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Re: Attribute returns_null

On 10/07/13 12:37, David Malcolm wrote:
On Mon, 2013-10-07 at 19:51 +0200, Marc Glisse wrote:
On Mon, 7 Oct 2013, David Malcolm wrote:

On Mon, 2013-10-07 at 16:17 +0200, Marc Glisse wrote:

this patch adds an attribute to let the compiler know that a function
never returns NULL. I saw some ECF_* flags, but the attribute seems
sufficient. I considered using nonnull(0), but then it would have been
confusing that the version of nonnull without arguments applies only to
parameters and not the return value.

I can't comment on the patch itself, but could there also be an
attribute "returns_null", for functions that *always* return NULL?
This may sound weird, but I know of at least one API that exposes such
functions: CPython's exception-handling API: see e.g.
and various other functions that have "Return value: Always NULL."
This allows the user to write one-liners like:

   return PyErr_NoMemory();

I didn't think about it very long, so I probably missed the best reasons,
but it doesn't sound like such a good idea. If PyErr_NoMemory always
returns NULL, why not make that clear in the code? It could be an inline
function, or even a macro that expands to

To me, attributes are there for when the language is insufficient, kind of
a last resort. Could you explain why you think it would be the best option
No, I can't :)   (that API predates my involvement in that project).
This seems to have dubious value to me -- ie, I don't think there's many functions that always return NULL in practice and of those, I doubt there's a lot of opportunity to optimize with that knowledge.

In contrast, knowing a function can not return null has a ton of value, both for optimization purposes and for improving warnings.


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