Compiled Lambda Expressions and AOT

[JamesNK]

Is it correct that compiled lambda expressions on UWP aren't really compiled and are instead interpreted? If so does that mean that on UWP and other AOT platforms like Xamarin/Unity with no support for IL emit it is faster to use MemberInfo and ConstructorInfo?

Related: JamesNK/Newtonsoft.Json#968

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If normal reflection is faster on AOT platforms I would like to know is if there is a way at runtime to discover that IL emit isn't supported?

A way to discover this capability at runtime will allow libraries to use compiled lambda expressions to boost on platforms it is available, and to fallback to traditional reflection on platforms where it isn't, while still using a single netstandard assembly.

[weshaggard]

cc @VSadov

That is correct for UWP the expressions are interpreted and not compiled at runtime.

[clairernovotny]

@JamesNK I know you'll hate this answer, but cross-compiling can ensure that each platform gets the most optimized version for it. With project.json and whatever's coming with the 1.1 tooling transitive dependencies will resolve correctly and ensure that the best version of your library wind up in the runtime dir.

[davidfowl]

This needs to be a runtime light up API that is netstandard. The project N toolchain and the JIT can treat it as an intrinsic and eliminate it at runtime.

[mjracca]

+1

This is a HUGE performance issue for us!

[JamesNK]

I know you'll hate this answer, but cross-compiling can ensure that each platform gets the most optimized version for it.

That would require knowing every platform that doesn't support compiling expressions and having a different version in the package for each of those platforms, UWP, iOS, Android, Unity, etc

And as new platforms come out they would get the slow netstandard binary until someone explicitly adds a different version in the package.

Or we could have a simple property that library authors can use to adapt the netstandard assembly to the platform at runtime. One assembly that works for now and forever.

[jkotas]

Agree - hardcoding the list of platforms is fragile. It does not work well for new platforms. Also, the answer can differ for given platform over time; or both interpreter-only and compiled System.Linq.Expressions can be available for some platforms.

Having an API that returns whether expressions are compiled sounds pretty reasonable.

[VSadov]

Since now any expression can be asked to be compiled for interpretation, the API would need to take a compiled expression.
We generally cannot tell if a delegate is a compiled expression (could be not related to expressions at all), but we can recognize interpreted expression delegates.

Would something like the following work for you?

/// returns true if d is a delegate that represents an interpreted expression
bool IsInterpretedExpression(Delegate d)

[JonHanna]

I read this as more an environment query; code being able to know if calling Compile(false) is going to result in interpretation or not.

[JamesNK]

Yes, I see it as an environment query.

IsInterpretedExpression might be useful for some people but I'm interested in discovering whether an environment supports compiled expressions before I choose to use expressions (or not, depending on the environment query result).

[davidfowl]

I read this as more an environment query; code being able to know if calling Compile(false) is going to result in interpretation or not.

Yes. It needs to be before not after. Framework code would likely switch logic altogether and use reflection invoke vs expression tree compilation.

[stratospheres]

Man, this really burned me - I simply ported some really well working code from an "old" style codebase to UWP and this absolutely hammered me... slowed down by roughly 20x.

+1 to fixing this... please.

[JamesNK]

Another person being hurt by this - JamesNK/Newtonsoft.Json#968 (comment)

[JonHanna]

Should such a property live in S.L.Expressions or perhaps elsewhere?

Am I right in thinking that a complementary CanInterpret would also be true now, and hence have no value?