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    SubjectRe: [patch 01/10] compiler: define __attribute_unused__
    On Wed, 2 May 2007, Rusty Russell wrote:

    > Adding this macro doesn't give us anything that simply saying
    > "__attribute__((unused))" doesn't give. But it does add a layer of
    > kernel-specific indirection.
    >

    That's obviously true since we're defining __attribute_unused__ to be
    __attribute__((unused)).

    We were trying to clean up the misconception that the current
    __attribute_used__ was created to suppress warnings when, in fact, that
    was not its purpose. It was created to emit the code for a function that
    appeared to be unreferenced and only suppressed warnings as a side-effect
    in gcc <3.4.

    > If we're going to get kernel-specific, I'd prefer to see:
    >
    > __needed: suppress warning and don't discard,

    That would be the current definition of __attribute_used__ (i.e. we're
    saying that we use the function in inline assembly even though it appears
    we don't use it at all).

    > __unneeded: suppress warning and might discard.
    >

    That would be the patched definition of __attribute_unused__.

    So let's go back to the problem this was initially supposed to fix from
    arch/i386/pci/init.c:

    static __init int pci_access_init(void)
    {
    int type = 0;

    #ifdef CONFIG_PCI_DIRECT
    type = pci_direct_probe();
    #endif
    #ifdef CONFIG_PCI_MMCONFIG
    pci_mmcfg_init(type);
    #endif
    ...

    and type is unreferenced for the remainder of the function. Obviously we
    could add #if defined(CONFIG_PCI_DIRECT) || defined(CONFIG_PCI_MMCONFIG)
    before the declaration of 'type', but that becomes sloppy pretty quickly.

    The patched version makes this:

    int type __attribute_unused__ = 0;

    which definitely tells you that you're using a compiler attribute that
    will be attached to that automatic. In your case:

    int type __unneeded = 0;

    doesn't say anything in this case. It doesn't resemble any attribute that
    a programmer might be familiar with and begs the question of why we've
    declared it if it's truly "unneeded"?

    By the way, there are tons of these instances where __attribute__((used))
    needs to be added in driver code to suppress unreferenced warnings.

    David
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