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    SubjectRe: [PATCH] prepare kconfig inline optimization for all architectures
    On Sun, Apr 27, 2008 at 02:14:12PM -0400, Christoph Hellwig wrote:
    > On Sun, Apr 27, 2008 at 08:09:57PM +0200, Sam Ravnborg wrote:
    > > With the config option we pass the inline hint to gcc (if enabled).
    > > So with the config option we have the possibility to pass a _hint_ to
    > > gcc about inlining.
    > >
    > > Before the config option there were no difference between
    > > static int alwyas_inline foo() {}
    > > and
    > > static int inline foo() {}
    > >
    > > With the config option we now have a situation where they actually
    > > differ as they should do (assuming gcc > 4.x).
    >
    > As Linus mentioned the hint doesn't make any sense because gcc will
    > get it wrong anyway. In fact when you look at kernel code it tends
    > to inline the everything and the kitchensink as long as there's just
    > one caller and this bloat the stack but doesn't inline where it needs
    > to. Better don't try to mess with that and do it explicit.

    The tendency is that gcc inline *more than we whish* - not less.
    Which is why we have noinline - to cover the cases where we do not
    want stuff inlined.

    What the patch in question does is to make a difference
    between always_inle and inline.
    Previously they were the same. With the patch applied and
    with a gcc > 4.0 inline is now a hint.

    Did you actually read the patch?

    >
    > > So you say that it is safe to assume all places where we really need
    > > always_inline are annotedted such - and we do not need a simple
    > > config option that the user can uncheck.
    >
    > I don't say it is that, it certainly isn't on powerpc and probably most
    > other architectures right now, because only x86 got the fixup so far.
    > But making it a user-visible option instead of an architecture opt
    > in/out selection doesn't make any sense.
    If you read the patch you will see that the architectures that
    want to enable this has to do an explicit HAVE_CC_INLINE_HINT
    so powerpc is not impacted by this until they request it.
    Exactly the reason why this was not widely enabled in the
    first place (but implemnted in a too x86 specific way).


    Updated patch below. I btw. renamed the option to say this is
    about a INLINE_HINT.

    Sam

    diff --git a/arch/sparc/Kconfig b/arch/sparc/Kconfig
    index 49590f8..9645a6f 100644
    --- a/arch/sparc/Kconfig
    +++ b/arch/sparc/Kconfig
    @@ -68,6 +68,7 @@ config SPARC
    default y
    select HAVE_IDE
    select HAVE_OPROFILE
    + select HAVE_CC_INLINE_HINT

    # Identify this as a Sparc32 build
    config SPARC32
    diff --git a/arch/sparc64/Kconfig b/arch/sparc64/Kconfig
    index 8acc5cc..3c8ba36 100644
    --- a/arch/sparc64/Kconfig
    +++ b/arch/sparc64/Kconfig
    @@ -9,6 +9,7 @@ config SPARC
    bool
    default y
    select HAVE_OPROFILE
    + select HAVE_CC_INLINE_HINT
    select HAVE_KPROBES
    select HAVE_KRETPROBES

    diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
    index 2fadf79..48e352b 100644
    --- a/arch/x86/Kconfig
    +++ b/arch/x86/Kconfig
    @@ -20,6 +20,7 @@ config X86
    def_bool y
    select HAVE_IDE
    select HAVE_OPROFILE
    + select HAVE_CC_INLINE_HINT
    select HAVE_KPROBES
    select HAVE_KRETPROBES
    select HAVE_KVM if ((X86_32 && !X86_VOYAGER && !X86_VISWS && !X86_NUMAQ) || X86_64)
    @@ -142,9 +143,6 @@ config AUDIT_ARCH
    config ARCH_SUPPORTS_AOUT
    def_bool y

    -config ARCH_SUPPORTS_OPTIMIZED_INLINING
    - def_bool y
    -
    # Use the generic interrupt handling code in kernel/irq/:
    config GENERIC_HARDIRQS
    bool
    diff --git a/arch/x86/Kconfig.debug b/arch/x86/Kconfig.debug
    index 5b1979a..23956ea 100644
    --- a/arch/x86/Kconfig.debug
    +++ b/arch/x86/Kconfig.debug
    @@ -258,15 +258,3 @@ config CPA_DEBUG

    endmenu

    -config OPTIMIZE_INLINING
    - bool "Allow gcc to uninline functions marked 'inline'"
    - default y
    - help
    - This option determines if the kernel forces gcc to inline the functions
    - developers have marked 'inline'. Doing so takes away freedom from gcc to
    - do what it thinks is best, which is desirable for the gcc 3.x series of
    - compilers. The gcc 4.x series have a rewritten inlining algorithm and
    - disabling this option will generate a smaller kernel there. Hopefully
    - this algorithm is so good that allowing gcc4 to make the decision can
    - become the default in the future, until then this option is there to
    - test gcc for this.
    diff --git a/include/linux/compiler-gcc.h b/include/linux/compiler-gcc.h
    index b2fd754..e49fade 100644
    --- a/include/linux/compiler-gcc.h
    +++ b/include/linux/compiler-gcc.h
    @@ -31,8 +31,7 @@
    /*
    * Force always-inline if the user requests it so via the .config:
    */
    -#if !defined(CONFIG_ARCH_SUPPORTS_OPTIMIZED_INLINING) || \
    - !defined(CONFIG_OPTIMIZE_INLINING) && (__GNUC__ >= 4)
    +#if !(defined(CONFIG_HAVE_CC_INLINE_HINT) && (__GNUC__ >= 4))
    # define inline inline __attribute__((always_inline))
    # define __inline__ __inline__ __attribute__((always_inline))
    # define __inline __inline __attribute__((always_inline))
    diff --git a/init/Kconfig b/init/Kconfig
    index ba3a389..3568647 100644
    --- a/init/Kconfig
    +++ b/init/Kconfig
    @@ -508,6 +508,13 @@ config CC_OPTIMIZE_FOR_SIZE

    If unsure, say N.

    +# archs shall "select HAVE_CC_INLINE_HINT"
    +# to avoid forcing all function annotated inline
    +# to be inline.
    +# Note: This is only enforced for gcc > 4.0
    +config HAVE_CC_INLINE_HINT
    + def_bool n
    +
    config SYSCTL
    bool


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