On Mon, 8 Aug 2016 00:12:37 -0400 (EDT) Nicolas Pitre <nicolas.pitre@xxxxxxxxxx> wrote: > On Mon, 8 Aug 2016, Nicholas Piggin wrote: > > > On Sun, 7 Aug 2016 01:33:45 -0400 (EDT) > > Nicolas Pitre <nicolas.pitre@xxxxxxxxxx> 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@xxxxxxxxx> > > > > > > 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 -- To unsubscribe from this list: send the line "unsubscribe linux-kbuild" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
