[PATCH 2/5] kbuild: allow archs to select build for link dead code/data elimination

[Nicholas Piggin]

On Mon, 8 Aug 2016 00:12:37 -0400 (EDT)
Nicolas Pitre <nicolas.pitre at linaro.org> wrote:

> On Mon, 8 Aug 2016, Nicholas Piggin wrote:
> 
> > On Sun, 7 Aug 2016 01:33:45 -0400 (EDT)
> > Nicolas Pitre <nicolas.pitre at linaro.org> wrote:
> >   
> > > On Fri, 5 Aug 2016, Nicholas Piggin wrote:
> > >   
> > > > Introduce LINKER_DCE option for architectures to select if they want
> > > > to build with -ffunction-sections, -fdata-sections, and link with
> > > > --gc-sections. It requires some work (documented) to ensure all
> > > > unreferenced entrypoints are live, and requires toolchain and
> > > > build verification, so it is made a per-arch option for now.
> > > > 
> > > > On a random powerpc64le build, this yelds a significant size saving,
> > > > it boots and runs fine, but there is a lot I haven't tested as yet,
> > > > so these savings may be reduced if there are bugs in the link.
> > > > 
> > > >     text      data        bss        dec   filename
> > > > 11169741   1180744    1923176	14273661   vmlinux
> > > > 10445269   1004127    1919707	13369103   vmlinux.dce
> > > > 
> > > > ~700K text, ~170K data, 6% removed from kernel image size.
> > > > 
> > > > Signed-off-by: Nicholas Piggin <npiggin at gmail.com>  
> > > 
> > > I played with that too. However this needs distinct sections for 
> > > exception tables and the like otherwise the backward references from the 
> > > final exception table to those functions responsible for those exception 
> > > entries has the effect of pulling in all those functions even if their 
> > > entry point is never referenced, making --gc-sections less effective.  
> > > I managed to fix this only with a change to gas (accepted upstream).
> > > 
> > > But once that is solved, you then have the missing forward reference 
> > > problem i.e. nothing actually references those individual exception 
> > > entry sections and ld happily drops them all. Having a KEEP() on each of 
> > > them is unworkable and defeats the purpose anyway.  That requires a 
> > > dummy reloc to trick ld into pulling in those sections when the parent 
> > > section is also pulled in.  
> > 
> > Right, although we don't *need* those things just for enabling
> > --gc-sections, do we? It may not be 100% optimal, but it's enough
> > to avoid the regression when switching to --whole-archive build
> > option.  
> 
> Oh absolutely.
> 
> > Your results are impressive, and I don't want to stand in the way of
> > either LTO or improving accuracy of --gc-sections. But both are things
> > that can be built on top of this patch, I think.  
> 
> Indeed.  Those patches are certainly welcome. They represent half of the 
> job already.  I just wanted to provide some insight about the whole 
> picture in case someone else notices those flaws I have identified.

Okay thanks, I appreciate you taking a look. I wanted to be sure I
wasn't missing some bug here.

Smaller kernel is nice for large systems because it means smaller
icache/dcache footprint and fewer branch trampolines, so I'm always
happy to see that effort. I will certainly help test LTO or some of
these other gc-sections improvements on powerpc.

Thanks,
Nick