SILOptimizer: Allow inlining of transparent functions in @backDeployed thunks.

In order for availability checks in iOS apps to be evaluated correctly when
running on macOS, the application binary must call a copy of
`_stdlib_isOSVersionAtLeast_AEIC()` that was emitted into the app, instead of
calling the `_stdlib_isOSVersionAtLeast()` function provided by the standard
library. This is because the call to the underlying compiler-rt function
`__isPlatformVersionAtLeast()` must be given the correct platform identifier
argument; if the call is not emitted into the client, then the macOS platform
identifier is used and the iOS version number will be mistakenly interpreted as
a macOS version number at runtime.

The `_stdlib_isOSVersionAtLeast()` function in the standard library is marked
`@_transparent` on iOS so that its call to `_stdlib_isOSVersionAtLeast_AEIC()`
is always inlined into the client. This works for the code generated by normal
`if #available` checks, but for the `@backDeployed` function thunks, the calls
to `_stdlib_isOSVersionAtLeast()` were not being inlined and that was causing
calls to `@backDeployed` functions to crash in iOS apps running on macOS since
their availability checks were being misevaluated.

The SIL optimizer has a heuristic which inhibits mandatory inlining in
functions that are classified as thunks, in order to save code size. This
heuristic needs to be relaxed in `@backDeployed` thunks, so that mandatory
inlining of `_stdlib_isOSVersionAtLeast()` can behave as expected. The change
should be safe since the only `@_transparent` function a `@backDeployed` thunk
is ever expected to call is `_stdlib_isOSVersionAtLeast()`.

Resolves rdar://134793410.

include/swift/SIL/SILFunction.h

@@ -56,7 +56,8 @@ enum IsThunk_t {
   IsNotThunk,
   IsThunk,
   IsReabstractionThunk,
-  IsSignatureOptimizedThunk
+  IsSignatureOptimizedThunk,
+  IsBackDeployedThunk,
 };
 enum IsDynamicallyReplaceable_t {
   IsNotDynamic,
@@ -368,7 +369,7 @@ private:
   ///
   /// The inliner uses this information to avoid inlining (non-trivial)
   /// functions into the thunk.
-  unsigned Thunk : 2;
+  unsigned Thunk : 3;
 
   /// The scope in which the parent class can be subclassed, if this is a method
   /// which is contained in the vtable of that class.
@@ -486,6 +487,7 @@ private:
       break;
     case IsThunk:
     case IsReabstractionThunk:
+    case IsBackDeployedThunk:
       thunkCanHaveSubclassScope = false;
       break;
     }

lib/SIL/IR/SILPrinter.cpp

@@ -3387,6 +3387,7 @@ void SILFunction::print(SILPrintContext &PrintCtx) const {
 
   switch (isThunk()) {
   case IsNotThunk: break;
+  case IsBackDeployedThunk: // FIXME: Give this a distinct label
   case IsThunk: OS << "[thunk] "; break;
   case IsSignatureOptimizedThunk:
     OS << "[signature_optimized_thunk] ";

lib/SILGen/SILGen.cpp

@@ -906,7 +906,7 @@ void SILGenModule::emitFunctionDefinition(SILDeclRef constant, SILFunction *f) {
     preEmitFunction(constant, f, loc);
     PrettyStackTraceSILFunction X("silgen emitBackDeploymentThunk", f);
     f->setBare(IsBare);
-    f->setThunk(IsThunk);
+    f->setThunk(IsBackDeployedThunk);
 
     SILGenFunction(*this, *f, dc).emitBackDeploymentThunk(constant);
 

lib/SILOptimizer/Mandatory/MandatoryInlining.cpp

@@ -1030,10 +1030,22 @@ class MandatoryInlining : public SILModuleTransform {
 
     SILOptFunctionBuilder FuncBuilder(*this);
     for (auto &F : *M) {
-      // Don't inline into thunks, even transparent callees.
-      if (F.isThunk())
+      switch (F.isThunk()) {
+      case IsThunk_t::IsThunk:
+      case IsThunk_t::IsReabstractionThunk:
+      case IsThunk_t::IsSignatureOptimizedThunk:
+        // Don't inline into most thunks, even transparent callees.
         continue;
 
