diff --git a/include/swift/SILOptimizer/Utils/Devirtualize.h b/include/swift/SILOptimizer/Utils/Devirtualize.h
index edcfbc13214..dfcfe5eabea 100644
--- a/include/swift/SILOptimizer/Utils/Devirtualize.h
+++ b/include/swift/SILOptimizer/Utils/Devirtualize.h
@@ -43,8 +43,7 @@ namespace swift {
 /// casted to produce a properly typed value (first element).
 typedef std::pair<ValueBase *, ApplySite> DevirtualizationResult;
 
-DevirtualizationResult tryDevirtualizeApply(FullApplySite AI);
-DevirtualizationResult tryDevirtualizeApply(FullApplySite AI,
+DevirtualizationResult tryDevirtualizeApply(ApplySite AI,
                                             ClassHierarchyAnalysis *CHA);
 bool canDevirtualizeApply(FullApplySite AI, ClassHierarchyAnalysis *CHA);
 bool isNominalTypeWithUnboundGenericParameters(SILType Ty, SILModule &M);
diff --git a/lib/SILOptimizer/SILCombiner/SILCombiner.h b/lib/SILOptimizer/SILCombiner/SILCombiner.h
index 114507541d0..81ff67262f1 100644
--- a/lib/SILOptimizer/SILCombiner/SILCombiner.h
+++ b/lib/SILOptimizer/SILCombiner/SILCombiner.h
@@ -226,7 +226,6 @@ public:
   SILInstruction *visitAllocRefDynamicInst(AllocRefDynamicInst *ARDI);
   SILInstruction *visitEnumInst(EnumInst *EI);
   SILInstruction *visitConvertFunctionInst(ConvertFunctionInst *CFI);
-  SILInstruction *visitWitnessMethodInst(WitnessMethodInst *WMI);
 
   /// Instruction visitor helpers.
   SILInstruction *optimizeBuiltinCanBeObjCClass(BuiltinInst *AI);
diff --git a/lib/SILOptimizer/SILCombiner/SILCombinerMiscVisitors.cpp b/lib/SILOptimizer/SILCombiner/SILCombinerMiscVisitors.cpp
index 6329a6b5fea..a20c9e891c0 100644
--- a/lib/SILOptimizer/SILCombiner/SILCombinerMiscVisitors.cpp
+++ b/lib/SILOptimizer/SILCombiner/SILCombinerMiscVisitors.cpp
@@ -1304,30 +1304,3 @@ SILInstruction *SILCombiner::visitEnumInst(EnumInst *EI) {
   return nullptr;
 }
 
-SILInstruction *SILCombiner::visitWitnessMethodInst(WitnessMethodInst *WMI) {
-  // Try to devirtualize a witness_method if it is statically possible.
-  // Many cases are handled by the inliner/devirtualizer, but certain
-  // special cases are not covered there, e.g. partial_apply(witness_method)
-  SILFunction *F;
-  SILWitnessTable *WT;
-
-  std::tie(F, WT) =
-      WMI->getModule().lookUpFunctionInWitnessTable(WMI->getConformance(),
-                                                    WMI->getMember());
-
-  if (!F)
-    return nullptr;
-
-  for (auto U = WMI->use_begin(), E = WMI->use_end(); U != E;) {
-    auto User = U->getUser();
-    ++U;
-    if (auto AI = ApplySite::isa(User)) {
-      auto Result = tryDevirtualizeWitnessMethod(AI);
-      if (Result.first) {
-        User->replaceAllUsesWith(Result.first);
-        eraseInstFromFunction(*User);
-      }
-    }
-  }
-  return nullptr;
-}
diff --git a/lib/SILOptimizer/Transforms/Devirtualizer.cpp b/lib/SILOptimizer/Transforms/Devirtualizer.cpp
index a652f5c8317..8dd42ba37a0 100644
--- a/lib/SILOptimizer/Transforms/Devirtualizer.cpp
+++ b/lib/SILOptimizer/Transforms/Devirtualizer.cpp
@@ -55,14 +55,14 @@ bool Devirtualizer::devirtualizeAppliesInFunction(SILFunction &F,
   llvm::SmallVector<SILInstruction *, 8> DeadApplies;
   llvm::SmallVector<ApplySite, 8> NewApplies;
 
-  SmallVector<FullApplySite, 16> Applies;
+  SmallVector<ApplySite, 16> Applies;
   for (auto &BB : F) {
     for (auto It = BB.begin(), End = BB.end(); It != End;) {
       auto &I = *It++;
 
       // Skip non-apply instructions.
 
