diff --git a/include/swift/SIL/SILFunction.h b/include/swift/SIL/SILFunction.h
index 2d68d46dbef..2f35b591288 100644
--- a/include/swift/SIL/SILFunction.h
+++ b/include/swift/SIL/SILFunction.h
@@ -40,7 +40,12 @@ class SILProfiler;
 enum IsBare_t { IsNotBare, IsBare };
 enum IsTransparent_t { IsNotTransparent, IsTransparent };
 enum Inline_t { InlineDefault, NoInline, AlwaysInline };
-enum IsThunk_t { IsNotThunk, IsThunk, IsReabstractionThunk };
+enum IsThunk_t {
+  IsNotThunk,
+  IsThunk,
+  IsReabstractionThunk,
+  IsSignatureOptimizedThunk
+};
 
 class SILSpecializeAttr final {
   friend SILFunction;
diff --git a/lib/SIL/SILPrinter.cpp b/lib/SIL/SILPrinter.cpp
index 949fc90ce1c..8714d654cea 100644
--- a/lib/SIL/SILPrinter.cpp
+++ b/lib/SIL/SILPrinter.cpp
@@ -2326,6 +2326,9 @@ void SILFunction::print(SILPrintContext &PrintCtx) const {
   switch (isThunk()) {
   case IsNotThunk: break;
   case IsThunk: OS << "[thunk] "; break;
+  case IsSignatureOptimizedThunk:
+    OS << "[signature_optimized_thunk] ";
+    break;
   case IsReabstractionThunk: OS << "[reabstraction_thunk] "; break;
   }
 
diff --git a/lib/SILOptimizer/FunctionSignatureTransforms/FunctionSignatureOpts.cpp b/lib/SILOptimizer/FunctionSignatureTransforms/FunctionSignatureOpts.cpp
index afe56a138c1..ed56089d029 100644
--- a/lib/SILOptimizer/FunctionSignatureTransforms/FunctionSignatureOpts.cpp
+++ b/lib/SILOptimizer/FunctionSignatureTransforms/FunctionSignatureOpts.cpp
@@ -529,7 +529,7 @@ void FunctionSignatureTransform::createFunctionSignatureOptimizedFunction() {
   }
 
   // Create the thunk body !
-  F->setThunk(IsThunk);
+  F->setThunk(IsSignatureOptimizedThunk);
   // The thunk now carries the information on how the signature is
   // optimized. If we inline the thunk, we will get the benefit of calling
   // the signature optimized function without additional setup on the
diff --git a/lib/Serialization/SILFormat.h b/lib/Serialization/SILFormat.h
index 7e7274a42ec..7f5f9cee42d 100644
--- a/lib/Serialization/SILFormat.h
+++ b/lib/Serialization/SILFormat.h
@@ -286,7 +286,7 @@ namespace sil_block {
       BCRecordLayout<SIL_FUNCTION, SILLinkageField,
                      BCFixed<1>,  // transparent
                      BCFixed<2>,  // serialized
-                     BCFixed<2>,  // thunk/reabstraction_thunk
+                     BCFixed<2>,  // thunks: signature optimized/reabstraction
                      BCFixed<1>,  // global_init
                      BCFixed<2>,  // inlineStrategy
                      BCFixed<2>,  // optimizationMode
diff --git a/test/SILOptimizer/dead_partial_apply_arg.swift b/test/SILOptimizer/dead_partial_apply_arg.swift
index e2ffc756361..8959ddd027e 100644
--- a/test/SILOptimizer/dead_partial_apply_arg.swift
+++ b/test/SILOptimizer/dead_partial_apply_arg.swift
@@ -8,7 +8,7 @@ extension Int32 {
 
 // This function has an unused metatype argument.
 
-// CHECK-LABEL: sil [thunk] [always_inline] @$Ss5Int32V4testE8lessthan3lhs3rhsSbAB_ABtFZ : $@convention(method) (Int32, Int32, @thin Int32.Type) -> Bool
+// CHECK-LABEL: sil [signature_optimized_thunk] [always_inline] @$Ss5Int32V4testE8lessthan3lhs3rhsSbAB_ABtFZ : $@convention(method) (Int32, Int32, @thin Int32.Type) -> Bool
   public static func lessthan (lhs: Int32, rhs: Int32) -> Bool {
     return lhs < rhs
   }
diff --git a/test/SILOptimizer/deadargsignatureopt.sil b/test/SILOptimizer/deadargsignatureopt.sil
index c0035ea60bc..d951917ecbb 100644
--- a/test/SILOptimizer/deadargsignatureopt.sil
+++ b/test/SILOptimizer/deadargsignatureopt.sil
@@ -5,7 +5,7 @@ import Swift
 
 // Specialize to a function with removed dead argument.
 
-// CHECK-LABEL: sil [thunk] [always_inline] @one_arg_dead
+// CHECK-LABEL: sil [signature_optimized_thunk] [always_inline] @one_arg_dead
 // CHECK: [[F:%[0-9]+]] = function_ref @$S12one_arg_deadTf4nnd_n
 // CHECK: [[A:%[0-9]+]] = apply [[F]](%0, %1)
 // CHECK: return [[A]]
@@ -18,7 +18,7 @@ bb0(%0 : $Int32, %1 : $Int32, %2 : $@thin Int32.Type):
   return %8 : $Bool
 }
 
-// CHECK-LABEL: sil [thunk] [always_inline] @always_inline_one_arg_dead
+// CHECK-LABEL: sil [signature_optimized_thunk] [always_inline] @always_inline_one_arg_dead
 // CHECK: [[F:%[0-9]+]] = function_ref @$S26always_inline_one_arg_deadTf4nnd_n
 // CHECK: [[A:%[0-9]+]] = apply [[F]](%0, %1)
 // CHECK: return [[A]]
@@ -33,7 +33,7 @@ bb0(%0 : $Int32, %1 : $Int32, %2 : $@thin Int32.Type):
 
 // Still delete only one dead arg as only one arg is partially applied.
 
