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    SubjectRe: [GIT] kbuild/lto changes for 3.15-rc1
    On Tue, Apr 15, 2014 at 01:36:02PM +0200, Markus Trippelsdorf wrote:
    > On 2014.04.15 at 13:19 +0200, Sam Ravnborg wrote:
    > > >
    > > > And while the code size reduction is less for MIPS than what others have
    > > > reported for their platforms (I'm still investigating) is still is enough
    > > > that embedded developers would commit murder for.
    > >
    > > I have experimented a little with a patch that links all of vmlinux in one step.
    > > I compared the text size of vmlinux without and with -ffunction-sections.
    > >
    > > With a defconfig build on x86 (32 bit) I got following results:
    > >
    > > size difference
    > > singlelink 10266506
    > > function-sections 9487369 779137 7,5%
    > >
    > > So this is a reduction of ~800 kb by enabling -ffunction-sections which
    > > allows the linker to throw away unused sections.
    > >
    > > I have not boot tested the kernel so chances are that too much was thrown out by the linker.
    > > But this is an option that has much smaller cost to use than lto.
    > > And seems to benefit nicely in size.
    > >
    > > I have not tried this wihtout my singlelink patch - but I assume similar results.
    >
    > No, it wouldn't work, because you cannot mix -r and --gc-sections (or
    > gold's --icf (identical code folding)).
    With make allnoconfig I see a 5% decrease in text size by applying -ffunction-sections.
    This is with latest mainline and no other than the following applied:

    diff --git a/arch/x86/Makefile b/arch/x86/Makefile
    index 602f57e..51bac0a 100644
    --- a/arch/x86/Makefile
    +++ b/arch/x86/Makefile
    @@ -171,6 +171,8 @@ KBUILD_CFLAGS += -fno-asynchronous-unwind-tables
    KBUILD_CFLAGS += $(call cc-option,-mno-sse -mno-mmx -mno-sse2 -mno-3dnow,)
    KBUILD_CFLAGS += $(call cc-option,-mno-avx,)

    +KBUILD_CFLAGS += -ffunction-sections
    +
    KBUILD_CFLAGS += $(mflags-y)
    KBUILD_AFLAGS += $(mflags-y)

    What can then explain this size difference?

    Sam


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