[Bug middle-end/32911] Function __attribute__ ((idempotent))

[matthew at wil dot cx]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=32911

Matthew Wilcox <matthew at wil dot cx> changed:

           What    |Removed                     |Added
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                 CC|                            |matthew at wil dot cx

--- Comment #6 from Matthew Wilcox <matthew at wil dot cx> ---
I actually have a use for a real idempotent function, that is f(f(x)) == f(x).
I think it's different from the behaviour wanted here which is more of an
initialiser functionality.

The concrete example is Linux's compound_head() macro which takes a pointer to
a struct page and returns a pointer to a (potentially different) struct page. 
A page is its own head, and calls to compound_head can be buried inside macros.

So I'd like gcc to know that given this program:

struct page *compound_head(struct page *) __attribute__((idempotent));

int g(struct page *page)
{
struct page *head = compound_head(page);
return page->refcount;
}

int f(struct page *page)
{
struct page *head = compound_head(page);
return g(head);
}

if it inlines g() into f(), it only needs to make one call to compound_head().