[U-Boot] Making U-Boot smaller
Tom Rini
trini at konsulko.com
Wed May 22 18:50:58 UTC 2019
On Wed, May 22, 2019 at 06:50:36PM +0200, Eugeniu Rosca wrote:
> Hi Tom,
>
> On Wed, May 22, 2019 at 11:09:54AM -0400, Tom Rini wrote:
> > On Wed, May 22, 2019 at 04:15:47PM +0200, Eugeniu Rosca wrote:
> > > cc: Yamada-san
> > >
> > > I dream of a (Kconfig/Kbuild-assisted) bloaty-like output [1] which
> > > would point out the culprit configs. This is hardly achievable, but
> > > looks good on the paper!
> > >
> > > CONFIG FILE SIZE
> > > ------------ --------------
> > > CONFIG_FEATURE_A 10.7Mi 37.1%
> > > CONFIG_FEATURE_B 5.39Mi 18.6%
> > > CONFIG_FEATURE_C 4.48Mi 15.5%
> > > CONFIG_FEATURE_D 1.86Mi 6.4%
> > > CONFIG_FEATURE_E 1.67Mi 5.8%
> > > CONFIG_FEATURE_F 1.61Mi 5.6%
> > > CONFIG_FEATURE_G 856Ki 2.9%
> > > CONFIG_FEATURE_H 470Ki 1.6%
> > > ....
> > > TOTAL 28.9Mi 100.0%
> > >
> > > [1] https://github.com/google/bloaty
> >
> > This is relatively easy to do today, with buildman and a local commit to
> > enable/disable CONFIG_FEATURE_A.
>
> Being a valid choice doesn't make it necessarily appealing, especially
> with 512 [1] configurations enabled in sandbox and knowing that U-Boot
> is not really randconfig-grade [2] (the latter is being improved).
>
> What I was alluding to is embedding the Kconfig symbol names into the
> ELF objects, such that utilities like 'nm' (currently producing a nice
> output of symbol sizes [3]) can also be made capable to report the exact
> Kconfig symbols contributing to the size of the objects passed as input.
> That would be, in my opinion, mind-blowingly useful.
>
> [1] grep "CONFIG.*=" .config | wc -l
> 512
>
> [2] https://patchwork.ozlabs.org/patch/1074420/
>
> [3] nm --print-size --size-sort --radix=d ./drivers/clk/clk-uclass.o
> 0000000000000421 0000000000000024 T clk_free
> 0000000000000961 0000000000000027 T clk_disable
> 0000000000000888 0000000000000027 T clk_enable
> 0000000000000000 0000000000000027 T clk_request
> 0000000000000503 0000000000000027 T clk_set_parent
> 0000000000000445 0000000000000029 T clk_get_rate
> 0000000000000474 0000000000000029 T clk_set_rate
Right. More numbers, and more easily readable is good. But to be
clear, since we use -ffunction-sections / -fdata-sections (and the
kernel doesn't normally), we get can get a lot more detail than I might
have implied. It's not just that you'll see that U-Boot shrank X bytes
with CONFIG_FEATURE_A disabled, it's that you'll see which functions
shrank by how much. What we don't have is the link between
"CONFIG_OPTION_X" and "is part of functions A/B/C". But we have a lot
of information like you would get out of nm already in u-boot*.map which
also includes "and we discarded these functions".
--
Tom
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