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@xxxxxxxxxxxxx> >>>> >>>> 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 >
