[AutoDiff] Fix derivative forwarding thunk linkage.

Make derivative forwarding thunks use original function's linkage instead of the
derivative function's, stripping external.

This is consistent with the linkage of differentiability witnesses.

Clarify AutoDiff linkage-related comments.

Resolves TF-1160: AutoDiff symbol TBDGen error.

Author: dan-zheng

lib/SILGen/SILGen.cpp

@@ -839,8 +839,8 @@ void SILGenModule::emitDifferentiabilityWitness(
   SILDifferentiabilityWitnessKey key{originalFunction->getName(), silConfig};
   auto *diffWitness = M.lookUpDifferentiabilityWitness(key);
   if (!diffWitness) {
-    // Strip external from linkage of original function.
-    // Necessary for Clang-imported functions, which have external linkage.
+    // Differentiability witnesses have the same linkage as the original
+    // function, stripping external.
     auto linkage = stripExternalFromLinkage(originalFunction->getLinkage());
     diffWitness = SILDifferentiabilityWitness::createDefinition(
         M, linkage, originalFunction, silConfig.parameterIndices,

lib/SILGen/SILGenPoly.cpp

@@ -3941,14 +3941,17 @@ SILFunction *SILGenModule::getOrCreateCustomDerivativeThunk(
 
   auto loc = customDerivativeFn->getLocation();
   SILGenFunctionBuilder fb(*this);
-  // This thunk is publicly exposed and cannot be transparent.
-  // Instead, mark it as "always inline" for optimization.
+  // Derivative thunks have the same linkage as the original function, stripping
+  // external.
+  auto linkage = stripExternalFromLinkage(originalFn->getLinkage());
   auto *thunk = fb.getOrCreateFunction(
-      loc, name, customDerivativeFn->getLinkage(), thunkFnTy, IsBare,
-      IsNotTransparent, customDerivativeFn->isSerialized(),
+      loc, name, linkage, thunkFnTy, IsBare, IsNotTransparent,
+      customDerivativeFn->isSerialized(),
       customDerivativeFn->isDynamicallyReplaceable(),
       customDerivativeFn->getEntryCount(), IsThunk,
       customDerivativeFn->getClassSubclassScope());
+  // This thunk may be publicly exposed and cannot be transparent.
+  // Instead, mark it as "always inline" for optimization.
   thunk->setInlineStrategy(AlwaysInline);
   if (!thunk->empty())
     return thunk;

test/AutoDiff/Sema/derivative_attr_type_checking.swift

@@ -837,6 +837,27 @@ fileprivate func _fileprivate_original_fileprivate_derivative(_ x: Float) -> (va
   fatalError()
 }
 
+func internal_original_usablefrominline_derivative(_ x: Float) -> Float { x }
+@usableFromInline
+@derivative(of: internal_original_usablefrominline_derivative)
+func _internal_original_usablefrominline_derivative(_ x: Float) -> (value: Float, pullback: (Float) -> Float) {
+  fatalError()
+}
+
+func internal_original_inlinable_derivative(_ x: Float) -> Float { x }
+@inlinable
+@derivative(of: internal_original_inlinable_derivative)
+func _internal_original_inlinable_derivative(_ x: Float) -> (value: Float, pullback: (Float) -> Float) {
+  fatalError()
+}
+
+func internal_original_alwaysemitintoclient_derivative(_ x: Float) -> Float { x }
+@_alwaysEmitIntoClient
+@derivative(of: internal_original_alwaysemitintoclient_derivative)
+func _internal_original_alwaysemitintoclient_derivative(_ x: Float) -> (value: Float, pullback: (Float) -> Float) {
+  fatalError()
+}
+
 // MARK: - Original function visibility < derivative function visibility
 
 @usableFromInline

test/AutoDiff/compiler_crashers_fixed/tf1160-derivative-usable-from-inline-mismatch.swift

@@ -0,0 +1,19 @@
+// RUN: %target-swift-frontend -c %s -verify
+// REQUIRES: asserts
+
+// TF-1160: Linker error for `@usableFromInline` derivative function but
+// non-`@usableFromInline` internal original function.
+
+import _Differentiation
+
+func internalOriginal(_ x: Float) -> Float { x }
+
+@usableFromInline
+@derivative(of: internalOriginal)
+func usableFromInlineDerivative(_ x: Float) -> (value: Float, pullback: (Float) -> Float) {
+  (x, { $0 })
+}
+
+// Original error: type-checking passes but TBDGen is not consistent with IRGen.
+// <unknown>:0: error: symbol 'AD__$s4main16internalOriginalyS2fF__vjp_src_0_wrt_0' (AD__$s4main16internalOriginalyS2fF__vjp_src_0_wrt_0) is in generated IR file, but not in TBD file
+// <unknown>:0: error: please file a radar or open a bug on bugs.swift.org with this code, and add -Xfrontend -validate-tbd-against-ir=none to squash the errors