[cfe-commits] [patch] Add the returns_twice attribute to know functions
rjmccall at apple.com
Tue Oct 11 16:13:18 PDT 2011
On Oct 11, 2011, at 3:49 PM, Rafael Ávila de Espíndola wrote:
>> Note that there *is* a subtyping relationship here: a normal
>> function can be safely used as a returns_twice function (which simply
>> never actually returns twice). But that's useless.
>> I agree that abstractly this ought to be a type attribute, and I
>> don't think it would be difficult to take advantage of the current
>> 'noreturn conversion' hooks to make the (unsafe) implicit conversion
>> work, but I'm not sure there's any real value to adding this
> OK, so I think the consensus is
> * The hard coded list will be removed after 3.0 branches and dragonegg
> producing this attribute too.
> * The attribute should be part of the type
> * We should try to avoid allowing unsafe conversions like
> int (*fptr)(jmp_buf) = &setjmp;
> but a returns_twice function pointer can point to a function with an
> otherwise matching type.
Sorry; re-reading myself, I can see how this is a plausible interpretation,
but this is actually not what I meant. Let me rephrase.
I think that, divorced from any other considerations, returns_twice would
belong in the type system. However, doing so would introduce
significant complexity for basically zero value — if nothing else, the
use restrictions on these functions make it extremely unlikely that they
will ever be called indirectly. Therefore I think it should stay a decl
attribute, and we can consider whether to make it a type attribute in
the off-chance that we ever see a compelling use case.
Note that even the template-metaprogramming use case will work
correctly with decl attributes.
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