-// CHECK-LABEL: sil [thunk] [always_inline] @two_args_dead
+// CHECK-LABEL: sil [signature_optimized_thunk] [always_inline] @two_args_dead
 // CHECK: [[F:%[0-9]+]] = function_ref @$S13two_args_deadTf4nnd_n
 // CHECK: [[A:%[0-9]+]] = apply [[F]](%0, %1)
 // CHECK: return [[A]]
diff --git a/test/SILOptimizer/funcsig_opaque.sil b/test/SILOptimizer/funcsig_opaque.sil
index 9446ac72557..d93837f813b 100644
--- a/test/SILOptimizer/funcsig_opaque.sil
+++ b/test/SILOptimizer/funcsig_opaque.sil
@@ -14,7 +14,7 @@ sil @testAggregateArgHelper : $@convention(thin) (@owned R<S>) -> ()
 
 // Test owned-to-guaranteed transformation of opaque arguments.
 //
-// CHECK-LABEL: sil [thunk] [always_inline] @testAggregateArg : $@convention(thin) (@in R<S>) -> @out () {
+// CHECK-LABEL: sil [signature_optimized_thunk] [always_inline] @testAggregateArg : $@convention(thin) (@in R<S>) -> @out () {
 // CHECK: bb0(%0 : $R<S>):
 // CHECK:   [[F:%.*]] = function_ref @$S16testAggregateArgTf4g_n : $@convention(thin) (@in_guaranteed R<S>) -> @out ()
 // CHECK:   [[CALL:%.*]] = apply [[F]](%0) : $@convention(thin) (@in_guaranteed R<S>) -> @out ()
diff --git a/test/SILOptimizer/functionsigopts.sil b/test/SILOptimizer/functionsigopts.sil
index b2bbc0e32df..4ef3bcb9a56 100644
--- a/test/SILOptimizer/functionsigopts.sil
+++ b/test/SILOptimizer/functionsigopts.sil
@@ -79,7 +79,7 @@ bb0:
   return %4 : $()
 }
 
-// CHECK-LABEL: sil [thunk] [always_inline] @argument_with_incomplete_epilogue_release
+// CHECK-LABEL: sil [signature_optimized_thunk] [always_inline] @argument_with_incomplete_epilogue_release
 // CHECK: [[IN2:%.*]] = struct_extract [[IN1:%.*]] : $goo, #goo.top
 // CHECK: function_ref @$S41argument_with_incomplete_epilogue_releaseTf4x_nTf4gn_n : $@convention(thin) (@guaranteed foo, @owned foo) -> ()
 // CHECK: release_value [[IN2]]
@@ -139,7 +139,7 @@ sil [serialized] @create_object : $@convention(thin) () -> Builtin.NativeObject
 // Make sure argument is exploded and the baz part is not passed in as argument, as its only use
 // is a release.
 //
-// CHECK-LABEL: sil [thunk] [always_inline] @dead_argument_due_to_only_release_user
+// CHECK-LABEL: sil [signature_optimized_thunk] [always_inline] @dead_argument_due_to_only_release_user
 // CHECK: [[IN:%.*]] = function_ref @$S38dead_argument_due_to_only_release_userTf4gX_n
 // CHECK: [[IN2:%.*]] = struct_extract [[IN1:%.*]] : $boo, #boo.a
 // CHECK: apply [[IN]]([[IN2]])
@@ -178,7 +178,7 @@ bb0(%0 : $boo):
 
 // Make sure argument is exploded and the baz part is not passed in as argument, as its only use
 // is a release.
-// CHECK-LABEL: sil [thunk] [always_inline] @dead_argument_due_to_only_release_user_but__exploded
+// CHECK-LABEL: sil [signature_optimized_thunk] [always_inline] @dead_argument_due_to_only_release_user_but__exploded
 // CHECK: [[FN1:%.*]] = function_ref @$S52dead_argument_due_to_only_release_user_but__explodedTf4gX_n
 // CHECK: [[IN1:%.*]] = struct_extract %0 : $lotsoffield, #lotsoffield.c
 // CHECK: [[IN2:%.*]] = struct_extract %0 : $lotsoffield, #lotsoffield.b
@@ -221,7 +221,7 @@ bb0(%0 : $lotsoffield):
 
 // Make sure argument is exploded and the baz part is not passed in as argument, as its only use
 // is a release.
-// CHECK-LABEL: sil [thunk] [always_inline] @dead_argument_due_to_more_than_release_user
+// CHECK-LABEL: sil [signature_optimized_thunk] [always_inline] @dead_argument_due_to_more_than_release_user
 // CHECK: [[FN1:%.*]] = function_ref @$S43dead_argument_due_to_more_than_release_userTf4gX_n : $@convention(thin) (@guaranteed baz, Int) -> (Int, Int)
 sil @dead_argument_due_to_more_than_release_user : $@convention(thin) (@owned boo) -> (Int, Int) {
 bb0(%0 : $boo):
@@ -303,7 +303,7 @@ bb3:
 
