha <richiisei@...il.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        GNU/Weeb Mailing List <gwml@...r.gnuweeb.org>
Subject: Re: [RFC PATCH v1 5/6] tools/nolibc/stdlib: Implement `malloc()`,
 `calloc()`, `realloc()` and `free()`

On Sun, Mar 20, 2022 at 11:36:55PM +0700, Ammar Faizi wrote:
> And this is what GCC doc says about __attribute__((__aligned__)):
> """
>   The aligned attribute specifies a minimum alignment for the variable
>   or structure field, measured in bytes. When specified, alignment must
>   be an integer constant power of 2. Specifying no alignment argument
>   implies the maximum alignment for the target, which is often, but by
>   no means always, 8 or 16 bytes.
> """
> 
> Link: https://gcc.gnu.org/onlinedocs/gcc-11.2.0/gcc/Common-Variable-Attributes.html#Common-Variable-Attributes

OK then that's fine, thank you. I thought it would force the alignment
to the type itself.

> Simple experiment on Linux x86-64...

That's even easier checked like this:

  $ cat > c.c <<EOF
  struct blah {
          char a;
          char b __attribute__((__aligned__));
  } blah;
  EOF
  $ gcc -g -c c.c
  $ pahole c.o
  struct blah {
          char                       a;                    /*     0     1 */
  
          /* XXX 15 bytes hole, try to pack */
  
          char                       b __attribute__((__aligned__(16))); /*    16     1 */
  
          /* size: 32, cachelines: 1, members: 2 */
          /* sum members: 2, holes: 1, sum holes: 15 */
          /* padding: 15 */
          /* forced alignments: 1, forced holes: 1, sum forced holes: 15 */
          /* last cacheline: 32 bytes */
  } __attribute__((__aligned__(16)));

Thank you!
Willy