diff --git a/toolchain/check/cpp/import.cpp b/toolchain/check/cpp/import.cpp
index de0bf1ec59fa4..11995bd807877 100644
--- a/toolchain/check/cpp/import.cpp
+++ b/toolchain/check/cpp/import.cpp
@@ -1738,14 +1738,15 @@ static auto ImportFunctionDecl(Context& context, SemIR::LocId loc_id,
           builder.Note(loc_id, InCppThunk);
         });
 
-    clang::FunctionDecl* thunk_clang_decl =
-        BuildCppThunk(context, function_info);
-    if (thunk_clang_decl) {
-      SemIR::FunctionId thunk_function_id = *ImportFunction(
-          context, loc_id, thunk_clang_decl, thunk_clang_decl->getNumParams());
-      SemIR::InstId thunk_function_decl_id =
-          context.functions().Get(thunk_function_id).first_owning_decl_id;
-      function_info.SetHasCppThunk(thunk_function_decl_id);
+    if (clang::FunctionDecl* thunk_clang_decl =
+            BuildCppThunk(context, function_info)) {
+      if (auto thunk_function_id =
+              ImportFunction(context, loc_id, thunk_clang_decl,
+                             thunk_clang_decl->getNumParams())) {
+        SemIR::InstId thunk_function_decl_id =
+            context.functions().Get(*thunk_function_id).first_owning_decl_id;
+        function_info.SetHasCppThunk(thunk_function_decl_id);
+      }
     }
   }
 
diff --git a/toolchain/check/cpp/thunk.cpp b/toolchain/check/cpp/thunk.cpp
index 6a53e3b592f61..b6e3b9ccb3c48 100644
--- a/toolchain/check/cpp/thunk.cpp
+++ b/toolchain/check/cpp/thunk.cpp
@@ -50,106 +50,30 @@ static auto GenerateThunkMangledName(
   return mangled_name_stream.TakeStr();
 }
 
-// Returns true if a C++ thunk is required for the given type. A C++ thunk is
-// required for any type except for void, pointer types and signed 32-bit and
-// 64-bit integers.
-static auto IsThunkRequiredForType(Context& context, SemIR::TypeId type_id)
-    -> bool {
-  if (!type_id.has_value() || type_id == SemIR::ErrorInst::TypeId) {
-    return false;
-  }
-
-  type_id = context.types().GetUnqualifiedType(type_id);
-
-  switch (context.types().GetAsInst(type_id).kind()) {
-    case SemIR::PointerType::Kind: {
-      return false;
-    }
-
-    case SemIR::ClassType::Kind: {
-      if (!context.types().IsComplete(type_id)) {
-        // Signed integers of 32 or 64 bits should be completed when imported.
-        return true;
-      }
-
-      auto int_info = context.types().TryGetIntTypeInfo(type_id);
-      if (!int_info || !int_info->bit_width.has_value()) {
-        return true;
-      }
-
-      llvm::APInt bit_width = context.ints().Get(int_info->bit_width);
-      return bit_width != 32 && bit_width != 64;
-    }
-
-    default:
-      return true;
-  }
-}
-
-auto IsCppThunkRequired(Context& context, const SemIR::Function& function)
-    -> bool {
-  if (!function.clang_decl_id.has_value()) {
-    return false;
-  }
-
-  // A thunk is required if any parameter or return type requires it. However,
-  // we don't generate a thunk if any relevant type is erroneous.
-  bool thunk_required = false;
-
-  const auto& decl_info = context.clang_decls().Get(function.clang_decl_id);
-  const auto* decl = cast<clang::FunctionDecl>(decl_info.key.decl);
-  if (decl_info.key.num_params !=
-      static_cast<int>(decl->getNumNonObjectParams())) {
-    // We require a thunk if the number of parameters we want isn't all of them.
-    // This happens if default arguments are in use, or (eventually) when
-    // calling a varargs function.
-    thunk_required = true;
-  } else {
-    // We require a thunk if any parameter is of reference type, even if the
-    // corresponding SemIR function has an acceptable parameter type.
-    // TODO: We should be able to avoid thunks for reference parameters.
-    for (auto* param : decl->parameters()) {
-      if (param->getType()->isReferenceType()) {
-        thunk_required = true;
-        break;
-      }
-    }
+// Returns whether the Carbon lowering for a parameter or return of this type is
+// known to match the C++ lowering.
+static auto IsSimpleAbiType(clang::ASTContext& ast_context,
+                            clang::QualType type, bool for_parameter) -> bool {
+  if (type->isVoidType() || type->isPointerType()) {
+    return true;
   }
 
-  SemIR::TypeId return_type_id =
-      function.GetDeclaredReturnType(context.sem_ir());
-  if (return_type_id.has_value()) {
-    if (return_type_id == SemIR::ErrorInst::TypeId) {
-      return false;
-    }
-    thunk_required =
-        thunk_required || IsThunkRequiredForType(context, return_type_id);
+  if (!for_parameter && type->isLValueReferenceType()) {
+    // An lvalue reference return type maps to a pointer, which uses the same
+    // lowering rule.
+    return true;
   }
 
-  for (auto param_id :
-       context.inst_blocks().GetOrEmpty(function.call_params_id)) {
-    if (param_id == SemIR::ErrorInst::InstId) {
+  if (const auto* enum_decl = type->getAsEnumDecl()) {
+    // An enum type has a simple ABI if its underlying type does.
+    type = enum_decl->getIntegerType();
+    if (type.isNull()) {
       return false;
     }
-    thunk_required =
-        thunk_required ||
-        IsThunkRequiredForType(
-            context, context.insts().GetAs<SemIR::AnyParam>(param_id).type_id);
-  }
-
-  return thunk_required;
-}
-
-// Returns whether the type is void, a pointer, or a signed int of 32 or 64
-// bits.
-static auto IsSimpleAbiType(clang::ASTContext& ast_context,
-                            clang::QualType type) -> bool {
-  if (type->isVoidType() || type->isPointerType()) {
-    return true;
   }
 
   if (const auto* builtin_type = type->getAs<clang::BuiltinType>()) {
-    if (builtin_type->isSignedInteger()) {
+    if (builtin_type->isIntegerType()) {
       uint64_t type_size = ast_context.getIntWidth(type);
       return type_size == 32 || type_size == 64;
     }
@@ -174,8 +98,8 @@ struct CalleeFunctionInfo {
     effective_return_type =
         is_ctor ? ast_context.getCanonicalTagType(method_decl->getParent())
                 : decl->getReturnType();
-    has_simple_return_type =
-        IsSimpleAbiType(ast_context, effective_return_type);
+    has_simple_return_type = IsSimpleAbiType(ast_context, effective_return_type,
+                                             /*for_parameter=*/false);
   }
 
   // Returns whether this callee has an implicit `this` parameter.
