[U-Boot] Making U-Boot smaller

[Tom Rini <trini@konsulko.com>]

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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