Re: [PATCH] x86: Align jump targets to 1 byte boundaries

Markus Trippelsdorf
2015-04-14T08:23:59Z

On 2015.04.14 at 07:38 +0200, Ingo Molnar wrote:
>
> Just to make sure, could you please also apply the 3 alignment patches
> attached below? There's a lot of noise from extra alignment.

Here's an updated table:

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)
8202614 970072 802816 9975502 ./vmlinux gcc-5 (lto + Ingo's patch)
8801016 978512 811008 10590536 ./vmlinux gcc-5 (Ingo's patch)
8733943 952088 798720 10484751 ./vmlinux gcc-5 (lto + -malign-data=abi)
9186105 958320 806912 10951337 ./vmlinux gcc-5 (-malign-data=abi)
8190327 952088 798720 9941135 ./vmlinux gcc-5 (lto + Ingo's patch + -malign-data=abi)
8784633 958320 806912 10549865 ./vmlinux gcc-5 (Ingo's patch + -malign-data=abi)

For the "lto + Ingo's patch + -malign-data=abi" combination there is a
10% text size reduction.

-malign-data is a new option for gcc-5 that controls how the compiler
aligns variables. "abi" aligns variables according to psABI and give the
tightest packing.
"compat" is the default and uses an increased alignment value compatible
with gcc-4.8. But this should be unnecessary for the kernel.
(The other possible value is "cache", which increases the alignment
value to match the cache line size.)

diff --git a/arch/x86/Makefile b/arch/x86/Makefile
index 5ba2d9ce82dc..93702eef1684 100644
--- a/arch/x86/Makefile
+++ b/arch/x86/Makefile
@@ -77,6 +77,9 @@ else
KBUILD_AFLAGS += -m64
KBUILD_CFLAGS += -m64

+ # Align variables according to psABI
+ KBUILD_CFLAGS += -malign-data=abi
+
# Don't autogenerate traditional x87 instructions
KBUILD_CFLAGS += $(call cc-option,-mno-80387)
KBUILD_CFLAGS += $(call cc-option,-mno-fp-ret-in-387)

> 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.

For a high level overview of LTO in gcc-5 see Honza's recent article:
http://hubicka.blogspot.de/2015/04/GCC5-IPA-LTO-news.html

I haven't looked at the generated code at all yet, because the kernel is
huge and I'm not sure where to best look for specific changes.

> _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.

This isn't possible, because kernel/built-in.o is a 'slim' lto object
file, that only contains compressed LTO sections with the compiler's
internal representation.

--
Markus