@@ -234,6 +158,52 @@ struct CalleeFunctionInfo {
 };
 }  // namespace
 
+auto IsCppThunkRequired(Context& context, const SemIR::Function& function)
+    -> bool {
+  if (!function.clang_decl_id.has_value()) {
+    return false;
+  }
+
+  const auto& decl_info = context.clang_decls().Get(function.clang_decl_id);
+  auto* decl = cast<clang::FunctionDecl>(decl_info.key.decl);
+  if (decl_info.key.num_params !=
+      static_cast<int>(decl->getNumNonObjectParams())) {
+    // We require a thunk if the number of parameters we want isn't all of them.
+    // This happens if default arguments are in use, or (eventually) when
+    // calling a varargs function.
+    return true;
+  }
+
+  CalleeFunctionInfo callee_info(decl, decl_info.key.num_params);
+  if (!callee_info.has_simple_return_type) {
+    return true;
+  }
+
+  auto& ast_context = context.ast_context();
+  if (callee_info.has_implicit_object_parameter()) {
+    // TODO: The object parameter is a reference parameter, but we don't force a
+    // thunk here like we do for explicit reference parameters in the case where
+    // we would map the parameter to an `addr` parameter. We should make this
+    // behavior consistent.
+    auto* method_decl = cast<clang::CXXMethodDecl>(decl);
+    if (method_decl->getRefQualifier() == clang::RQ_RValue ||
+        method_decl->getMethodQualifiers().hasConst()) {
+      return true;
+    }
+  }
+
+  const auto* function_type =
+      decl->getType()->castAs<clang::FunctionProtoType>();
+  for (int i : llvm::seq(decl->getNumParams())) {
+    if (!IsSimpleAbiType(ast_context, function_type->getParamType(i),
+                         /*for_parameter=*/true)) {
+      return true;
+    }
+  }
+
+  return false;
+}
+
 // Given a pointer type, returns the corresponding _Nonnull-qualified pointer
 // type.
 static auto GetNonnullType(clang::ASTContext& ast_context,
@@ -255,7 +225,7 @@ static auto GetNonNullablePointerType(clang::ASTContext& ast_context,
 static auto GetThunkParameterType(clang::ASTContext& ast_context,
                                   clang::QualType callee_type)
     -> clang::QualType {
-  if (IsSimpleAbiType(ast_context, callee_type)) {
+  if (IsSimpleAbiType(ast_context, callee_type, /*for_parameter=*/true)) {
     return callee_type;
   }
   return GetNonNullablePointerType(ast_context, callee_type);
diff --git a/toolchain/check/testdata/interop/cpp/enum/anonymous.carbon b/toolchain/check/testdata/interop/cpp/enum/anonymous.carbon
index cd6d20f9dc540..bcb9f50f851de 100644
--- a/toolchain/check/testdata/interop/cpp/enum/anonymous.carbon
+++ b/toolchain/check/testdata/interop/cpp/enum/anonymous.carbon
@@ -47,9 +47,8 @@ fn G() {
 // CHECK:STDOUT:   %F.cpp_overload_set.value: %F.cpp_overload_set.type = cpp_overload_set_value @F.cpp_overload_set [concrete]
 // CHECK:STDOUT:   %.4f0: type = class_type @.1 [concrete]
 // CHECK:STDOUT:   %int_1.81a: %.4f0 = int_value 1 [concrete]
-// CHECK:STDOUT:   %ptr.793: type = ptr_type %.4f0 [concrete]
-// CHECK:STDOUT:   %F__carbon_thunk.type.eda1ac.1: type = fn_type @F__carbon_thunk.1 [concrete]
-// CHECK:STDOUT:   %F__carbon_thunk.0cd6a8.1: %F__carbon_thunk.type.eda1ac.1 = struct_value () [concrete]
+// CHECK:STDOUT:   %F.type: type = fn_type @F [concrete]
+// CHECK:STDOUT:   %F: %F.type = struct_value () [concrete]
 // CHECK:STDOUT:   %C: type = class_type @C [concrete]
 // CHECK:STDOUT:   %C.C.cpp_overload_set.type: type = cpp_overload_set_type @C.C.cpp_overload_set [concrete]
 // CHECK:STDOUT:   %C.C.cpp_overload_set.value: %C.C.cpp_overload_set.type = cpp_overload_set_value @C.C.cpp_overload_set [concrete]
@@ -60,19 +59,12 @@ fn G() {
 // CHECK:STDOUT:   %C.F.cpp_overload_set.value: %C.F.cpp_overload_set.type = cpp_overload_set_value @C.F.cpp_overload_set [concrete]
 // CHECK:STDOUT:   %.bb7: type = class_type @.2 [concrete]
 // CHECK:STDOUT:   %int_1.1d6: %.bb7 = int_value 1 [concrete]
-// CHECK:STDOUT:   %ptr.73d: type = ptr_type %.bb7 [concrete]
-// CHECK:STDOUT:   %F__carbon_thunk.type.eda1ac.2: type = fn_type @F__carbon_thunk.2 [concrete]
-// CHECK:STDOUT:   %F__carbon_thunk.0cd6a8.2: %F__carbon_thunk.type.eda1ac.2 = struct_value () [concrete]
+// CHECK:STDOUT:   %C.F.type: type = fn_type @C.F [concrete]
+// CHECK:STDOUT:   %C.F: %C.F.type = struct_value () [concrete]
 // CHECK:STDOUT:   %type_where: type = facet_type <type where .Self impls <CanDestroy>> [concrete]
-// CHECK:STDOUT:   %facet_value.597: %type_where = facet_value %.bb7, () [concrete]
-// CHECK:STDOUT:   %DestroyT.binding.as_type.as.Destroy.impl.Op.type.52d: type = fn_type @DestroyT.binding.as_type.as.Destroy.impl.Op, @DestroyT.binding.as_type.as.Destroy.impl(%facet_value.597) [concrete]
