> On Dec 1, 2016, at 11:48 AM, Andrew Trick <atr...@apple.com> wrote:
> Responding on swift-dev to what turned into an interesting discussion…
>> On Dec 1, 2016, at 11:19 AM, Joe Groff <jgr...@apple.com> wrote:
>>> On Dec 1, 2016, at 11:18 AM, Joe Groff <jgr...@apple.com> wrote:
>>>> On Dec 1, 2016, at 11:16 AM, Andrew Trick <atr...@apple.com> wrote:
>>>>> On Dec 1, 2016, at 10:34 AM, Jordan Rose <jordan_r...@apple.com> wrote:
>>>>>> On Nov 30, 2016, at 18:16, Joe Groff <jgr...@apple.com> wrote:
>>>>>>> On Nov 30, 2016, at 6:10 PM, Andrew Trick <atr...@apple.com> wrote:
>>>>>>> We definitely want an inline-at-Onone, but I assumed we were treating 
>>>>>>> "@inline(__always)" as a normal heuristic because:
>>>>>>> - @transparent already does inline-at-Onone.
>>>>>>> - @inline(__always) without @transparent negatively affects the debug 
>>>>>>> experience.
>>>>>>> - @inline(__always) isn't useful if no optimizations are run after 
>>>>>>> inlining.
>>>>>>> That said, if the common expectation is for @inline(__always) to kick 
>>>>>>> in independent of optimization, then we should honor that.
>>>>>>> It's easy for me to add an inlining pass to runSILPassesForOnone. I'm 
>>>>>>> just not sure that will have the effect people are looking for. It's 
>>>>>>> unlikely to help performance since nothing is going to run after the 
>>>>>>> inlining (no capture promotion or alloc box-to-stack). The only 
>>>>>>> optimization we currently run at Onone after diagnostics is 
>>>>>>> prespecialization.
> Actually, the inliner specializes as it goes, so inlining after mandatory SIL 
> passes would still be quite useful.
> My concern is really that the user may expect inline(__always) to be as 
> effective a performance tool as @transparent, while preserving debug line 
> locations.
> If it runs late, it doesn’t expose capture promotion, box-to-stack, 
> guaranteed ARC optimization, etc.
>>>>>>> If someone is using inline(__always) for -Onone performance reasons, I 
>>>>>>> think they really need to use @transparent.
>>>>>>> An alternative is to have @inline(__always) be another way to spell 
>>>>>>> @transparent. People would need to implicitly know that this would 
>>>>>>> affect diagnostics of the inlined method.
>>>>>>> Any thoughts?
>>>>>> `@transparent` doesn't just mean "inline-at-Onone", it also allows 
>>>>>> diagnostic passes to constant-fold through the implementation and 
>>>>>> actively eliminates debug information. It was never intended to be used 
>>>>>> for performance, but only for the limited set of things that need it. I 
>>>>>> imagine that the specialization you get by inlining some things is 
>>>>>> pretty important by itself. I suspect closure inlining would also be 
>>>>>> helpful for a lot of low-level "with*" combinators; doesn't transparent 
>>>>>> inlining do that, and could inline-always-inlining do the same?
>>>>> For reference: 
>>>>> https://github.com/apple/swift/blob/master/docs/TransparentAttr.rst
>>>> The only remaining question: is it ok for a non-transparent function to be 
>>>> inlined prior to running some dataflow diagnostics?
>>> Doing so changes the set of programs that are accepted, since it may expose 
>>> more code to diagnostic passes. We don't want diagnostics to depend on 
>>> optimizations.
>> Perhaps we could avoid this by changing the diagnostic passes to 
>> intentionally ignore instructions with inlined source locations, though that 
>> seems brittle.
>> -Joe
> Well, inline(__always) would be mandatory, not an optimization. But I think 
> your point is that the user does not expect that attribute to affect program 
> legality. Having data-flow diagnostics “ignore” certain instructions sounds 
> horrible. Is there an obvious problem, or is this hypothetical? i.e. is there 
> some diagnostic that is currently enforced at the function boundary but would 
> be considered legal code after inlining?

An example of something that depends on dataflow diagnostics would be something 
that's constant-foldable causing static overflow diagnostics:

func double(_ x: UInt8) -> UInt8 {
  return x + x

_ = double(128)

Making `double` transparent causes the compiler to statically reject the call:

/Users/jgroff/bar.swift:6:5: error: arithmetic operation '128 + 128' (on 
unsigned 8-bit integer type) results in an overflow
_ = double(128)

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