[swift-dev] No return functions and program exit

[Michael Gottesman]

> On Feb 7, 2017, at 11:56 AM, Andrew Trick <atrick at apple.com> wrote:
> 
>> 
>> On Feb 6, 2017, at 12:19 PM, Michael Gottesman via swift-dev <swift-dev at swift.org <mailto:swift-dev at swift.org>> wrote:
>> 
>> 
>>> On Feb 6, 2017, at 11:44 AM, Jordan Rose <jordan_rose at apple.com <mailto:jordan_rose at apple.com>> wrote:
>>> 
>>> 
>>>> On Feb 6, 2017, at 11:25, Joe Groff via swift-dev <swift-dev at swift.org <mailto:swift-dev at swift.org>> wrote:
>>>> 
>>>> 
>>>>> On Feb 6, 2017, at 11:22 AM, Michael Gottesman <mgottesman at apple.com <mailto:mgottesman at apple.com>> wrote:
>>>>> 
>>>>> Here is my suggestion:
>>>>> 
>>>>> 1. We assume by default the leaking case.
>>>>> 2. We change noreturn functions from C to maybe have a special semantic tag on them that says that cleanups should occur before them (i.e. UIApplicationMain).
>>> 
>>> I'm not sure what you mean by this. Functions from C exist in both groups, and I don't see why one assumption is better than the other.
>>> 
>>> 
>>>> 
>>>> I feel that "clean up before" is the safer ground case, and if we do any work to whitelist a group, it should be for the common "leakable" noreturns, like exit/_exit/abort/fatalError. That way, we momentarily burn some pointless cycles in the case we get it "wrong" rather than permanently leak memory.
>>> 
>>> I don't like this because of the reverse issue: under -Onone, you may want to pop back up the stack in the debugger and see what values you had, and they won't be available. It's almost always possible to get things released sooner; usually more awkward to get them to stay alive.
>> 
>> On the other hand, this is safe to do in the short term. We can special case asserts. One thing to consider though is if this should be provided to users. If not, we can just use semantics. Otherwise, we would need to discuss how to surface this at the language level.
>> 
>> Michael
> 
> Sorry I didn't jump in yesterday. I'm afraid I don't follow most of the reasoning expressed in the thread. I do completely understand Jordan's points.
> 
> 'noreturn' functions are called from may-return functions. Guaranteeing cleanup would result in inconsistent behavior as a result of optimization.
> 
> The optimizer can always shorten lifetimes when it determines that the caller can't access the object. But I don't see what that has to do with 'noreturn'.
> 
> I agree that we *could* add a special "cleanup before" semantic tag for some C functions, but I'm not aware of a need to do that and there are definite drawbacks.

No worries. (TBH I was just trying to get a decision and was posting straw men proposals). Basically after further talk off list, we got in agreement to go with the scope splitting approach (i.e. no cleanups).

What I really just wanted was something definitive that I could explicitly document. I added a small blurb to ./docs/ARCOptimization.*. I need to add an example and make it slightly clearer.

Michael

> 
> -Andy

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