linux-kernel - Re: [PATCH] drm/bridge/tc358767: make the array ext

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Date:   Thu, 19 Aug 2021 07:40:41 -0700
From:   Joe Perches <joe@...ches.com>
To:     Colin Ian King <colin.king@...onical.com>,
        Andrzej Hajda <a.hajda@...sung.com>,
        Neil Armstrong <narmstrong@...libre.com>,
        Robert Foss <robert.foss@...aro.org>,
        Laurent Pinchart <Laurent.pinchart@...asonboard.com>,
        Jonas Karlman <jonas@...boo.se>,
        Jernej Skrabec <jernej.skrabec@...il.com>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        dri-devel@...ts.freedesktop.org
Cc:     kernel-janitors@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] drm/bridge/tc358767: make the array ext_div static
 const, makes object smaller

On Thu, 2021-08-19 at 14:54 +0100, Colin Ian King wrote:
> On 19/08/2021 14:51, Joe Perches wrote:
> > On Thu, 2021-08-19 at 14:38 +0100, Colin King wrote:
> > > From: Colin Ian King <colin.king@...onical.com>
> > > 
> > > Don't populate the array ext_div on the stack but instead it
> > > static const. Makes the object code smaller by 118 bytes:
> > > 
> > > Before:
> > >    text    data    bss     dec    hex filename
> > >   39449   17500    128   57077   def5 ./drivers/gpu/drm/bridge/tc358767.o
> > > 
> > > After:
> > >    text    data    bss     dec    hex filename
> > >   39235   17596    128   56959   de7f ./drivers/gpu/drm/bridge/tc358767.o
> > 
> > Why is text smaller and data larger with this change?
> 
> There are less instructions being used with the change since it's not
> shoving the array data onto the stack at run time. Instead the array is
> being stored in the data section and there is less object code required
> to access the data.

Ah.  It's really because it's not a minimal compilation ala defconfig.

I think you should really stop making these size comparisons with
.config uses that are not based on a defconfig as a whole lot of other
things are going on.

Please notice that the object sizes are significantly smaller below:

So with an x86-64 defconfig and this compilation unit enabled with
CONFIG_OF enabled and CONFIG_DRM_TOSHIBA_TC358767=y, with gcc 10.3
and this change the object size actually increases a bit.

$ size drivers/gpu/drm/bridge/tc358767.o*
   text	   data	    bss	    dec	    hex	filename
  13554	    268	      1	  13823	   35ff	drivers/gpu/drm/bridge/tc358767.o.new
  13548	    268	      1	  13817	   35f9	drivers/gpu/drm/bridge/tc358767.o.old

objdump -h shows these differences:

.old:
  0 .text         00001e1f  0000000000000000  0000000000000000  00000040  2**4
                  CONTENTS, ALLOC, LOAD, RELOC, READONLY, CODE
[...]
 14 .rodata       000005ae  0000000000000000  0000000000000000  000046e0  2**5
                  CONTENTS, ALLOC, LOAD, RELOC, READONLY, DATA

.new:
  0 .text         00001e05  0000000000000000  0000000000000000  00000040  2**4
                  CONTENTS, ALLOC, LOAD, RELOC, READONLY, CODE
[...]
 11 .rodata       000005ce  0000000000000000  0000000000000000  00004600  2**5
                  CONTENTS, ALLOC, LOAD, RELOC, READONLY, DATA

cheers, Joe