[U-Boot] [RFC PATCH] ppc4xx: Use gc-sections to reduce image size

[Detlev Zundel]

Hi Stefan,

> This patch enables gc-sections for PPC4xx. This is done to
> generate smaller U-Boot images. For this the linker script
> also needs some tweaking:
>
> - For example change *(text) to *(text*)
> - Add KEEP to some of the symbols, especially "resetvec"
>
> This patch is tested on the following boards and reduces
> the image size by the these values:
>
> Canyonlands: 11,642
> Katmai:       9,170
> Kilauea:     10,742
> Sequoia:     11,398

Interesting.

Just for whoever is interested - I wondered how to find out what is
actually unused to find pointers to unused crufty code.

Actually this is pretty easy by adding "--print-gc-sections" to the
PLATFORM_LDFLAGS:

-----8<-----
diff --git a/arch/powerpc/cpu/ppc4xx/config.mk b/arch/powerpc/cpu/ppc4xx/config.mk
index b0d346a..f1e15c9 100644
--- a/arch/powerpc/cpu/ppc4xx/config.mk
+++ b/arch/powerpc/cpu/ppc4xx/config.mk
@@ -36,7 +36,7 @@ endif
 # Enable gc-sections to enable generation of smaller images.
 # Please note that the linker scripts may need some tweaking with this
 # change as well.
-PLATFORM_LDFLAGS += --gc-sections
+PLATFORM_LDFLAGS += --gc-sections --print-gc-sections
 PLATFORM_RELFLAGS += -ffunction-sections -fdata-sections
 
 # Use default linker script.  Board port can override in board/*/config.mk
-----8<-----

Using this, the linker prints lots of message slike this:

ppc_4xxFP-ld: Removing unused section '.data' in file 'arch/powerpc/cpu/ppc4xx/start.o'
ppc_4xxFP-ld: Removing unused section '.bss' in file 'arch/powerpc/cpu/ppc4xx/start.o'
ppc_4xxFP-ld: Removing unused section '.text' in file 'board/amcc/sequoia/init.o'
ppc_4xxFP-ld: Removing unused section '.data' in file 'board/amcc/sequoia/init.o'
ppc_4xxFP-ld: Removing unused section '.bss' in file 'board/amcc/sequoia/init.o'
ppc_4xxFP-ld: Removing unused section '.text' in file 'arch/powerpc/cpu/ppc4xx/resetvec.o'
ppc_4xxFP-ld: Removing unused section '.data' in file 'arch/powerpc/cpu/ppc4xx/resetvec.o'
ppc_4xxFP-ld: Removing unused section '.bss' in file 'arch/powerpc/cpu/ppc4xx/resetvec.o'

I believe they can be gladly ignored, because the "generic" segments are
now empty thanks to the "-ffunction-sections -fdata-sections"
directives.  What is really interesting are messages like this:

ppc_4xxFP-ld: Removing unused section '.text.gpio_read_out_bit' in file 'arch/powerpc/cpu/ppc4xx/libppc4xx.a(gpio.o)'
ppc_4xxFP-ld: Removing unused section '.text.gpio_read_in_bit' in file 'arch/powerpc/cpu/ppc4xx/libppc4xx.a(gpio.o)'
ppc_4xxFP-ld: Removing unused section '.text.gpio_config' in file 'arch/powerpc/cpu/ppc4xx/libppc4xx.a(gpio.o)'
ppc_4xxFP-ld: Removing unused section '.text.gpio_write_bit' in file 'arch/powerpc/cpu/ppc4xx/libppc4xx.a(gpio.o)'

They essentially show the functions that are not used in this link run.

Cheers
  Detlev

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