linux-kernel - Re: [PATCH 1/2] sh: align .bss section padding to 8-byte boundary

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Message-ID: <20250313093838.10749599@pumpkin>
Date: Thu, 13 Mar 2025 09:38:38 +0000
From: David Laight <david.laight.linux@...il.com>
To: Geert Uytterhoeven <geert@...ux-m68k.org>
Cc: Uros Bizjak <ubizjak@...il.com>, John Paul Adrian Glaubitz
 <glaubitz@...sik.fu-berlin.de>, Artur Rojek <contact@...ur-rojek.eu>,
 Yoshinori Sato <ysato@...rs.sourceforge.jp>, Rich Felker <dalias@...c.org>,
 Daniel Lezcano <daniel.lezcano@...aro.org>, Thomas Gleixner
 <tglx@...utronix.de>, Geert Uytterhoeven <geert+renesas@...der.be>, "D .
 Jeff Dionne" <jeff@...esemi.io>, Rob Landley <rob@...dley.net>,
 linux-sh@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] sh: align .bss section padding to 8-byte boundary

On Wed, 12 Mar 2025 10:47:00 +0100
Geert Uytterhoeven <geert@...ux-m68k.org> wrote:

> Hi Uros,
> 
> On Wed, 12 Mar 2025 at 09:32, Uros Bizjak <ubizjak@...il.com> wrote:
> > On Wed, Mar 12, 2025 at 9:22 AM John Paul Adrian Glaubitz
> > <glaubitz@...sik.fu-berlin.de> wrote:  
....
> > > OK. FWIW, do you understand what SBSS is for? I couldn't find any explanation
> > > for it.  
> >
> > Small BSS section. The compiler can put data objects under a certain
> > size threshold to the .sbss section. Looking at GCC sh config, sh does
> > not use this section.  

The .sbss (and .sdata) sections are used by some architectures (eg Nios2)
for data that can be accessed using a 16bit offset from a fixed register.
(Although using the register as global register variable generates better code.)
Historically they may have been used for data at the top/bottom of the
addresses space (for the same reason).

There is no reason to just use it for 'small' (eg 8 bytes or less)
data, that is just some empirical default.

I guess they could also be used on cpu like the strongarm for memory
that used the 'small data cache' (useful for screen buffer memory!).

	David