Messages in this thread Patch in this message | | | Date | Tue, 14 Apr 2015 07:38:21 +0200 | From | Ingo Molnar <> | Subject | Re: [PATCH] x86: Align jump targets to 1 byte boundaries |
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* Markus Trippelsdorf <markus@trippelsdorf.de> wrote:
> On 2015.04.13 at 11:31 -0700, Linus Torvalds wrote: > > On Mon, Apr 13, 2015 at 10:26 AM, Markus Trippelsdorf > > <markus@trippelsdorf.de> wrote: > > > > > > I must have made a measurement mistake above, because the actual code > > > size savings are roughly 5%: > > > > Can you check against the -fno-guess-branch-probability output? > > text data bss dec filename > 8746230 970072 802816 10519118 ./vmlinux gcc-5 (lto) > 9202488 978512 811008 10992008 ./vmlinux gcc-5 > 8036915 970296 802816 9810027 ./vmlinux gcc-5 (lto -fno-guess-branch-probability) > 8593615 978512 811008 10383135 ./vmlinux gcc-5 (-fno-guess-branch-probability)
Just to make sure, could you please also apply the 3 alignment patches attached below? There's a lot of noise from extra alignment.
Having said that, LTO should have three main effects:
1) better cross-unit inlining decisions
2) better register allocation and clobbering knowledge (if a small function is known not to clobber caller-saved registers, then the saving can be skipped)
3) better dead code elimination
1)-2) is probably worth the price, 3) in isolation isn't. So we'd have to estimate which one is how significant, to judge the value of LTO - but I haven't seen any effort so far to disambiguate it.
_Possibly_ if you build kernel/built-in.o only, and compared its sizes, that would help a bit, because the core kernel has very little dead code, giving a fairer estimation of 'true' optimizations.
Thanks,
Ingo
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arch/x86/Makefile | 9 +++++++++ 1 file changed, 9 insertions(+)
diff --git a/arch/x86/Makefile b/arch/x86/Makefile index 5ba2d9ce82dc..10989a73b986 100644 --- a/arch/x86/Makefile +++ b/arch/x86/Makefile @@ -77,6 +77,15 @@ else KBUILD_AFLAGS += -m64 KBUILD_CFLAGS += -m64 + # Pack jump targets tightly, don't align them to the default 16 bytes: + KBUILD_CFLAGS += -falign-jumps=1 + + # Pack functions tightly as well: + KBUILD_CFLAGS += -falign-functions=1 + + # Pack loops tightly as well: + KBUILD_CFLAGS += -falign-loops=1 + # Don't autogenerate traditional x87 instructions KBUILD_CFLAGS += $(call cc-option,-mno-80387) KBUILD_CFLAGS += $(call cc-option,-mno-fp-ret-in-387)
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