IRGen: Don't emit capture descriptor for stack allocated closures

[aschwaighofer]

No description provided.

[aschwaighofer]

@swift-ci Please test and merge

[jrose-apple]

Does this affect debugging?

[aschwaighofer]

Does this affect debugging?

lldb does not use the closure context capture descriptor as far as I can tell:

The only place that I found that reads the capture descriptor is ReflectionContext::getInstanceTypeInfo.

The only place where lldb uses this function is in SwiftLanguageRuntime::GetMemberVariableOffset to get the offset of a member by name.

You would need this information if you wanted to display what is captured by closure. I don't see that we are currently doing this.

No tests fail.

cc @dcci @slavapestov

[jrose-apple]

I don't know how many tests we have for optimized code, but I guess that's because we haven't made that a focus yet. It's possible we're regressing that behavior, but it's probably worth it for this optimization.

[aschwaighofer]

This is unrelated to optimized or unoptimized code. We can have stack allocated closures under either mode. The pass that makes closures stack allocated runs in the mandatory pipeline.

[jrose-apple]

Ah, then it may be worth adding explicit checks to LLDB for printing a stack-allocated closure's captures. We shouldn't regress on that.

[aschwaighofer]

You can print the captured variables inside of a closure during executing of the closure without the capture descriptor - they are passed as arguments to the function that implements the closure:

$ /Library/Developer/Toolchains/swift-DEVELOPMENT-SNAPSHOT-2019-01-24-a.xctoolchain/usr/bin/lldb ./TestClosureDebugging
Process 82636 stopped
* thread #1, queue = 'com.apple.main-thread', stop reason = breakpoint 1.2
    frame #0: 0x0000000100001af5 TestClosureDebugging`closure #1 in testClosure(a=(x = 1), b=(x = 2)) at TestClosureDebugging.swift:14:13
   11  	
   12  	func testClosure(_ a: A, _ b: B) {
   13  	  useClosure {
-> 14  	    let x = a
   15  	    let y = b
   16  	    print(a.x)
   17  	    print(b.x)
Target 0: (TestClosureDebugging) stopped.
(lldb) print a
(TestClosureDebugging.A) $R0 = 0x0000000100808af0 (x = 1)
(lldb) print b
(TestClosureDebugging.B) $R2 = 0x0000000100808be0 (x = 2)

lldb does not seem to print a closure's captures when you print the closure itself which would require the capture descriptor:

* thread #1, queue = 'com.apple.main-thread', stop reason = step in
    frame #0: 0x00000001000015c9 TestClosureDebugging`useClosure(cl=0x0000000100001c60 TestClosureDebugging`partial apply forwarder for closure #1 () -> () in TestClosureDebugging.testClosure(TestClosureDebugging.A, TestClosureDebugging.B) -> () at <compiler-generated>) at TestClosureDebugging.swift:2:3
   1   	func useClosure(_ cl: () -> ()) {
-> 2   	  cl()
   3   	}
   4   	
   5   	class A {
   6   	  var x = 1
   7   	}
Target 0: (TestClosureDebugging) stopped.
(lldb) print cl
error: <EXPR>:3:1: error: non-escaping value 'cl' may only be called
cl

When you print an escaping closure it does not seem to print the captures either:

* thread #1, queue = 'com.apple.main-thread', stop reason = step in
    frame #0: 0x000000010000154d TestClosureDebugging`useClosure(cl=0x0000000100001c50 TestClosureDebugging`partial apply forwarder for closure #1 () -> () in TestClosureDebugging.testClosure(TestClosureDebugging.A, TestClosureDebugging.B) -> () at <compiler-generated>) at TestClosureDebugging.swift:3:3
   1   	func useClosure(_ cl: @escaping () -> ()) {
   2   	  let f = cl
-> 3   	  f()
   4   	}
   5   	
   6   	class A {
   7   	  var x = 1
Target 0: (TestClosureDebugging) stopped.
(lldb) print f
() -> () $R0 = 0x0000000100001c50 TestClosureDebugging`partial apply forwarder for closure #1 () -> () in TestClosureDebugging.testClosure(TestClosureDebugging.A, TestClosureDebugging.B) -> () at <compiler-generated>
(lldb) print cl
() -> () $R2 = 0x0000000100001c50 TestClosureDebugging`partial apply forwarder for closure #1 () -> () in TestClosureDebugging.testClosure(TestClosureDebugging.A, TestClosureDebugging.B) -> () at <compiler-generated>

[jrose-apple]

Ahh, thanks for the clarification!