 // Make sure %0 is a dead argument.
 //
-// CHECK-LABEL: sil [thunk] [always_inline] @exploded_release_to_dead_argument
+// CHECK-LABEL: sil [signature_optimized_thunk] [always_inline] @exploded_release_to_dead_argument
 // CHECK: bb0([[INPUT_ARG0:%[0-9]+]] : $boo):
 // CHECK: [[IN1:%.*]] = function_ref @$S33exploded_release_to_dead_argumentTf4d_n
 // CHECK: apply [[IN1]]()
@@ -348,7 +348,7 @@ bb0(%0 : $boo):
 
 // Make sure %0 is not a dead argument, but gets converted to a guaranteed arg.
 //
-// CHECK-LABEL: sil [thunk] [always_inline] @exploded_release_to_guaranteed_param
+// CHECK-LABEL: sil [signature_optimized_thunk] [always_inline] @exploded_release_to_guaranteed_param
 // CHECK: bb0([[INPUT_ARG0:%[0-9]+]] : $boo):
 // CHECK: [[IN1:%.*]] = function_ref @$S36exploded_release_to_guaranteed_paramTf4gX_n
 // CHECK: release_value [[INPUT_ARG0]]
@@ -390,7 +390,7 @@ bb0(%0 : $boo):
   return %6 : $()
 }
 
-// CHECK-LABEL: sil [thunk] [always_inline] @single_owned_return_value
+// CHECK-LABEL: sil [signature_optimized_thunk] [always_inline] @single_owned_return_value
 // CHECK: bb0([[INPUT_ARG0:%[0-9]+]] : $boo):
 // CHECK: [[IN1:%.*]] = function_ref @$S25single_owned_return_valueTf4n_g
 // CHECK: [[IN2:%.*]] = apply [[IN1]]([[INPUT_ARG0]]
@@ -426,7 +426,7 @@ bb0(%0 : $boo):
 }
 
 
-// CHECK-LABEL: sil [thunk] [always_inline] @single_owned_return_value_with_self_recursion
+// CHECK-LABEL: sil [signature_optimized_thunk] [always_inline] @single_owned_return_value_with_self_recursion
 // CHECK: function_ref @$S45single_owned_return_value_with_self_recursionTf4n_g
 // CHECK: [[RET:%.*]] = apply
 // CHECK: retain_value [[RET]]
@@ -468,7 +468,7 @@ bb3(%4 : $boo):
   return %4 : $boo
 }
 
-// CHECK-LABEL: sil [thunk] [always_inline] @single_owned_return_value_with_interfering_release
+// CHECK-LABEL: sil [signature_optimized_thunk] [always_inline] @single_owned_return_value_with_interfering_release
 // CHECK: bb0([[INPUT_ARG0:%[0-9]+]] : $boo):
 // CHECK: [[IN1:%.*]] = function_ref @$S50single_owned_return_value_with_interfering_releaseTf4x_nTf4gnn_n
 // CHECK-NOT: retain_value
@@ -508,7 +508,7 @@ bb0(%0 : $boo):
 
 // Make sure we do not move the retain_value in the throw block.
 //
-// CHECK-LABEL: sil [serialized] [thunk] [always_inline] @owned_to_unowned_retval_with_error_result : $@convention(thin) (@owned boo) -> (@owned boo, @error Error) {
+// CHECK-LABEL: sil [serialized] [signature_optimized_thunk] [always_inline] @owned_to_unowned_retval_with_error_result : $@convention(thin) (@owned boo) -> (@owned boo, @error Error) {
 // CHECK: function_ref @$S41owned_to_unowned_retval_with_error_resultTfq4n_g : $@convention(thin) (@owned boo) -> (boo, @error Error)
 // CHECK: bb1
 // CHECK-NOT: retain_value
@@ -550,7 +550,7 @@ bb2:
   throw undef : $Error
 }
 
-// CHECK-LABEL: sil [serialized] [thunk] [always_inline] @dead_arg_with_callsites : $@convention(thin) (Builtin.NativeObject, Builtin.NativeObject) -> () {
+// CHECK-LABEL: sil [serialized] [signature_optimized_thunk] [always_inline] @dead_arg_with_callsites : $@convention(thin) (Builtin.NativeObject, Builtin.NativeObject) -> () {
 // CHECK: bb0
 // CHECK-NEXT: function_ref
 // CHECK-NEXT: function_ref @$S23dead_arg_with_callsitesTfq4dn_n : $@convention(thin) (Builtin.NativeObject) -> ()
@@ -818,7 +818,7 @@ bb0(%0 : $goo):
 //========================================
 // @owned => @guaranteed with error result
 
-// CHECK-LABEL: sil [serialized] [thunk] [always_inline] @owned_to_guaranteed_with_error_result : $@convention(thin) (@owned Builtin.NativeObject, Int) -> (Int, @error Error) {
+// CHECK-LABEL: sil [serialized] [signature_optimized_thunk] [always_inline] @owned_to_guaranteed_with_error_result : $@convention(thin) (@owned Builtin.NativeObject, Int) -> (Int, @error Error) {
 // CHECK: [[FUNC_REF:%[0-9]+]] = function_ref @$S37owned_to_guaranteed_with_error_resultTfq4gn_n : $@convention(thin) (@guaranteed Builtin.NativeObject, Int)
 // CHECK: try_apply [[FUNC_REF]]({{.*}}, normal bb1, error bb2
 // CHECK: bb1([[NORMRET:%[0-9]+]] : $Int):
@@ -903,7 +903,7 @@ bb0(%0 : $Builtin.NativeObject, %1 : $Int):
 // @owned => @guaranteed
 