-      auto Apply = FullApplySite::isa(&I);
+      auto Apply = ApplySite::isa(&I);
       if (!Apply)
         continue;
       Applies.push_back(Apply);
@@ -102,11 +102,12 @@ bool Devirtualizer::devirtualizeAppliesInFunction(SILFunction &F,
     auto *CalleeFn = Apply.getReferencedFunction();
     assert(CalleeFn && "Expected devirtualized callee!");
 
-    // FIXME: Until we link everything in up front we need to ensure
-    // that we link after devirtualizing in order to pull in
-    // everything we reference from the stdlib. After we do that we
-    // can move the notification code below back into the main loop
-    // above.
+    // We need to ensure that we link after devirtualizing in order to pull in
+    // everything we reference from another module. This is especially important
+    // for transparent functions, because if transparent functions are not
+    // inlined for some reason, we need to generate code for them.
+    // Note that functions, which are only referenced from witness/vtables, are
+    // not linked upfront by the SILLinker.
     if (!CalleeFn->isDefinition())
       F.getModule().linkFunction(CalleeFn, SILModule::LinkingMode::LinkAll);
 
diff --git a/lib/SILOptimizer/Utils/Devirtualize.cpp b/lib/SILOptimizer/Utils/Devirtualize.cpp
index e8fb220ee3b..4fa10940045 100644
--- a/lib/SILOptimizer/Utils/Devirtualize.cpp
+++ b/lib/SILOptimizer/Utils/Devirtualize.cpp
@@ -1080,7 +1080,7 @@ DevirtualizationResult swift::tryDevirtualizeWitnessMethod(ApplySite AI) {
 /// Attempt to devirtualize the given apply if possible, and return a
 /// new instruction in that case, or nullptr otherwise.
 DevirtualizationResult
-swift::tryDevirtualizeApply(FullApplySite AI, ClassHierarchyAnalysis *CHA) {
+swift::tryDevirtualizeApply(ApplySite AI, ClassHierarchyAnalysis *CHA) {
   DEBUG(llvm::dbgs() << "    Trying to devirtualize: " << *AI.getInstruction());
 
   // Devirtualize apply instructions that call witness_method instructions:
@@ -1091,6 +1091,11 @@ swift::tryDevirtualizeApply(FullApplySite AI, ClassHierarchyAnalysis *CHA) {
   if (isa<WitnessMethodInst>(AI.getCallee()))
     return tryDevirtualizeWitnessMethod(AI);
 
+  // TODO: check if we can also de-virtualize partial applies of class methods.
+  FullApplySite FAS = FullApplySite::isa(AI.getInstruction());
+  if (!FAS)
+    return std::make_pair(nullptr, ApplySite());
+
   /// Optimize a class_method and alloc_ref pair into a direct function
   /// reference:
   ///
@@ -1107,8 +1112,8 @@ swift::tryDevirtualizeApply(FullApplySite AI, ClassHierarchyAnalysis *CHA) {
   ///  into
   ///
   /// %YY = function_ref @...
-  if (auto *CMI = dyn_cast<ClassMethodInst>(AI.getCallee())) {
-    auto &M = AI.getModule();
+  if (auto *CMI = dyn_cast<ClassMethodInst>(FAS.getCallee())) {
+    auto &M = FAS.getModule();
     auto Instance = stripUpCasts(CMI->getOperand());
     auto ClassType = Instance->getType();
     if (ClassType.is<MetatypeType>())
@@ -1116,34 +1121,34 @@ swift::tryDevirtualizeApply(FullApplySite AI, ClassHierarchyAnalysis *CHA) {
 
     auto *CD = ClassType.getClassOrBoundGenericClass();
 
-    if (isEffectivelyFinalMethod(AI, ClassType, CD, CHA))
-      return tryDevirtualizeClassMethod(AI, Instance);
+    if (isEffectivelyFinalMethod(FAS, ClassType, CD, CHA))
+      return tryDevirtualizeClassMethod(FAS, Instance);
 
     // Try to check if the exact dynamic type of the instance is statically
     // known.
     if (auto Instance = getInstanceWithExactDynamicType(CMI->getOperand(),
                                                         CMI->getModule(),
                                                         CHA))
-      return tryDevirtualizeClassMethod(AI, Instance);
+      return tryDevirtualizeClassMethod(FAS, Instance);
 
     if (auto ExactTy = getExactDynamicType(CMI->getOperand(), CMI->getModule(),
                                            CHA)) {
       if (ExactTy == CMI->getOperand()->getType())
-        return tryDevirtualizeClassMethod(AI, CMI->getOperand());
+        return tryDevirtualizeClassMethod(FAS, CMI->getOperand());
     }
   }
 