-// CHECK:STDOUT:   %DestroyT.binding.as_type.as.Destroy.impl.Op.afc: %DestroyT.binding.as_type.as.Destroy.impl.Op.type.52d = struct_value () [concrete]
-// CHECK:STDOUT:   %facet_value.b21: %type_where = facet_value %C, () [concrete]
-// CHECK:STDOUT:   %DestroyT.binding.as_type.as.Destroy.impl.Op.type.b92: type = fn_type @DestroyT.binding.as_type.as.Destroy.impl.Op, @DestroyT.binding.as_type.as.Destroy.impl(%facet_value.b21) [concrete]
+// CHECK:STDOUT:   %facet_value: %type_where = facet_value %C, () [concrete]
+// CHECK:STDOUT:   %DestroyT.binding.as_type.as.Destroy.impl.Op.type.b92: type = fn_type @DestroyT.binding.as_type.as.Destroy.impl.Op, @DestroyT.binding.as_type.as.Destroy.impl(%facet_value) [concrete]
 // CHECK:STDOUT:   %DestroyT.binding.as_type.as.Destroy.impl.Op.841: %DestroyT.binding.as_type.as.Destroy.impl.Op.type.b92 = struct_value () [concrete]
-// CHECK:STDOUT:   %facet_value.21d: %type_where = facet_value %.4f0, () [concrete]
-// CHECK:STDOUT:   %DestroyT.binding.as_type.as.Destroy.impl.Op.type.02d: type = fn_type @DestroyT.binding.as_type.as.Destroy.impl.Op, @DestroyT.binding.as_type.as.Destroy.impl(%facet_value.21d) [concrete]
-// CHECK:STDOUT:   %DestroyT.binding.as_type.as.Destroy.impl.Op.246: %DestroyT.binding.as_type.as.Destroy.impl.Op.type.02d = struct_value () [concrete]
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: imports {
@@ -84,7 +76,7 @@ fn G() {
 // CHECK:STDOUT:   }
 // CHECK:STDOUT:   %F.cpp_overload_set.value: %F.cpp_overload_set.type = cpp_overload_set_value @F.cpp_overload_set [concrete = constants.%F.cpp_overload_set.value]
 // CHECK:STDOUT:   %int_1.81a: %.4f0 = int_value 1 [concrete = constants.%int_1.81a]
-// CHECK:STDOUT:   %F__carbon_thunk.decl.e1b8ec.1: %F__carbon_thunk.type.eda1ac.1 = fn_decl @F__carbon_thunk.1 [concrete = constants.%F__carbon_thunk.0cd6a8.1] {
+// CHECK:STDOUT:   %F.decl: %F.type = fn_decl @F [concrete = constants.%F] {
 // CHECK:STDOUT:     <elided>
 // CHECK:STDOUT:   } {
 // CHECK:STDOUT:     <elided>
@@ -98,7 +90,7 @@ fn G() {
 // CHECK:STDOUT:   }
 // CHECK:STDOUT:   %C.F.cpp_overload_set.value: %C.F.cpp_overload_set.type = cpp_overload_set_value @C.F.cpp_overload_set [concrete = constants.%C.F.cpp_overload_set.value]
 // CHECK:STDOUT:   %int_1.1d6: %.bb7 = int_value 1 [concrete = constants.%int_1.1d6]
-// CHECK:STDOUT:   %F__carbon_thunk.decl.e1b8ec.2: %F__carbon_thunk.type.eda1ac.2 = fn_decl @F__carbon_thunk.2 [concrete = constants.%F__carbon_thunk.0cd6a8.2] {
+// CHECK:STDOUT:   %C.F.decl: %C.F.type = fn_decl @C.F [concrete = constants.%C.F] {
 // CHECK:STDOUT:     <elided>
 // CHECK:STDOUT:   } {
 // CHECK:STDOUT:     <elided>
@@ -115,10 +107,7 @@ fn G() {
 // CHECK:STDOUT:   %F.ref.loc8: %F.cpp_overload_set.type = name_ref F, imports.%F.cpp_overload_set.value [concrete = constants.%F.cpp_overload_set.value]
 // CHECK:STDOUT:   %Cpp.ref.loc8_9: <namespace> = name_ref Cpp, imports.%Cpp [concrete = imports.%Cpp]
 // CHECK:STDOUT:   %b.ref: %.4f0 = name_ref b, imports.%int_1.81a [concrete = constants.%int_1.81a]
-// CHECK:STDOUT:   %.loc8_12.1: ref %.4f0 = temporary_storage
-// CHECK:STDOUT:   %.loc8_12.2: ref %.4f0 = temporary %.loc8_12.1, %b.ref
-// CHECK:STDOUT:   %addr.loc8_14: %ptr.793 = addr_of %.loc8_12.2
-// CHECK:STDOUT:   %F__carbon_thunk.call.loc8: init %empty_tuple.type = call imports.%F__carbon_thunk.decl.e1b8ec.1(%addr.loc8_14)
+// CHECK:STDOUT:   %F.call: init %empty_tuple.type = call imports.%F.decl(%b.ref)
 // CHECK:STDOUT:   %Cpp.ref.loc10_3: <namespace> = name_ref Cpp, imports.%Cpp [concrete = imports.%Cpp]
 // CHECK:STDOUT:   %C.ref.loc10_6: type = name_ref C, imports.%C.decl [concrete = constants.%C]
 // CHECK:STDOUT:   %C.ref.loc10_8: %C.C.cpp_overload_set.type = name_ref C, imports.%C.C.cpp_overload_set.value [concrete = constants.%C.C.cpp_overload_set.value]
@@ -133,25 +122,12 @@ fn G() {
 // CHECK:STDOUT:   %C.ref.loc10_18: type = name_ref C, imports.%C.decl [concrete = constants.%C]
 // CHECK:STDOUT:   %e.ref: %.bb7 = name_ref e, imports.%int_1.1d6 [concrete = constants.%int_1.1d6]
 // CHECK:STDOUT:   %addr.loc10_11.2: %ptr.d9e = addr_of %.loc10_11.3
-// CHECK:STDOUT:   %.loc10_20.1: ref %.bb7 = temporary_storage
-// CHECK:STDOUT:   %.loc10_20.2: ref %.bb7 = temporary %.loc10_20.1, %e.ref
-// CHECK:STDOUT:   %addr.loc10_22: %ptr.73d = addr_of %.loc10_20.2
-// CHECK:STDOUT:   %F__carbon_thunk.call.loc10: init %empty_tuple.type = call imports.%F__carbon_thunk.decl.e1b8ec.2(%addr.loc10_11.2, %addr.loc10_22)