 // We should optimize this call.
-// CHECK-LABEL: sil [serialized] [thunk] [always_inline] @owned_to_guaranteed_simple_singlebb_callee : $@convention(thin) (@owned Builtin.NativeObject) -> () {
+// CHECK-LABEL: sil [serialized] [signature_optimized_thunk] [always_inline] @owned_to_guaranteed_simple_singlebb_callee : $@convention(thin) (@owned Builtin.NativeObject) -> () {
 // CHECK: [[FUNC_REF:%[0-9]+]] = function_ref @$S42owned_to_guaranteed_simple_singlebb_calleeTfq4g_n : $@convention(thin) (@guaranteed Builtin.NativeObject) -> ()
 // CHECK: apply [[FUNC_REF]]
 // CHECK-NOT: fix_lifetime
@@ -1052,7 +1052,7 @@ bb0(%0 : $Builtin.NativeObject):
 // We should be able to optimize this callee since the release is in the exit
 // BB. Additionally this test makes sure that we delete dead parameters before
 // invalidating the owned argument -> release map.
-// CHECK-LABEL: sil [serialized] [thunk] [always_inline] @owned_to_guaranteed_multibb_callee_with_release_in_exit : $@convention(thin) (Builtin.Int1, @owned Builtin.NativeObject) -> () {
+// CHECK-LABEL: sil [serialized] [signature_optimized_thunk] [always_inline] @owned_to_guaranteed_multibb_callee_with_release_in_exit : $@convention(thin) (Builtin.Int1, @owned Builtin.NativeObject) -> () {
 // CHECK-NOT: fix_lifetime
 // CHECK: release_value %1 : $Builtin.NativeObject
 sil [serialized] @owned_to_guaranteed_multibb_callee_with_release_in_exit : $@convention(thin) (Builtin.Int1, @owned Builtin.NativeObject) -> () {
@@ -1135,7 +1135,7 @@ bb2:
   return %1 : $()
 }
 
-// CHECK-LABEL: sil [serialized] [thunk] [always_inline] @owned_to_guaranteed_simple_singlebb_multiple_arg_callee : $@convention(thin) (Builtin.Int1, @owned Builtin.NativeObject, Builtin.Int1) -> () {
+// CHECK-LABEL: sil [serialized] [signature_optimized_thunk] [always_inline] @owned_to_guaranteed_simple_singlebb_multiple_arg_callee : $@convention(thin) (Builtin.Int1, @owned Builtin.NativeObject, Builtin.Int1) -> () {
 sil [serialized] @owned_to_guaranteed_simple_singlebb_multiple_arg_callee : $@convention(thin) (Builtin.Int1, @owned Builtin.NativeObject, Builtin.Int1) -> () {
 bb0(%0 : $Builtin.Int1, %1 : $Builtin.NativeObject, %2 : $Builtin.Int1):
   // make it a non-trivial function
@@ -1186,7 +1186,7 @@ bb3:
   return %3 : $()
 }
 
-// CHECK-LABEL: sil [serialized] [thunk] [always_inline] @owned_to_guaranteed_multibb_callee_with_release_in_exit_two_args_1 : $@convention(thin) (@owned Builtin.NativeObject, @owned Builtin.NativeObject) -> () {
+// CHECK-LABEL: sil [serialized] [signature_optimized_thunk] [always_inline] @owned_to_guaranteed_multibb_callee_with_release_in_exit_two_args_1 : $@convention(thin) (@owned Builtin.NativeObject, @owned Builtin.NativeObject) -> () {
 // CHECK: bb0([[INPUT_PTR1:%.*]] : $Builtin.NativeObject, [[INPUT_PTR2:%.*]] : $Builtin.NativeObject):
 // CHECK: release_value [[INPUT_PTR2]] : $Builtin.NativeObject
 // CHECK: release_value [[INPUT_PTR1]] : $Builtin.NativeObject
@@ -1231,7 +1231,7 @@ bb2:
   return %2 : $()
 }
 
-// CHECK-LABEL: sil [serialized] [thunk] [always_inline] @owned_to_guaranteed_multibb_callee_with_release_in_exit_two_args_2 : $@convention(thin) (@owned Builtin.NativeObject, @owned Builtin.NativeObject) -> () {
+// CHECK-LABEL: sil [serialized] [signature_optimized_thunk] [always_inline] @owned_to_guaranteed_multibb_callee_with_release_in_exit_two_args_2 : $@convention(thin) (@owned Builtin.NativeObject, @owned Builtin.NativeObject) -> () {
 // CHECK: bb0([[INPUT_PTR1:%.*]] : $Builtin.NativeObject, [[INPUT_PTR2:%.*]] : $Builtin.NativeObject):
 // CHECK: release_value [[INPUT_PTR2]] : $Builtin.NativeObject
 // CHECK: release_value [[INPUT_PTR1]] : $Builtin.NativeObject
@@ -1378,7 +1378,7 @@ bb0(%0 : $foo):
 
 // We should remove the array semantic from specialized calls.
 
