Messages in this thread Patch in this message | | | Date | Thu, 14 May 2015 20:17:07 +0200 | From | Ingo Molnar <> | Subject | Re: [PATCH] x86: Align jump targets to 1 byte boundaries |
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* Denys Vlasenko <dvlasenk@redhat.com> wrote:
> > What do you guys think about this? I think we should seriously > > consider relaxing our alignment defaults. > > Looks like nobody objected. I think it's ok to submit > this patch for real.
Yeah, so my plan is to apply the following three changes from that discussion:
--- tip.orig/arch/x86/Makefile +++ tip/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) ... and not do -fno-guess-branch-probability, because it destroys likely()/unlikely() annotations.
Which is a pity, considering the size effect on defconfig:
text data bss dec filename 12566383 1617840 1089536 15273759 vmlinux.expect=10 [==vanilla] 11923529 1617840 1089536 14630905 vmlinux.-fno-guess-branch-probability 11903663 1617840 1089536 14611039 vmlinux.align=1 11646102 1617840 1089536 14353478 vmlinux.align=1+fno-guess-branch-probability
I.e. 2.6% of savings on top of the above three patches, while the effect of our hot/cold branch annotations is only around 0.4%, so if GCC preserved our annotations under -fno-guess-branch-probability we'd be good by at least 2%.
But GCC doesn't.
There were also these other changes I tested:
+ # 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
We could maybe consider -fno-caller-saves. What do you think about that option?
-fno-inline-small-functions is probably a bad idea, and -fno-tree-ch is probably a bad idea as well and is a dangerously rare option in any case that could break in unexpected ways.
Thanks,
Ingo
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