-// CHECK:STDOUT:   %DestroyT.binding.as_type.as.Destroy.impl.Op.bound.loc10_20: <bound method> = bound_method %.loc10_20.2, constants.%DestroyT.binding.as_type.as.Destroy.impl.Op.afc
+// CHECK:STDOUT:   %C.F.call: init %empty_tuple.type = call imports.%C.F.decl(%addr.loc10_11.2, %e.ref)
+// CHECK:STDOUT:   %DestroyT.binding.as_type.as.Destroy.impl.Op.bound: <bound method> = bound_method %.loc10_11.3, constants.%DestroyT.binding.as_type.as.Destroy.impl.Op.841
 // CHECK:STDOUT:   <elided>
-// CHECK:STDOUT:   %bound_method.loc10_20: <bound method> = bound_method %.loc10_20.2, %DestroyT.binding.as_type.as.Destroy.impl.Op.specific_fn.1
-// CHECK:STDOUT:   %addr.loc10_20: %ptr.73d = addr_of %.loc10_20.2
-// CHECK:STDOUT:   %DestroyT.binding.as_type.as.Destroy.impl.Op.call.loc10_20: init %empty_tuple.type = call %bound_method.loc10_20(%addr.loc10_20)
-// CHECK:STDOUT:   %DestroyT.binding.as_type.as.Destroy.impl.Op.bound.loc10_11: <bound method> = bound_method %.loc10_11.3, constants.%DestroyT.binding.as_type.as.Destroy.impl.Op.841
-// CHECK:STDOUT:   <elided>
-// CHECK:STDOUT:   %bound_method.loc10_11: <bound method> = bound_method %.loc10_11.3, %DestroyT.binding.as_type.as.Destroy.impl.Op.specific_fn.2
+// CHECK:STDOUT:   %bound_method.loc10_11: <bound method> = bound_method %.loc10_11.3, %DestroyT.binding.as_type.as.Destroy.impl.Op.specific_fn
 // CHECK:STDOUT:   %addr.loc10_11.3: %ptr.d9e = addr_of %.loc10_11.3
-// CHECK:STDOUT:   %DestroyT.binding.as_type.as.Destroy.impl.Op.call.loc10_11: init %empty_tuple.type = call %bound_method.loc10_11(%addr.loc10_11.3)
-// CHECK:STDOUT:   %DestroyT.binding.as_type.as.Destroy.impl.Op.bound.loc8: <bound method> = bound_method %.loc8_12.2, constants.%DestroyT.binding.as_type.as.Destroy.impl.Op.246
-// CHECK:STDOUT:   <elided>
-// CHECK:STDOUT:   %bound_method.loc8: <bound method> = bound_method %.loc8_12.2, %DestroyT.binding.as_type.as.Destroy.impl.Op.specific_fn.3
-// CHECK:STDOUT:   %addr.loc8_12: %ptr.793 = addr_of %.loc8_12.2
-// CHECK:STDOUT:   %DestroyT.binding.as_type.as.Destroy.impl.Op.call.loc8: init %empty_tuple.type = call %bound_method.loc8(%addr.loc8_12)
+// CHECK:STDOUT:   %DestroyT.binding.as_type.as.Destroy.impl.Op.call: init %empty_tuple.type = call %bound_method.loc10_11(%addr.loc10_11.3)
 // CHECK:STDOUT:   return
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
diff --git a/toolchain/check/testdata/interop/cpp/function/decayed_param.carbon b/toolchain/check/testdata/interop/cpp/function/decayed_param.carbon
index ee7baec860d74..fd6768045cdf4 100644
--- a/toolchain/check/testdata/interop/cpp/function/decayed_param.carbon
+++ b/toolchain/check/testdata/interop/cpp/function/decayed_param.carbon
@@ -117,8 +117,8 @@ fn F() {
 // CHECK:STDOUT:   %OptionalStorage.impl_witness.01b: <witness> = impl_witness imports.%OptionalStorage.impl_witness_table.377, @ptr.as.OptionalStorage.impl(%i32) [concrete]
 // CHECK:STDOUT:   %OptionalStorage.facet: %OptionalStorage.type = facet_value %ptr.235, (%OptionalStorage.impl_witness.01b) [concrete]
 // CHECK:STDOUT:   %Optional.8fd: type = class_type @Optional, @Optional(%OptionalStorage.facet) [concrete]
-// CHECK:STDOUT:   %TakesArray__carbon_thunk.type: type = fn_type @TakesArray__carbon_thunk [concrete]
-// CHECK:STDOUT:   %TakesArray__carbon_thunk: %TakesArray__carbon_thunk.type = struct_value () [concrete]
+// CHECK:STDOUT:   %TakesArray.type: type = fn_type @TakesArray [concrete]
+// CHECK:STDOUT:   %TakesArray: %TakesArray.type = struct_value () [concrete]
 // CHECK:STDOUT:   %ImplicitAs.type.6cc: type = facet_type <@ImplicitAs, @ImplicitAs(%Optional.8fd)> [concrete]
 // CHECK:STDOUT:   %ImplicitAs.Convert.type.770: type = fn_type @ImplicitAs.Convert, @ImplicitAs(%Optional.8fd) [concrete]
 // CHECK:STDOUT:   %T.binding.as_type.as.ImplicitAs.impl.Convert.type.6f7: type = fn_type @T.binding.as_type.as.ImplicitAs.impl.Convert.2, @T.binding.as_type.as.ImplicitAs.impl.3a5(%T.76d) [symbolic]
@@ -153,7 +153,7 @@ fn F() {
 // CHECK:STDOUT:   %Core.import_ref.6db = import_ref Core//prelude/types/optional, loc{{\d+_\d+}}, unloaded
 // CHECK:STDOUT:   %Core.import_ref.5a7 = import_ref Core//prelude/types/optional, loc{{\d+_\d+}}, unloaded
 // CHECK:STDOUT:   %OptionalStorage.impl_witness_table.377 = impl_witness_table (%Core.import_ref.8c0, %Core.import_ref.566, %Core.import_ref.637, %Core.import_ref.6db, %Core.import_ref.5a7), @ptr.as.OptionalStorage.impl [concrete]
-// CHECK:STDOUT:   %TakesArray__carbon_thunk.decl: %TakesArray__carbon_thunk.type = fn_decl @TakesArray__carbon_thunk [concrete = constants.%TakesArray__carbon_thunk] {
+// CHECK:STDOUT:   %TakesArray.decl: %TakesArray.type = fn_decl @TakesArray [concrete = constants.%TakesArray] {