-// CHECK-LABEL: sil [serialized] [thunk] [always_inline] [_semantics "array.foobar"] @array_semantic : $@convention(method) (@owned Builtin.NativeObject) -> () {
+// CHECK-LABEL: sil [serialized] [signature_optimized_thunk] [always_inline] [_semantics "array.foobar"] @array_semantic : $@convention(method) (@owned Builtin.NativeObject) -> () {
 // CHECK: [[FUNC_REF:%[0-9]+]] = function_ref @$S14array_semanticTfq4g_n : $@convention(thin) (@guaranteed Builtin.NativeObject) -> ()
 // CHECK: apply [[FUNC_REF]]
 // CHECK: release_value
@@ -1444,7 +1444,7 @@ bb0(%0 : $Builtin.Int32):
   return %9999 : $()
 }
 
-// CHECK-LABEL: sil [serialized] [thunk] [always_inline] @s2 : $@convention(thin) (Builtin.Int32) -> () {
+// CHECK-LABEL: sil [serialized] [signature_optimized_thunk] [always_inline] @s2 : $@convention(thin) (Builtin.Int32) -> () {
 sil [serialized] @s2 : $@convention(thin) (Builtin.Int32) -> () {
 bb0(%0 : $Builtin.Int32):
   // make it a non-trivial function
@@ -1481,7 +1481,7 @@ bb0(%0 : $Builtin.Int32):
   return %9999 : $()
 }
 
-// CHECK-LABEL: sil [serialized] [thunk] [always_inline] @s1 : $@convention(thin) (Builtin.Int32) -> () {
+// CHECK-LABEL: sil [serialized] [signature_optimized_thunk] [always_inline] @s1 : $@convention(thin) (Builtin.Int32) -> () {
 sil [serialized] @s1 : $@convention(thin) (Builtin.Int32) -> () {
 bb0(%0 : $Builtin.Int32):
   // make it a non-trivial function
@@ -1512,13 +1512,13 @@ bb0(%0 : $Builtin.Int32):
   return %9999 : $()
 }
 
-// CHECK-LABEL: sil hidden [thunk] [always_inline] @generic_owned_to_guaranteed : $@convention(thin) <T where T : KlassFoo> (@owned T) -> Int64
+// CHECK-LABEL: sil hidden [signature_optimized_thunk] [always_inline] @generic_owned_to_guaranteed : $@convention(thin) <T where T : KlassFoo> (@owned T) -> Int64
 // CHECK: function_ref @$S27generic_owned_to_guaranteedTf4g_n 
 // CHECK: apply
 // CHECK: release_value
 // CHECK: end sil function 'generic_owned_to_guaranteed'
 
-// CHECK-LABEL: sil [thunk] [always_inline] @generic_owned_to_guaranteed_caller : $@convention(thin) <T where T : KlassFoo> (@owned T) -> Int64 
+// CHECK-LABEL: sil [signature_optimized_thunk] [always_inline] @generic_owned_to_guaranteed_caller : $@convention(thin) <T where T : KlassFoo> (@owned T) -> Int64
 // CHECK: function_ref @$S34generic_owned_to_guaranteed_callerTf4g_n 
 // CHECK: apply
 // CHECK: release_value
@@ -1552,13 +1552,13 @@ bb0(%0 : $T):
   return %15 : $Int64
 }
 
-// CHECK-LABEL: sil hidden [thunk] [always_inline] @generic_in_to_guaranteed : $@convention(thin) <T where T : P> (@in T) -> Int64
+// CHECK-LABEL: sil hidden [signature_optimized_thunk] [always_inline] @generic_in_to_guaranteed : $@convention(thin) <T where T : P> (@in T) -> Int64
 // CHECK: function_ref @$S24generic_in_to_guaranteedTf4g_n 
 // CHECK: apply
 // CHECK: destroy_addr
 // CHECK: end sil function 'generic_in_to_guaranteed'
 
-// CHECK-LABEL: sil [thunk] [always_inline] @generic_in_to_guaranteed_caller : $@convention(thin) <T where T : P> (@in T) -> Int64 
+// CHECK-LABEL: sil [signature_optimized_thunk] [always_inline] @generic_in_to_guaranteed_caller : $@convention(thin) <T where T : P> (@in T) -> Int64
 // CHECK: function_ref @$S31generic_in_to_guaranteed_callerTf4g_n 
 // CHECK: apply
 // CHECK: destroy_addr
@@ -1591,7 +1591,7 @@ bb0(%0 : $*T):
   return %15 : $Int64
 }
 
-// CHECK-LABEL: sil [thunk] [always_inline] @generic_func_with_dead_non_generic_arg : $@convention(thin) <T> (@owned foo, @in T) -> ()
+// CHECK-LABEL: sil [signature_optimized_thunk] [always_inline] @generic_func_with_dead_non_generic_arg : $@convention(thin) <T> (@owned foo, @in T) -> ()
 // CHECK: function_ref @$S027generic_func_with_dead_non_A4_argTf4dd_n : $@convention(thin) () -> ()
 // Call the specialization which is not polymorphic.
 // CHECK: apply
@@ -1604,7 +1604,7 @@ bb0(%0 : $foo, %1 : $*T):
   return %r : $()
 }
 
-// CHECK-LABEL: sil [thunk] [always_inline] @generic_func_with_dead_generic_arg : $@convention(thin) <T> (Int64, @in T) -> Int64
+// CHECK-LABEL: sil [signature_optimized_thunk] [always_inline] @generic_func_with_dead_generic_arg : $@convention(thin) <T> (Int64, @in T) -> Int64
 // CHECK: function_ref @$S023generic_func_with_dead_A4_argTf4nd_n : $@convention(thin) (Int64) -> Int64 
 // Call the specialization which is not polymorphic.
 // CHECK: apply
@@ -1616,7 +1616,7 @@ bb0(%0 : $Int64, %1 : $*T):
   return %0 : $Int64
 }
 
