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

Ingo Molnar
2015-04-14T09:16:37Z

* Markus Trippelsdorf <markus@trippelsdorf.de> wrote:

> 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.

Lets rename "Ingo's patch" to "code_align=1". The interesting one
would be to compare:

code_align=1 + -fno-guess-branch-probability
vs.
lto + code_align=1 + -fno-guess-branch-probability

Or rather, you could try "Ingo's combo patch" further below, with and
without LTO.

I'd expect LTO to still be in the 5% reduction range.

> -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.

I'm not so sure about that one, our data access patterns are usually a
lot more well thought out than our code alignment (which is really
mostly compiler controlled). It also gives limited savings:

9202488 vmlinux gcc-5
9186105 vmlinux gcc-5 (-malign-data=abi)

Which is 0.1%. I've got a handful of options in that size range:

+ # Reduces vmlinux size by 0.25%:
+ KBUILD_CFLAGS += -fno-caller-saves
+
+ # Reduces vmlinux size by 1.10%:
+ KBUILD_CFLAGS += -fno-inline-small-functions
+
+ # Reduces vmlinux size by about 0.95%:
+ KBUILD_CFLAGS += -fno-tree-ch

but obviously they are more obscure and thus riskier. Find below an
updated "Ingo's combo patch". It gives more than 10% savings here on
x86 defconfig using gcc 4.9, without LTO.

Thanks,

Ingo

---------------->
arch/x86/Makefile | 24 ++++++++++++++++++++++++
1 file changed, 24 insertions(+)

diff --git a/arch/x86/Makefile b/arch/x86/Makefile
index 5ba2d9ce82dc..999e94685d12 100644
--- a/arch/x86/Makefile
+++ b/arch/x86/Makefile
@@ -77,10 +77,34 @@ 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)

+ #
+ # Don't guess branch probabilities, follow the code and unlikely()/likely() hints,
+ # which reduces vmlinux size by about 5.4%:
+ #
+ KBUILD_CFLAGS += -fno-guess-branch-probability
+
+ # Reduces vmlinux size by 0.25%:
+ KBUILD_CFLAGS += -fno-caller-saves
+
+ # Reduces vmlinux size by 1.10%:
+ KBUILD_CFLAGS += -fno-inline-small-functions
+
+ # Reduces vmlinux size by about 0.95%:
+ KBUILD_CFLAGS += -fno-tree-ch
+
# Use -mpreferred-stack-boundary=3 if supported.
KBUILD_CFLAGS += $(call cc-option,-mpreferred-stack-boundary=3)