 // CHECK:STDOUT:     <elided>
 // CHECK:STDOUT:   } {
 // CHECK:STDOUT:     <elided>
@@ -207,7 +207,7 @@ fn F() {
 // CHECK:STDOUT:   %.loc11_18.2: init %Optional.8fd = converted %addr.loc11_18.1, %T.binding.as_type.as.ImplicitAs.impl.Convert.call
 // CHECK:STDOUT:   %.loc11_18.3: ref %Optional.8fd = temporary %.loc11_18.1, %.loc11_18.2
 // CHECK:STDOUT:   %.loc11_18.4: %Optional.8fd = bind_value %.loc11_18.3
-// CHECK:STDOUT:   %TakesArray__carbon_thunk.call: init %empty_tuple.type = call imports.%TakesArray__carbon_thunk.decl(%.loc11_18.4)
+// CHECK:STDOUT:   %TakesArray.call: init %empty_tuple.type = call imports.%TakesArray.decl(%.loc11_18.4)
 // CHECK:STDOUT:   %DestroyT.binding.as_type.as.Destroy.impl.Op.bound.loc11: <bound method> = bound_method %.loc11_18.3, constants.%DestroyT.binding.as_type.as.Destroy.impl.Op.af3
 // CHECK:STDOUT:   <elided>
 // CHECK:STDOUT:   %bound_method.loc11_18.3: <bound method> = bound_method %.loc11_18.3, %DestroyT.binding.as_type.as.Destroy.impl.Op.specific_fn.1
diff --git a/toolchain/check/testdata/interop/cpp/function/pointer.carbon b/toolchain/check/testdata/interop/cpp/function/pointer.carbon
index af7024e809317..2630edff12fe5 100644
--- a/toolchain/check/testdata/interop/cpp/function/pointer.carbon
+++ b/toolchain/check/testdata/interop/cpp/function/pointer.carbon
@@ -663,8 +663,8 @@ fn F() {
 // CHECK:STDOUT:   %OptionalStorage.impl_witness.707: <witness> = impl_witness imports.%OptionalStorage.impl_witness_table.377, @ptr.as.OptionalStorage.impl(%S) [concrete]
 // CHECK:STDOUT:   %OptionalStorage.facet: %OptionalStorage.type = facet_value %ptr.5c7, (%OptionalStorage.impl_witness.707) [concrete]
 // CHECK:STDOUT:   %Optional.143: type = class_type @Optional, @Optional(%OptionalStorage.facet) [concrete]
-// CHECK:STDOUT:   %foo__carbon_thunk.type: type = fn_type @foo__carbon_thunk [concrete]
-// CHECK:STDOUT:   %foo__carbon_thunk: %foo__carbon_thunk.type = struct_value () [concrete]
+// CHECK:STDOUT:   %foo.type: type = fn_type @foo [concrete]
+// CHECK:STDOUT:   %foo: %foo.type = struct_value () [concrete]
 // CHECK:STDOUT:   %ImplicitAs.type.06b: type = facet_type <@ImplicitAs, @ImplicitAs(%Optional.143)> [concrete]
 // CHECK:STDOUT:   %ImplicitAs.Convert.type.e1b: type = fn_type @ImplicitAs.Convert, @ImplicitAs(%Optional.143) [concrete]
 // CHECK:STDOUT:   %T.binding.as_type.as.ImplicitAs.impl.Convert.type.6f7: type = fn_type @T.binding.as_type.as.ImplicitAs.impl.Convert.1, @T.binding.as_type.as.ImplicitAs.impl.3a5(%T.76d) [symbolic]
@@ -699,7 +699,7 @@ fn F() {
 // CHECK:STDOUT:   %Core.import_ref.6db = import_ref Core//prelude/types/optional, loc{{\d+_\d+}}, unloaded
 // CHECK:STDOUT:   %Core.import_ref.5a7 = import_ref Core//prelude/types/optional, loc{{\d+_\d+}}, unloaded
 // CHECK:STDOUT:   %OptionalStorage.impl_witness_table.377 = impl_witness_table (%Core.import_ref.8c0, %Core.import_ref.566, %Core.import_ref.637, %Core.import_ref.6db, %Core.import_ref.5a7), @ptr.as.OptionalStorage.impl [concrete]
-// CHECK:STDOUT:   %foo__carbon_thunk.decl: %foo__carbon_thunk.type = fn_decl @foo__carbon_thunk [concrete = constants.%foo__carbon_thunk] {
+// CHECK:STDOUT:   %foo.decl: %foo.type = fn_decl @foo [concrete = constants.%foo] {
 // CHECK:STDOUT:     <elided>
 // CHECK:STDOUT:   } {
 // CHECK:STDOUT:     <elided>
@@ -743,7 +743,7 @@ fn F() {
 // CHECK:STDOUT:   %.loc9_11.2: init %Optional.143 = converted %addr.loc9_11.1, %T.binding.as_type.as.ImplicitAs.impl.Convert.call
 // CHECK:STDOUT:   %.loc9_11.3: ref %Optional.143 = temporary %.loc9_11.1, %.loc9_11.2
 // CHECK:STDOUT:   %.loc9_11.4: %Optional.143 = bind_value %.loc9_11.3
-// CHECK:STDOUT:   %foo__carbon_thunk.call: init %empty_tuple.type = call imports.%foo__carbon_thunk.decl(%.loc9_11.4)
+// CHECK:STDOUT:   %foo.call: init %empty_tuple.type = call imports.%foo.decl(%.loc9_11.4)
 // CHECK:STDOUT:   %DestroyT.binding.as_type.as.Destroy.impl.Op.bound.loc9: <bound method> = bound_method %.loc9_11.3, constants.%DestroyT.binding.as_type.as.Destroy.impl.Op.465
 // CHECK:STDOUT:   <elided>
 // CHECK:STDOUT:   %bound_method.loc9_11.3: <bound method> = bound_method %.loc9_11.3, %DestroyT.binding.as_type.as.Destroy.impl.Op.specific_fn.1
@@ -925,8 +925,8 @@ fn F() {
 // CHECK:STDOUT:   %OptionalStorage.facet: %OptionalStorage.type = facet_value %ptr.5c7, (%OptionalStorage.impl_witness.707) [concrete]
 // CHECK:STDOUT:   %Optional.143: type = class_type @Optional, @Optional(%OptionalStorage.facet) [concrete]
 // CHECK:STDOUT:   %pattern_type.aa5: type = pattern_type %Optional.143 [concrete]
-// CHECK:STDOUT:   %Direct__carbon_thunk.type: type = fn_type @Direct__carbon_thunk [concrete]