-// CHECK-LABEL: sil [thunk] [always_inline] @generic_func_with_unused_generic_param_and_non_generic_arg : $@convention(thin) <T> (Int64) -> Int64 
+// CHECK-LABEL: sil [signature_optimized_thunk] [always_inline] @generic_func_with_unused_generic_param_and_non_generic_arg : $@convention(thin) <T> (Int64) -> Int64
 // CHECK: function_ref @$S025generic_func_with_unused_a15_param_and_non_A4_argTf4n_n : $@convention(thin) (Int64) -> Int64 
 // Call the specialization which is not polymorphic.
 // CHECK: apply
@@ -1633,7 +1633,7 @@ bb0(%0 : $Int64):
   return %r : $Int64
 }
 
-// CHECK-LABEL: sil [thunk] [always_inline] @generic_func_with_unused_generic_param_and_dead_non_generic_arg : $@convention(thin) <T> (Int64, Int64) -> Int64
+// CHECK-LABEL: sil [signature_optimized_thunk] [always_inline] @generic_func_with_unused_generic_param_and_dead_non_generic_arg : $@convention(thin) <T> (Int64, Int64) -> Int64
 // CHECK: function_ref @$S025generic_func_with_unused_a20_param_and_dead_non_A4_argTf4nd_n : $@convention(thin) (Int64) -> Int64 
 // Call the specialization which is not polymorphic.
 // CHECK: apply
@@ -1650,7 +1650,7 @@ bb0(%0 : $Int64):
   return %r : $Int64
 }
 
-// CHECK-LABEL: sil [thunk] [always_inline] @generic_func_with_unused_generic_param_and_dead_generic_arg : $@convention(thin) <T> (@in_guaranteed T) -> ()
+// CHECK-LABEL: sil [signature_optimized_thunk] [always_inline] @generic_func_with_unused_generic_param_and_dead_generic_arg : $@convention(thin) <T> (@in_guaranteed T) -> ()
 // CHECK: function_ref @$S025generic_func_with_unused_a16_param_and_dead_A4_argTf4d_n : $@convention(thin) () -> ()
 // Call the specialization which is not polymorphic.
 // CHECK: apply
@@ -1669,7 +1669,7 @@ bb0(%0 : $*T):
   return %r : $()
 }
 
-// CHECK-LABEL: sil [thunk] [always_inline] @generic_func_with_unused_generic_param_and_dead_owned_generic_arg : $@convention(thin) <T where T : KlassFoo> (@owned T) -> ()
+// CHECK-LABEL: sil [signature_optimized_thunk] [always_inline] @generic_func_with_unused_generic_param_and_dead_owned_generic_arg : $@convention(thin) <T where T : KlassFoo> (@owned T) -> ()
 // CHECK: function_ref @$S025generic_func_with_unused_a22_param_and_dead_owned_A4_argTf4d_n : $@convention(thin) () -> ()
 // Call the specialization which is not polymorphic.
 // CHECK: apply
@@ -1690,7 +1690,7 @@ bb0(%0 : $T):
   return %r : $()
 }
 
-// CHECK-LABEL: sil [thunk] [always_inline] @generic_func_with_multple_generic_args_and_dead_generic_arg : $@convention(thin) <T, S> (@in T, @in S) -> ()
+// CHECK-LABEL: sil [signature_optimized_thunk] [always_inline] @generic_func_with_multple_generic_args_and_dead_generic_arg : $@convention(thin) <T, S> (@in T, @in S) -> ()
 // CHECK: function_ref @$S026generic_func_with_multple_a15_args_and_dead_A4_argTf4nd_n : $@convention(thin) <τ_0_0, τ_0_1> (@in τ_0_0) -> ()
 // Call the specialization which has only one function argument, because another one is dead
 // and was eliminated.
diff --git a/test/SILOptimizer/functionsigopts_sroa.sil b/test/SILOptimizer/functionsigopts_sroa.sil
index 25cc8973335..51b95396a97 100644
--- a/test/SILOptimizer/functionsigopts_sroa.sil
+++ b/test/SILOptimizer/functionsigopts_sroa.sil
@@ -136,7 +136,7 @@ struct SingleFieldLvl3 {
 
 /// This checks the case where we have a single level hierarchy and the root is
 /// dead.
-// CHECK-LABEL: sil [serialized] [thunk] [always_inline] @single_level_dead_root_callee : $@convention(thin) (S1) -> Builtin.Int32 {
+// CHECK-LABEL: sil [serialized] [signature_optimized_thunk] [always_inline] @single_level_dead_root_callee : $@convention(thin) (S1) -> Builtin.Int32 {
 // CHECK: bb0([[INPUT:%[0-9]+]] : $S1):
 // CHECK: [[FN:%[0-9]+]] = function_ref @$S29single_level_dead_root_calleeTfq4x_n : $@convention(thin) (Builtin.Int32) -> Builtin.Int32
 // CHECK: [[ARG:%.*]] = struct_extract [[INPUT]] : $S1, #S1.f2
@@ -182,7 +182,7 @@ bb0(%0 : $S1):
   return %9999 : $()
 }
 
