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    SubjectRe: [PATCH] micro optimization in kernel/params.c; don't call to_module_kobject before we really have to.
    On Sat, 29 Jan 2005, Brian Gerst wrote:

    > Jesper Juhl wrote:
    > > In kernel/params.c::module_attr_show and kernel/params.c::module_attr_store
    > > we call to_module_kobject() and save the result in a local variable right
    > > before a conditional statement that does not depend on the call to
    > > to_module_kobject() and may cause the function to return. If the function
    > > returns before we use the result of to_module_kobject() then the call is
    > > just a waste of time. The patch moves the call to to_module_kobject() down
    > > just before it is actually used, thus we save a call to the function in a
    > > few cases. I doubt this is in any way measurable, but I see no reason to not
    > > move the call - it should be an infinitesimal improvement with no ill
    > > sideeffects.
    > > Please consider applying.
    > >
    > >
    > > Signed-off-by: Jesper Juhl <juhl-lkml@dif.dk>
    > >
    > > diff -up linux-2.6.11-rc2-bk7-orig/kernel/params.c
    > > linux-2.6.11-rc2-bk7/kernel/params.c
    > > --- linux-2.6.11-rc2-bk7-orig/kernel/params.c 2005-01-29 23:54:53.000000000
    > > +0100
    > > +++ linux-2.6.11-rc2-bk7/kernel/params.c 2005-01-30 00:27:08.000000000
    > > +0100
    > > @@ -625,11 +625,12 @@ static ssize_t module_attr_show(struct k
    > > int ret;
    > > attribute = to_module_attr(attr);
    > > - mk = to_module_kobject(kobj);
    > > if (!attribute->show)
    > > return -EPERM;
    > > + mk = to_module_kobject(kobj);
    > > +
    > > if (!try_module_get(mk->mod))
    > > return -ENODEV;
    > > @@ -649,11 +650,12 @@ static ssize_t module_attr_store(struct int
    > > ret;
    > > attribute = to_module_attr(attr);
    > > - mk = to_module_kobject(kobj);
    > > if (!attribute->store)
    > > return -EPERM;
    > > + mk = to_module_kobject(kobj);
    > > +
    > > if (!try_module_get(mk->mod))
    > > return -ENODEV;
    > >
    > >
    >
    > I'd bet that the compiler already reorders the assignment since there is no
    > side effect to to_module_kobject(). Even with a patch like this you can't
    > control exactly when the assignment is done. There may not even be an
    > assignment, since mk is a constant offset of kobj.
    >
    True, the compiler is free to be clever, but I still think it's best to
    write the code in the most optimal way as seen from a C perspective.
    I just took a look at the compiled object files with and without the
    patch, and it makes no difference what-so-ever - gcc generates the exact
    same code. So you are right, gcc is clever about it.
    Hmm, it's Rusty's code as far as I can see, I'll leave it up to him to
    apply the patch or not.

    --
    Jesper


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