-// CHECK:STDOUT:   %Direct__carbon_thunk: %Direct__carbon_thunk.type = struct_value () [concrete]
+// CHECK:STDOUT:   %Direct.type: type = fn_type @Direct [concrete]
+// CHECK:STDOUT:   %Direct: %Direct.type = struct_value () [concrete]
 // CHECK:STDOUT:   %ImplicitAs.type.cc7: type = generic_interface_type @ImplicitAs [concrete]
 // CHECK:STDOUT:   %ImplicitAs.generic: %ImplicitAs.type.cc7 = struct_value () [concrete]
 // CHECK:STDOUT:   %ImplicitAs.type.06b: type = facet_type <@ImplicitAs, @ImplicitAs(%Optional.143)> [concrete]
@@ -977,7 +977,7 @@ fn F() {
 // CHECK:STDOUT:   %Core.import_ref.6db = import_ref Core//prelude/types/optional, loc{{\d+_\d+}}, unloaded
 // CHECK:STDOUT:   %Core.import_ref.5a7 = import_ref Core//prelude/types/optional, loc{{\d+_\d+}}, unloaded
 // CHECK:STDOUT:   %OptionalStorage.impl_witness_table.377 = impl_witness_table (%Core.import_ref.8c0, %Core.import_ref.566, %Core.import_ref.637, %Core.import_ref.6db, %Core.import_ref.5a7), @ptr.as.OptionalStorage.impl [concrete]
-// CHECK:STDOUT:   %Direct__carbon_thunk.decl: %Direct__carbon_thunk.type = fn_decl @Direct__carbon_thunk [concrete = constants.%Direct__carbon_thunk] {
+// CHECK:STDOUT:   %Direct.decl: %Direct.type = fn_decl @Direct [concrete = constants.%Direct] {
 // CHECK:STDOUT:     <elided>
 // CHECK:STDOUT:   } {
 // CHECK:STDOUT:     %OptionalStorage.facet.loc11_16.1: %OptionalStorage.type = facet_value constants.%ptr.5c7, (constants.%OptionalStorage.impl_witness.707) [concrete = constants.%OptionalStorage.facet]
@@ -1036,8 +1036,8 @@ fn F() {
 // CHECK:STDOUT:   %.loc11_14.2: init %Optional.143 = converted %addr.loc11_14.1, %T.binding.as_type.as.ImplicitAs.impl.Convert.call
 // CHECK:STDOUT:   %.loc11_14.3: ref %Optional.143 = temporary %.loc11_14.1, %.loc11_14.2
 // CHECK:STDOUT:   %.loc11_14.4: %Optional.143 = bind_value %.loc11_14.3
-// CHECK:STDOUT:   %Direct__carbon_thunk.call: init %Optional.143 = call imports.%Direct__carbon_thunk.decl(%.loc11_14.4) to %.loc11_16.1
-// CHECK:STDOUT:   %.loc11_16.2: ref %Optional.143 = temporary %.loc11_16.1, %Direct__carbon_thunk.call
+// CHECK:STDOUT:   %Direct.call: init %Optional.143 = call imports.%Direct.decl(%.loc11_14.4) to %.loc11_16.1
+// CHECK:STDOUT:   %.loc11_16.2: ref %Optional.143 = temporary %.loc11_16.1, %Direct.call
 // CHECK:STDOUT:   name_binding_decl {
 // CHECK:STDOUT:     %a.patt: %pattern_type.aa5 = value_binding_pattern a [concrete]
 // CHECK:STDOUT:   }
diff --git a/toolchain/lower/testdata/interop/cpp/parameters.carbon b/toolchain/lower/testdata/interop/cpp/parameters.carbon
index 9ebc2832d64d4..70a357b747b53 100644
--- a/toolchain/lower/testdata/interop/cpp/parameters.carbon
+++ b/toolchain/lower/testdata/interop/cpp/parameters.carbon
@@ -121,49 +121,43 @@ fn PassValueExpr(y: Cpp.Y) {
 // CHECK:STDOUT:   %.loc7_22.3.temp = alloca i16, align 2, !dbg !11
 // CHECK:STDOUT:   %.loc9_21.3.temp = alloca i8, align 1, !dbg !12
 // CHECK:STDOUT:   %.loc9_24.3.temp = alloca i16, align 2, !dbg !13
-// CHECK:STDOUT:   %.loc9_27.3.temp = alloca i32, align 4, !dbg !14
-// CHECK:STDOUT:   %.loc9_30.3.temp = alloca i64, align 8, !dbg !15
 // CHECK:STDOUT:   call void @llvm.lifetime.start.p0(ptr %.loc7_19.3.temp), !dbg !10
 // CHECK:STDOUT:   store i8 1, ptr %.loc7_19.3.temp, align 1, !dbg !10
 // CHECK:STDOUT:   call void @llvm.lifetime.start.p0(ptr %.loc7_22.3.temp), !dbg !11
 // CHECK:STDOUT:   store i16 2, ptr %.loc7_22.3.temp, align 2, !dbg !11
-// CHECK:STDOUT:   call void @_Z11pass_signedasil.carbon_thunk(ptr %.loc7_19.3.temp, ptr %.loc7_22.3.temp, i32 3, i64 4), !dbg !16
+// CHECK:STDOUT:   call void @_Z11pass_signedasil.carbon_thunk(ptr %.loc7_19.3.temp, ptr %.loc7_22.3.temp, i32 3, i64 4), !dbg !14
 // CHECK:STDOUT:   call void @llvm.lifetime.start.p0(ptr %.loc9_21.3.temp), !dbg !12
 // CHECK:STDOUT:   store i8 1, ptr %.loc9_21.3.temp, align 1, !dbg !12
 // CHECK:STDOUT:   call void @llvm.lifetime.start.p0(ptr %.loc9_24.3.temp), !dbg !13
 // CHECK:STDOUT:   store i16 2, ptr %.loc9_24.3.temp, align 2, !dbg !13
-// CHECK:STDOUT:   call void @llvm.lifetime.start.p0(ptr %.loc9_27.3.temp), !dbg !14
-// CHECK:STDOUT:   store i32 3, ptr %.loc9_27.3.temp, align 4, !dbg !14
-// CHECK:STDOUT:   call void @llvm.lifetime.start.p0(ptr %.loc9_30.3.temp), !dbg !15
-// CHECK:STDOUT:   store i64 4, ptr %.loc9_30.3.temp, align 4, !dbg !15
-// CHECK:STDOUT:   call void @_Z13pass_unsignedhtjm.carbon_thunk(ptr %.loc9_21.3.temp, ptr %.loc9_24.3.temp, ptr %.loc9_27.3.temp, ptr %.loc9_30.3.temp), !dbg !17
-// CHECK:STDOUT:   ret void, !dbg !18