-// CHECK-LABEL: sil [serialized] [thunk] [always_inline] @single_level_live_root_callee : $@convention(thin) (S1) -> Builtin.Int32 {
+// CHECK-LABEL: sil [serialized] [signature_optimized_thunk] [always_inline] @single_level_live_root_callee : $@convention(thin) (S1) -> Builtin.Int32 {
 // CHECK: bb0([[INPUT:%[0-9]+]] : $S1):
 // CHECK: [[FN:%[0-9]+]] = function_ref @$S29single_level_live_root_calleeTfq4x_n : $@convention(thin) (Builtin.Int16, Builtin.Int32) -> Builtin.Int32
 // CHECK: [[ARG2:%.*]] = struct_extract [[INPUT]] : $S1, #S1.f2
@@ -234,7 +234,7 @@ bb0(%0 : $S1):
 // This test checks where we have a multiple level hierarchy, the root is dead,
 // but the root has all fields used. This means that we should extract
 // everything, but we should not "reform" the aggregate.
-// CHECK-LABEL: sil [serialized] [thunk] [always_inline] @multiple_level_all_root_fields_used_callee : $@convention(thin) (S2) -> (Builtin.Int16, Builtin.Int64) {
+// CHECK-LABEL: sil [serialized] [signature_optimized_thunk] [always_inline] @multiple_level_all_root_fields_used_callee : $@convention(thin) (S2) -> (Builtin.Int16, Builtin.Int64) {
 // CHECK: bb0([[INPUT:%.*]] : $S2):
 // CHECK: [[FN:%.*]] = function_ref @$S42multiple_level_all_root_fields_used_calleeTfq4x_n : $@convention(thin) (Builtin.Int16, Builtin.Int64) -> (Builtin.Int16, Builtin.Int64)
 // CHECK: [[EXT1:%.*]] = struct_extract [[INPUT]] : $S2, #S2.f2
@@ -292,7 +292,7 @@ bb0(%0 : $S2):
 }
 
 /// This test checks a multiple level hierarchy where the root has no fields used.
-// CHECK-LABEL: sil [serialized] [thunk] [always_inline] @multiple_level_no_root_fields_have_direct_uses_callee : $@convention(thin) (S3) -> (Builtin.Int16, Builtin.Int64) {
+// CHECK-LABEL: sil [serialized] [signature_optimized_thunk] [always_inline] @multiple_level_no_root_fields_have_direct_uses_callee : $@convention(thin) (S3) -> (Builtin.Int16, Builtin.Int64) {
 // CHECK: bb0([[IN:%.*]] : $S3):
 // CHECK: [[FN:%.*]] = function_ref @$S53multiple_level_no_root_fields_have_direct_uses_calleeTfq4x_n : $@convention(thin) (Builtin.Int16, Builtin.Int64) -> (Builtin.Int16, Builtin.Int64)
 // CHECK: [[EXT1:%.*]] = struct_extract [[IN]] : $S3, #S3.f2
@@ -355,7 +355,7 @@ bb0(%0 : $S3):
 
 // This test checks a multiple level hierarchy where the root has its own use
 // and needs to be reformed via a struct.
-// CHECK-LABEL: sil [serialized] [thunk] [always_inline] @multiple_level_root_must_be_reformed_callee : $@convention(thin) (S2) -> (Builtin.Int16, Builtin.Int64) {
+// CHECK-LABEL: sil [serialized] [signature_optimized_thunk] [always_inline] @multiple_level_root_must_be_reformed_callee : $@convention(thin) (S2) -> (Builtin.Int16, Builtin.Int64) {
 // CHECK: bb0([[IN:%.*]] : $S2):
 // CHECK: [[FN:%.*]] = function_ref @$S43multiple_level_root_must_be_reformed_calleeTfq4x_n : $@convention(thin) (Builtin.Int16, Builtin.Int32, Builtin.Int64) -> (Builtin.Int16, Builtin.Int64)
 // CHECK: [[EXT1:%.*]] = struct_extract [[IN]] : $S2, #S2.f2
@@ -439,7 +439,7 @@ bb0(%0 : $S2):
 }
 
 // This test checks if we can handle @owned structs correctly
-// CHECK-LABEL: sil [serialized] [thunk] [always_inline] @owned_struct_1_callee : $@convention(thin) (@owned S5, @owned S5) -> (Builtin.Int16, Builtin.Int32, Builtin.Int16, Builtin.Int32) {
+// CHECK-LABEL: sil [serialized] [signature_optimized_thunk] [always_inline] @owned_struct_1_callee : $@convention(thin) (@owned S5, @owned S5) -> (Builtin.Int16, Builtin.Int32, Builtin.Int16, Builtin.Int32) {
 // CHECK: bb0([[IN1:%.*]] : $S5, [[IN2:%.*]] : $S5):
 // CHECK: [[FN:%.*]] = function_ref @$S21owned_struct_1_calleeTfq4dgX_n : $@convention(thin) (@guaranteed S4, Builtin.Int16, Builtin.Int32) -> (Builtin.Int16, Builtin.Int32, Builtin.Int16, Builtin.Int32)
 // CHECK: [[EXT1:%.*]] = struct_extract [[IN2]] : $S5, #S5.f2
@@ -505,7 +505,7 @@ bb0(%0 : $S5):
 }
 
 // This test checks if we can properly insert arguments in between dead arguments.
-// CHECK-LABEL: sil [serialized] [thunk] [always_inline] @owned_struct_2_callee : $@convention(thin) (Builtin.Int256, Builtin.Int256, @owned S5, Builtin.Int128, Builtin.Int128) -> (Builtin.Int256, Builtin.Int16, Builtin.Int32, Builtin.Int128) {
+// CHECK-LABEL: sil [serialized] [signature_optimized_thunk] [always_inline] @owned_struct_2_callee : $@convention(thin) (Builtin.Int256, Builtin.Int256, @owned S5, Builtin.Int128, Builtin.Int128) -> (Builtin.Int256, Builtin.Int16, Builtin.Int32, Builtin.Int128) {
 // CHECK: bb0([[IN1:%.*]] : $Builtin.Int256, [[IN2:%.*]] : $Builtin.Int256, [[IN3:%.*]] : $S5, [[IN4:%.*]] : $Builtin.Int128, [[IN5:%.*]] : $Builtin.Int128):
 // CHECK: [[FN:%.*]] = function_ref @$S21owned_struct_2_calleeTfq4ndgXdn_n : $@convention(thin) (Builtin.Int256, @guaranteed S4, Builtin.Int16, Builtin.Int32, Builtin.Int128) -> (Builtin.Int256, Builtin.Int16, Builtin.Int32, Builtin.Int128)
 // CHECK: [[EXT1:%.*]] = struct_extract [[IN3]] : $S5, #S5.f2
@@ -567,7 +567,7 @@ bb0(%0 : $Builtin.Int256, %1 : $S5, %2 : $Builtin.Int128):
 }
 