-  if (isa<SuperMethodInst>(AI.getCallee())) {
-    if (AI.hasSelfArgument()) {
-      return tryDevirtualizeClassMethod(AI, AI.getSelfArgument());
+  if (isa<SuperMethodInst>(FAS.getCallee())) {
+    if (FAS.hasSelfArgument()) {
+      return tryDevirtualizeClassMethod(FAS, FAS.getSelfArgument());
     }
 
     // It is an invocation of a class method.
     // Last operand is the metatype that should be used for dispatching.
-    return tryDevirtualizeClassMethod(AI, AI.getArguments().back());
+    return tryDevirtualizeClassMethod(FAS, FAS.getArguments().back());
   }
 
-  return std::make_pair(nullptr, FullApplySite());
+  return std::make_pair(nullptr, ApplySite());
 }
 
 bool swift::canDevirtualizeApply(FullApplySite AI, ClassHierarchyAnalysis *CHA) {
diff --git a/test/SILOptimizer/devirt_default_witness_method.sil b/test/SILOptimizer/devirt_default_witness_method.sil
index b5fc06cf1f4..d4ce7669bbe 100644
--- a/test/SILOptimizer/devirt_default_witness_method.sil
+++ b/test/SILOptimizer/devirt_default_witness_method.sil
@@ -1,4 +1,4 @@
-// RUN: %target-sil-opt -assume-parsing-unqualified-ownership-sil -enable-sil-verify-all %s -sil-combine -enable-resilience | %FileCheck %s
+// RUN: %target-sil-opt -assume-parsing-unqualified-ownership-sil -enable-sil-verify-all %s -devirtualizer -sil-combine -enable-resilience | %FileCheck %s
 sil_stage canonical
 
 import Builtin
diff --git a/test/SILOptimizer/devirt_static_witness_method.sil b/test/SILOptimizer/devirt_static_witness_method.sil
index 45b16d1f08b..47e9aa10d6f 100644
--- a/test/SILOptimizer/devirt_static_witness_method.sil
+++ b/test/SILOptimizer/devirt_static_witness_method.sil
@@ -1,4 +1,4 @@
-// RUN: %target-sil-opt -assume-parsing-unqualified-ownership-sil -enable-sil-verify-all %s -sil-combine | %FileCheck %s
+// RUN: %target-sil-opt -assume-parsing-unqualified-ownership-sil -enable-sil-verify-all %s -devirtualizer -sil-combine | %FileCheck %s
 sil_stage canonical
 
 import Builtin
diff --git a/test/SILOptimizer/existential_type_propagation.sil b/test/SILOptimizer/existential_type_propagation.sil
index 5554b044cd0..814ae81aae9 100644
--- a/test/SILOptimizer/existential_type_propagation.sil
+++ b/test/SILOptimizer/existential_type_propagation.sil
@@ -1,4 +1,4 @@
-// RUN: %target-sil-opt -assume-parsing-unqualified-ownership-sil -enable-sil-verify-all %s -sil-combine -inline -sil-combine | %FileCheck %s
+// RUN: %target-sil-opt -assume-parsing-unqualified-ownership-sil -enable-sil-verify-all %s -sil-combine -devirtualizer -inline -sil-combine | %FileCheck %s
 