+// CHECK:STDOUT:   call void @_Z13pass_unsignedhtjm.carbon_thunk(ptr %.loc9_21.3.temp, ptr %.loc9_24.3.temp, i32 3, i64 4), !dbg !15
+// CHECK:STDOUT:   ret void, !dbg !16
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: declare i16 @_CMakeShort.Main()
 // CHECK:STDOUT:
-// CHECK:STDOUT: define void @_CPassShort.Main(i16 %a, ptr %b) !dbg !19 {
+// CHECK:STDOUT: define void @_CPassShort.Main(i16 %a, ptr %b) !dbg !17 {
 // CHECK:STDOUT: entry:
-// CHECK:STDOUT:   %.loc17_18.1.temp = alloca i16, align 2, !dbg !20
-// CHECK:STDOUT:   %.loc18_18.3.temp = alloca i16, align 2, !dbg !21
-// CHECK:STDOUT:   %.loc19_18.2.temp = alloca i16, align 2, !dbg !22
-// CHECK:STDOUT:   %.loc20_28.3.temp = alloca i16, align 2, !dbg !23
-// CHECK:STDOUT:   call void @llvm.lifetime.start.p0(ptr %.loc17_18.1.temp), !dbg !20
-// CHECK:STDOUT:   store i16 %a, ptr %.loc17_18.1.temp, align 2, !dbg !20
-// CHECK:STDOUT:   call void @_Z10pass_shorts.carbon_thunk(ptr %.loc17_18.1.temp), !dbg !24
-// CHECK:STDOUT:   %.loc18_18.2 = load i16, ptr %b, align 2, !dbg !21
-// CHECK:STDOUT:   call void @llvm.lifetime.start.p0(ptr %.loc18_18.3.temp), !dbg !21
-// CHECK:STDOUT:   store i16 %.loc18_18.2, ptr %.loc18_18.3.temp, align 2, !dbg !21
-// CHECK:STDOUT:   call void @_Z10pass_shorts.carbon_thunk(ptr %.loc18_18.3.temp), !dbg !25
-// CHECK:STDOUT:   %.loc19_18.1 = load i16, ptr @_Cc.Main, align 2, !dbg !22
-// CHECK:STDOUT:   call void @llvm.lifetime.start.p0(ptr %.loc19_18.2.temp), !dbg !22
-// CHECK:STDOUT:   store i16 %.loc19_18.1, ptr %.loc19_18.2.temp, align 2, !dbg !22
-// CHECK:STDOUT:   call void @_Z10pass_shorts.carbon_thunk(ptr %.loc19_18.2.temp), !dbg !26
-// CHECK:STDOUT:   %MakeShort.call = call i16 @_CMakeShort.Main(), !dbg !23
-// CHECK:STDOUT:   call void @llvm.lifetime.start.p0(ptr %.loc20_28.3.temp), !dbg !23
-// CHECK:STDOUT:   store i16 %MakeShort.call, ptr %.loc20_28.3.temp, align 2, !dbg !23
-// CHECK:STDOUT:   call void @_Z10pass_shorts.carbon_thunk(ptr %.loc20_28.3.temp), !dbg !27
-// CHECK:STDOUT:   ret void, !dbg !28
+// CHECK:STDOUT:   %.loc17_18.1.temp = alloca i16, align 2, !dbg !18
+// CHECK:STDOUT:   %.loc18_18.3.temp = alloca i16, align 2, !dbg !19
+// CHECK:STDOUT:   %.loc19_18.2.temp = alloca i16, align 2, !dbg !20
+// CHECK:STDOUT:   %.loc20_28.3.temp = alloca i16, align 2, !dbg !21
+// CHECK:STDOUT:   call void @llvm.lifetime.start.p0(ptr %.loc17_18.1.temp), !dbg !18
+// CHECK:STDOUT:   store i16 %a, ptr %.loc17_18.1.temp, align 2, !dbg !18
+// CHECK:STDOUT:   call void @_Z10pass_shorts.carbon_thunk(ptr %.loc17_18.1.temp), !dbg !22
+// CHECK:STDOUT:   %.loc18_18.2 = load i16, ptr %b, align 2, !dbg !19
+// CHECK:STDOUT:   call void @llvm.lifetime.start.p0(ptr %.loc18_18.3.temp), !dbg !19
+// CHECK:STDOUT:   store i16 %.loc18_18.2, ptr %.loc18_18.3.temp, align 2, !dbg !19
+// CHECK:STDOUT:   call void @_Z10pass_shorts.carbon_thunk(ptr %.loc18_18.3.temp), !dbg !23
+// CHECK:STDOUT:   %.loc19_18.1 = load i16, ptr @_Cc.Main, align 2, !dbg !20
+// CHECK:STDOUT:   call void @llvm.lifetime.start.p0(ptr %.loc19_18.2.temp), !dbg !20
+// CHECK:STDOUT:   store i16 %.loc19_18.1, ptr %.loc19_18.2.temp, align 2, !dbg !20
+// CHECK:STDOUT:   call void @_Z10pass_shorts.carbon_thunk(ptr %.loc19_18.2.temp), !dbg !24
+// CHECK:STDOUT:   %MakeShort.call = call i16 @_CMakeShort.Main(), !dbg !21
+// CHECK:STDOUT:   call void @llvm.lifetime.start.p0(ptr %.loc20_28.3.temp), !dbg !21
+// CHECK:STDOUT:   store i16 %MakeShort.call, ptr %.loc20_28.3.temp, align 2, !dbg !21
+// CHECK:STDOUT:   call void @_Z10pass_shorts.carbon_thunk(ptr %.loc20_28.3.temp), !dbg !25
+// CHECK:STDOUT:   ret void, !dbg !26
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: ; Function Attrs: nocallback nofree nosync nounwind willreturn memory(argmem: readwrite)
@@ -193,25 +187,23 @@ fn PassValueExpr(y: Cpp.Y) {
 // CHECK:STDOUT: declare void @_Z11pass_signedasil(i8 signext, i16 signext, i32, i64) #2
 // CHECK:STDOUT:
 // CHECK:STDOUT: ; Function Attrs: alwaysinline mustprogress
-// CHECK:STDOUT: define dso_local void @_Z13pass_unsignedhtjm.carbon_thunk(ptr %0, ptr %1, ptr %2, ptr %3) #1 {
+// CHECK:STDOUT: define dso_local void @_Z13pass_unsignedhtjm.carbon_thunk(ptr %0, ptr %1, i32 %2, i64 %3) #1 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   %.addr = alloca ptr, align 8
 // CHECK:STDOUT:   %.addr1 = alloca ptr, align 8
-// CHECK:STDOUT:   %.addr2 = alloca ptr, align 8
-// CHECK:STDOUT:   %.addr3 = alloca ptr, align 8
+// CHECK:STDOUT:   %.addr2 = alloca i32, align 4
+// CHECK:STDOUT:   %.addr3 = alloca i64, align 8
 // CHECK:STDOUT:   store ptr %0, ptr %.addr, align 8
 // CHECK:STDOUT:   store ptr %1, ptr %.addr1, align 8
