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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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