 /// This test makes sure that we ignore pointer arguments for now.
-// CHECK-LABEL: sil [serialized] [thunk] [always_inline] @ignore_ptrs_callee : $@convention(thin) (@in S1, S1, S1) -> (Builtin.Int16, Builtin.Int16) {
+// CHECK-LABEL: sil [serialized] [signature_optimized_thunk] [always_inline] @ignore_ptrs_callee : $@convention(thin) (@in S1, S1, S1) -> (Builtin.Int16, Builtin.Int16) {
 // CHECK: bb0([[IN1:%.*]] : $*S1, [[IN2:%.*]] : $S1, [[IN3:%.*]] : $S1):
 // CHECK: [[FN:%.*]] = function_ref @$S18ignore_ptrs_calleeTfq4nxx_n : $@convention(thin) (@in S1, Builtin.Int16, Builtin.Int16) -> (Builtin.Int16, Builtin.Int16)
 // CHECK: [[EXT1:%.*]] = struct_extract [[IN2]] : $S1, #S1.f1
@@ -677,7 +677,7 @@ bb0(%0 : $FakeStaticString, %1 : $FakeString, %2 : $FakeStaticString):
 // This test makes sure that we handle cases where the callee has field uses
 // that are processed in a different order than the fields are layed out in the
 // structure.
-// CHECK-LABEL: sil [serialized] [thunk] [always_inline] @check_out_of_order_uses_callee : $@convention(thin) (S1) -> () {
+// CHECK-LABEL: sil [serialized] [signature_optimized_thunk] [always_inline] @check_out_of_order_uses_callee : $@convention(thin) (S1) -> () {
 // CHECK: bb0([[IN:%.*]] : $S1):
 // CHECK: [[FN:%.*]] = function_ref @$S30check_out_of_order_uses_calleeTfq4x_n : $@convention(thin) (Builtin.Int16, Builtin.Int32) -> ()
 // CHECK: [[EXT1:%.*]] = struct_extract [[IN]] : $S1, #S1.f2
@@ -735,7 +735,7 @@ bb0(%0 : $S1):
 }
 
 // Make sure that we do not SROA classes.
-// CHECK-LABEL: sil [serialized] [thunk] [always_inline] @class_callee_1 : $@convention(thin) (@owned C1, Builtin.Int32) -> Builtin.Int32 {
+// CHECK-LABEL: sil [serialized] [signature_optimized_thunk] [always_inline] @class_callee_1 : $@convention(thin) (@owned C1, Builtin.Int32) -> Builtin.Int32 {
 // CHECK: bb0([[IN1:%.*]] : $C1, [[IN2:%.*]] : $Builtin.Int32):
 // CHECK: [[FN:%.*]] = function_ref @$S14class_callee_1Tfq4gn_n : $@convention(thin) (@guaranteed C1, Builtin.Int32) -> Builtin.Int32
 // CHECK: apply [[FN]]([[IN1]], [[IN2]]) : $@convention(thin) (@guaranteed C1, Builtin.Int32) -> Builtin.Int32
@@ -774,7 +774,7 @@ bb0(%0 : $C1, %1 : $Builtin.Int32):
   return %1 : $Builtin.Int32
 }
 
-// CHECK-LABEL: sil [serialized] [thunk] [always_inline] @class_caller_1 : $@convention(thin) (@owned C1, Builtin.Int32) -> Builtin.Int32 {
+// CHECK-LABEL: sil [serialized] [signature_optimized_thunk] [always_inline] @class_caller_1 : $@convention(thin) (@owned C1, Builtin.Int32) -> Builtin.Int32 {
 // CHECK: bb0([[IN1:%.*]] : $C1, [[IN2:%.*]] : $Builtin.Int32):
 // CHECK: [[FN:%.*]] = function_ref @$S14class_callee_1Tfq4gn_n : $@convention(thin) (@guaranteed C1, Builtin.Int32) -> Builtin.Int32
 // CHECK: apply [[FN]]([[IN1]], [[IN2]]) : $@convention(thin) (@guaranteed C1, Builtin.Int32) -> Builtin.Int32
@@ -856,7 +856,7 @@ bb0(%0 : $ThirtySixFieldStruct):
   return %9999 : $()
 }
 
-// CHECK-LABEL: sil [serialized] [thunk] [always_inline] @more_than_32_type_sized_callee : $@convention(thin) (ThirtySixFieldStruct) -> Builtin.Int32 {
+// CHECK-LABEL: sil [serialized] [signature_optimized_thunk] [always_inline] @more_than_32_type_sized_callee : $@convention(thin) (ThirtySixFieldStruct) -> Builtin.Int32 {
 sil [serialized] @more_than_32_type_sized_callee : $@convention(thin) (ThirtySixFieldStruct) -> Builtin.Int32 {
 bb0(%0 : $ThirtySixFieldStruct):
   // make it a non-trivial function