+      case IsThunk_t::IsNotThunk:
+      case IsThunk_t::IsBackDeployedThunk:
+        // For correctness, inlining _stdlib_isOSVersionAtLeast() when it is
+        // declared transparent is mandatory in the thunks of @backDeployed
+        // functions. These thunks will not contain calls to other transparent
+        // functions.
+        break;
+      }
+
       // Skip deserialized functions.
       if (F.wasDeserializedCanonical())
         continue;

lib/Serialization/ModuleFormat.h

@@ -58,7 +58,7 @@ const uint16_t SWIFTMODULE_VERSION_MAJOR = 0;
 /// describe what change you made. The content of this comment isn't important;
 /// it just ensures a conflict if two people change the module format.
 /// Don't worry about adhering to the 80-column limit for this line.
-const uint16_t SWIFTMODULE_VERSION_MINOR = 885; // opened existentials
+const uint16_t SWIFTMODULE_VERSION_MINOR = 886; // SIL function thunk kind
 
 /// A standard hash seed used for all string hashes in a serialized module.
 ///

lib/Serialization/SILFormat.h

@@ -292,7 +292,7 @@ namespace sil_block {
       BCRecordLayout<SIL_FUNCTION, SILLinkageField,
                      BCFixed<1>,  // transparent
                      BCFixed<2>,  // serializedKind
-                     BCFixed<2>,  // thunks: signature optimized/reabstraction
+                     BCFixed<3>,  // thunk kind
                      BCFixed<1>,  // without_actually_escaping
                      BCFixed<3>,  // specialPurpose
                      BCFixed<2>,  // inlineStrategy

test/IRGen/Inputs/back_deployed.swift

@@ -0,0 +1,6 @@
+@backDeployed(before: SwiftStdlib 6.0)
+public func backDeployedFunc() {
+  otherFunc()
+}
+
+@usableFromInline internal func otherFunc() {}

test/IRGen/back_deployed_Onone.swift

@@ -0,0 +1,17 @@
+// RUN: %empty-directory(%t)
+// RUN: %target-swift-frontend -emit-module %S/Inputs/back_deployed.swift -o %t/ -swift-version 5 -enable-library-evolution
+// RUN: %target-swift-frontend -emit-ir %s -I %t -Onone | %FileCheck %s
+
+// _stdlib_isOSVersionAtLeast() is not @_transparent on macOS, watchOS, and tvOS
+// REQUIRES: OS=macosx || OS=watchos || OS=tvos
+
+import back_deployed
+
+public func test() {
+  backDeployedFunc()
+}
+
+// CHECK: define{{.*}} hidden swiftcc void @"$s13back_deployed0A12DeployedFuncyyFTwb"
+// CHECK:   call swiftcc i1 @"$ss26_stdlib_isOSVersionAtLeastyBi1_Bw_BwBwtF"
+// CHECK:   call swiftcc void @"$s13back_deployed0A12DeployedFuncyyFTwB"
+// CHECK:   call swiftcc void @"$s13back_deployed0A12DeployedFuncyyF"

test/IRGen/back_deployed_Onone_transparent.swift

@@ -0,0 +1,20 @@
+// RUN: %empty-directory(%t)
+// RUN: %target-swift-frontend -emit-module %S/Inputs/back_deployed.swift -o %t/ -swift-version 5 -enable-library-evolution
+// RUN: %target-swift-frontend -emit-ir %s -I %t -Onone | %FileCheck %s
+
+// _stdlib_isOSVersionAtLeast() is @_transparent on iOS
+// REQUIRES: OS=ios
+
+import back_deployed
+
+public func test() {
+  backDeployedFunc()
+}
+
+// CHECK: define{{.*}} hidden swiftcc void @"$s13back_deployed0A12DeployedFuncyyFTwb"
+// CHECK:   call swiftcc i1 @"$ss31_stdlib_isOSVersionAtLeast_AEICyBi1_Bw_BwBwtF"
+// CHECK:   call swiftcc void @"$s13back_deployed0A12DeployedFuncyyFTwB"
+// CHECK:   call swiftcc void @"$s13back_deployed0A12DeployedFuncyyF"
+
+// CHECK: define{{.*}} hidden swiftcc i1 @"$ss31_stdlib_isOSVersionAtLeast_AEICyBi1_Bw_BwBwtF"
+// CHECK:   call i32 @__isPlatformVersionAtLeast