 // Check that type propagation is performed correctly for existentials.
 // The concrete type set in init_existential instructions should be propagated
@@ -235,3 +235,70 @@ sil_witness_table public_external ArrayClassReaderWriter: ReaderWriterType modul
   method #ReaderWriterType.read!1: @_TTWC28existential_type_propagation22ArrayClassReaderWriterS_16ReaderWriterTypeS_FS1_4readuRq_S1__fq_FSiSi
   method #ReaderWriterType.write!1: @_TTWC28existential_type_propagation22ArrayClassReaderWriterS_16ReaderWriterTypeS_FS1_5writeuRq_S1__fRq_FTSi5valueSi_T_
 }
+
+protocol PPP : class {
+  func foo()
+}
+
+class BBB: PPP {
+  @inline(never)
+  func foo()
+}
+
+final class XXX : BBB {
+  @inline(never)
+  override func foo()
+}
+
+// Check that sil-combine does not crash on this example and does not generate a wrong
+// upcast.
+// CHECK-LABEL: sil @silcombine_dont_generate_wrong_upcasts_during_devirt
+// CHECK-NOT: upcast
+// CHECK: witness_method $XXX, #PPP.foo!1 : {{.*}} : $@convention(witness_method) <τ_0_0 where τ_0_0 : PPP> (@guaranteed τ_0_0) -> ()
+// CHECK-NOT: upcast
+// CHECK: return
+sil @silcombine_dont_generate_wrong_upcasts_during_devirt: $@convention(thin) (@owned BBB) -> () {
+bb0(%0 : $BBB):
+  strong_retain %0 : $BBB
+  %3 = init_existential_ref %0 : $BBB : $BBB, $PPP
+  %5 = open_existential_ref %3 : $PPP to $@opened("0AC9A62E-926E-11E6-8BF5-685B35C48C83") PPP
+  %6 = witness_method $XXX, #PPP.foo!1, %5 : $@opened("0AC9A62E-926E-11E6-8BF5-685B35C48C83") PPP : $@convention(witness_method) <τ_0_0 where τ_0_0 : PPP> (@guaranteed τ_0_0) -> ()
+  %7 = apply %6<@opened("0AC9A62E-926E-11E6-8BF5-685B35C48C83") PPP>(%5) : $@convention(witness_method) <τ_0_0 where τ_0_0 : PPP> (@guaranteed τ_0_0) -> ()
+  %8 = apply %6<@opened("0AC9A62E-926E-11E6-8BF5-685B35C48C83") PPP>(%5) : $@convention(witness_method) <τ_0_0 where τ_0_0 : PPP> (@guaranteed τ_0_0) -> ()
+  strong_release %3 : $PPP
+  %9 = tuple ()
+  strong_release %0 : $BBB
+  %11 = tuple ()
+  return %11 : $()
+}
+
+// Check that both applies can be devirtualized by means of propagating the concrete
+// type of the existential into witness_method and apply instructions.
+// CHECK-LABEL: sil @silcombine_devirt_both_applies_of_witness_method
+// CHECK-NOT: open_existential_ref
+// CHECK-NOT: witness_method
+// CHECK: [[FR1:%.*]] = function_ref @_TTWC4nix23XXXS_3PPPS_FS1_3foofT_T_
+// CHECK: apply [[FR1]](%0)
+// CHECK: [[FR2:%.*]] = function_ref @_TTWC4nix23XXXS_3PPPS_FS1_3foofT_T_
+// CHECK: apply [[FR2]](%0)
+// CHECK: return
+sil @silcombine_devirt_both_applies_of_witness_method : $@convention(thin) (@owned XXX) -> () {
+bb0(%0 : $XXX):
+  strong_retain %0 : $XXX
+  %3 = init_existential_ref %0 : $XXX : $XXX, $PPP
+  %5 = open_existential_ref %3 : $PPP to $@opened("0AC9A62E-926E-11E6-8BF5-685B35C48C83") PPP
+  %6 = witness_method $@opened("0AC9A62E-926E-11E6-8BF5-685B35C48C83") PPP, #PPP.foo!1, %5 : $@opened("0AC9A62E-926E-11E6-8BF5-685B35C48C83") PPP : $@convention(witness_method) <τ_0_0 where τ_0_0 : PPP> (@guaranteed τ_0_0) -> ()
+  %7 = apply %6<@opened("0AC9A62E-926E-11E6-8BF5-685B35C48C83") PPP>(%5) : $@convention(witness_method) <τ_0_0 where τ_0_0 : PPP> (@guaranteed τ_0_0) -> ()
+  %8 = apply %6<@opened("0AC9A62E-926E-11E6-8BF5-685B35C48C83") PPP>(%5) : $@convention(witness_method) <τ_0_0 where τ_0_0 : PPP> (@guaranteed τ_0_0) -> ()
+  strong_release %3 : $PPP
+  %9 = tuple ()
+  strong_release %0 : $XXX
+  %11 = tuple ()
+  return %11 : $()
+}
+
+sil @_TTWC4nix23XXXS_3PPPS_FS1_3foofT_T_ : $@convention(witness_method) (@guaranteed XXX) -> ()
+
+sil_witness_table XXX: PPP module nix2 {
+  method #PPP.foo!1: @_TTWC4nix23XXXS_3PPPS_FS1_3foofT_T_	// protocol witness for PPP.foo() -> () in conformance XXX
+}
diff --git a/test/SILOptimizer/sil_combine.sil b/test/SILOptimizer/sil_combine.sil
index 2113a743401..511a6b2e7da 100644
--- a/test/SILOptimizer/sil_combine.sil
+++ b/test/SILOptimizer/sil_combine.sil
@@ -3137,69 +3137,3 @@ bb0(%0 : $VV):
   return %26 : $()
 }
 
