[PATCH] drm/bridge/tc358767: make the array ext_div static const, makes object smaller
Colin Ian King
colin.king at canonical.com
Thu Aug 19 14:51:00 UTC 2021
On 19/08/2021 15:40, Joe Perches wrote:
> 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 at canonical.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.
I'm using allmodconfig, which I believe is a legitimate configuration,
especially since distros so build kernels with lots of modules.
I'll double check on this though in case I've made a mistake.
>
> 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
ACK. Understood. Even so, it still makes sense for these kind of
janitorial changes as it makes sense to constify arrays when they are
read-only and making them static is sensible for const data.
>
> cheers, Joe
>
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