-protocol PPP : class {
-  func foo()
-}
-
-class BBB: PPP {
-  @inline(never)
-  func foo()
-}
-
-final class XXX : BBB {
-  @inline(never)
-  override func foo()
-}
-
-// Check that sil-combine does not crash on this example and does not generate a wrong
-// upcast.
-// CHECK-LABEL: sil @silcombine_dont_generate_wrong_upcasts_during_devirt
-// CHECK-NOT: upcast
-// CHECK: witness_method $XXX, #PPP.foo!1 : {{.*}} : $@convention(witness_method) <τ_0_0 where τ_0_0 : PPP> (@guaranteed τ_0_0) -> ()
-// CHECK-NOT: upcast
-// CHECK: return
-sil @silcombine_dont_generate_wrong_upcasts_during_devirt: $@convention(thin) (@owned BBB) -> () {
-bb0(%0 : $BBB):
-  strong_retain %0 : $BBB
-  %3 = init_existential_ref %0 : $BBB : $BBB, $PPP
-  %5 = open_existential_ref %3 : $PPP to $@opened("0AC9A62E-926E-11E6-8BF5-685B35C48C83") PPP
-  %6 = witness_method $XXX, #PPP.foo!1, %5 : $@opened("0AC9A62E-926E-11E6-8BF5-685B35C48C83") PPP : $@convention(witness_method) <τ_0_0 where τ_0_0 : PPP> (@guaranteed τ_0_0) -> ()
-  %7 = apply %6<@opened("0AC9A62E-926E-11E6-8BF5-685B35C48C83") PPP>(%5) : $@convention(witness_method) <τ_0_0 where τ_0_0 : PPP> (@guaranteed τ_0_0) -> ()
-  %8 = apply %6<@opened("0AC9A62E-926E-11E6-8BF5-685B35C48C83") PPP>(%5) : $@convention(witness_method) <τ_0_0 where τ_0_0 : PPP> (@guaranteed τ_0_0) -> ()
-  strong_release %3 : $PPP
-  %9 = tuple ()
-  strong_release %0 : $BBB
-  %11 = tuple ()
-  return %11 : $()
-}
-
-// Check that both applies can be devirtualized by means of propagating the concrete
-// type of the existential into witness_method and apply instructions.
-// CHECK-LABEL: sil @silcombine_devirt_both_applies_of_witness_method
-// CHECK-NOT: open_existential_ref
-// CHECK-NOT: witness_method
-// CHECK: [[FR1:%.*]] = function_ref @_TTWC4nix23XXXS_3PPPS_FS1_3foofT_T_
-// CHECK: apply [[FR1]](%0)
-// CHECK: [[FR2:%.*]] = function_ref @_TTWC4nix23XXXS_3PPPS_FS1_3foofT_T_
-// CHECK: apply [[FR2]](%0)
-// CHECK: return
-sil @silcombine_devirt_both_applies_of_witness_method : $@convention(thin) (@owned XXX) -> () {
-bb0(%0 : $XXX):
-  strong_retain %0 : $XXX
-  %3 = init_existential_ref %0 : $XXX : $XXX, $PPP
-  %5 = open_existential_ref %3 : $PPP to $@opened("0AC9A62E-926E-11E6-8BF5-685B35C48C83") PPP
-  %6 = witness_method $@opened("0AC9A62E-926E-11E6-8BF5-685B35C48C83") PPP, #PPP.foo!1, %5 : $@opened("0AC9A62E-926E-11E6-8BF5-685B35C48C83") PPP : $@convention(witness_method) <τ_0_0 where τ_0_0 : PPP> (@guaranteed τ_0_0) -> ()
-  %7 = apply %6<@opened("0AC9A62E-926E-11E6-8BF5-685B35C48C83") PPP>(%5) : $@convention(witness_method) <τ_0_0 where τ_0_0 : PPP> (@guaranteed τ_0_0) -> ()
-  %8 = apply %6<@opened("0AC9A62E-926E-11E6-8BF5-685B35C48C83") PPP>(%5) : $@convention(witness_method) <τ_0_0 where τ_0_0 : PPP> (@guaranteed τ_0_0) -> ()
-  strong_release %3 : $PPP
-  %9 = tuple ()
-  strong_release %0 : $XXX
-  %11 = tuple ()
-  return %11 : $()
-}
-
-sil @_TTWC4nix23XXXS_3PPPS_FS1_3foofT_T_ : $@convention(witness_method) (@guaranteed XXX) -> ()
-
-sil_witness_table XXX: PPP module nix2 {
-  method #PPP.foo!1: @_TTWC4nix23XXXS_3PPPS_FS1_3foofT_T_	// protocol witness for PPP.foo() -> () in conformance XXX
-}