-// CHECK:STDOUT:   store ptr %2, ptr %.addr2, align 8
-// CHECK:STDOUT:   store ptr %3, ptr %.addr3, align 8
+// CHECK:STDOUT:   store i32 %2, ptr %.addr2, align 4
+// CHECK:STDOUT:   store i64 %3, ptr %.addr3, align 8
 // CHECK:STDOUT:   %4 = load ptr, ptr %.addr, align 8
 // CHECK:STDOUT:   %5 = load i8, ptr %4, align 1
 // CHECK:STDOUT:   %6 = load ptr, ptr %.addr1, align 8
 // CHECK:STDOUT:   %7 = load i16, ptr %6, align 2
-// CHECK:STDOUT:   %8 = load ptr, ptr %.addr2, align 8
-// CHECK:STDOUT:   %9 = load i32, ptr %8, align 4
-// CHECK:STDOUT:   %10 = load ptr, ptr %.addr3, align 8
-// CHECK:STDOUT:   %11 = load i64, ptr %10, align 8
-// CHECK:STDOUT:   call void @_Z13pass_unsignedhtjm(i8 zeroext %5, i16 zeroext %7, i32 %9, i64 %11)
+// CHECK:STDOUT:   %8 = load i32, ptr %.addr2, align 4
+// CHECK:STDOUT:   %9 = load i64, ptr %.addr3, align 8
+// CHECK:STDOUT:   call void @_Z13pass_unsignedhtjm(i8 zeroext %5, i16 zeroext %7, i32 %8, i64 %9)
 // CHECK:STDOUT:   ret void
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
@@ -231,7 +223,7 @@ fn PassValueExpr(y: Cpp.Y) {
 // CHECK:STDOUT: declare void @_Z10pass_shorts(i16 signext) #2
 // CHECK:STDOUT:
 // CHECK:STDOUT: ; uselistorder directives
-// CHECK:STDOUT: uselistorder ptr @llvm.lifetime.start.p0, { 9, 8, 7, 6, 5, 4, 3, 2, 1, 0 }
+// CHECK:STDOUT: uselistorder ptr @llvm.lifetime.start.p0, { 7, 6, 5, 4, 3, 2, 1, 0 }
 // CHECK:STDOUT:
 // CHECK:STDOUT: attributes #0 = { nocallback nofree nosync nounwind willreturn memory(argmem: readwrite) }
 // CHECK:STDOUT: attributes #1 = { alwaysinline mustprogress "min-legal-vector-width"="0" "no-trapping-math"="true" "stack-protector-buffer-size"="0" "target-cpu"="x86-64" "target-features"="+cmov,+cx8,+fxsr,+mmx,+sse,+sse2,+x87" "tune-cpu"="generic" }
@@ -254,21 +246,19 @@ fn PassValueExpr(y: Cpp.Y) {
 // CHECK:STDOUT: !11 = !DILocation(line: 7, column: 22, scope: !7)
 // CHECK:STDOUT: !12 = !DILocation(line: 9, column: 21, scope: !7)
 // CHECK:STDOUT: !13 = !DILocation(line: 9, column: 24, scope: !7)
-// CHECK:STDOUT: !14 = !DILocation(line: 9, column: 27, scope: !7)
-// CHECK:STDOUT: !15 = !DILocation(line: 9, column: 30, scope: !7)
-// CHECK:STDOUT: !16 = !DILocation(line: 7, column: 3, scope: !7)
-// CHECK:STDOUT: !17 = !DILocation(line: 9, column: 3, scope: !7)
-// CHECK:STDOUT: !18 = !DILocation(line: 6, column: 1, scope: !7)
-// CHECK:STDOUT: !19 = distinct !DISubprogram(name: "PassShort", linkageName: "_CPassShort.Main", scope: null, file: !6, line: 16, type: !8, spFlags: DISPFlagDefinition, unit: !5)
-// CHECK:STDOUT: !20 = !DILocation(line: 17, column: 18, scope: !19)
-// CHECK:STDOUT: !21 = !DILocation(line: 18, column: 18, scope: !19)
-// CHECK:STDOUT: !22 = !DILocation(line: 19, column: 18, scope: !19)
-// CHECK:STDOUT: !23 = !DILocation(line: 20, column: 18, scope: !19)
-// CHECK:STDOUT: !24 = !DILocation(line: 17, column: 3, scope: !19)
-// CHECK:STDOUT: !25 = !DILocation(line: 18, column: 3, scope: !19)
-// CHECK:STDOUT: !26 = !DILocation(line: 19, column: 3, scope: !19)
-// CHECK:STDOUT: !27 = !DILocation(line: 20, column: 3, scope: !19)
-// CHECK:STDOUT: !28 = !DILocation(line: 16, column: 1, scope: !19)
+// CHECK:STDOUT: !14 = !DILocation(line: 7, column: 3, scope: !7)
+// CHECK:STDOUT: !15 = !DILocation(line: 9, column: 3, scope: !7)
+// CHECK:STDOUT: !16 = !DILocation(line: 6, column: 1, scope: !7)
+// CHECK:STDOUT: !17 = distinct !DISubprogram(name: "PassShort", linkageName: "_CPassShort.Main", scope: null, file: !6, line: 16, type: !8, spFlags: DISPFlagDefinition, unit: !5)
+// CHECK:STDOUT: !18 = !DILocation(line: 17, column: 18, scope: !17)
+// CHECK:STDOUT: !19 = !DILocation(line: 18, column: 18, scope: !17)
+// CHECK:STDOUT: !20 = !DILocation(line: 19, column: 18, scope: !17)
+// CHECK:STDOUT: !21 = !DILocation(line: 20, column: 18, scope: !17)
+// CHECK:STDOUT: !22 = !DILocation(line: 17, column: 3, scope: !17)
+// CHECK:STDOUT: !23 = !DILocation(line: 18, column: 3, scope: !17)
+// CHECK:STDOUT: !24 = !DILocation(line: 19, column: 3, scope: !17)
+// CHECK:STDOUT: !25 = !DILocation(line: 20, column: 3, scope: !17)
+// CHECK:STDOUT: !26 = !DILocation(line: 16, column: 1, scope: !17)
 // CHECK:STDOUT: ; ModuleID = 'import_struct.carbon'
 // CHECK:STDOUT: source_filename = "import_struct.carbon"
 // CHECK:STDOUT: target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-i128:128-f80:128-n8:16:32:64-S128"
diff --git a/toolchain/lower/testdata/interop/cpp/reference.carbon b/toolchain/lower/testdata/interop/cpp/reference.carbon
index 5f1cc90535f41..efbd96a8fa4df 100644
--- a/toolchain/lower/testdata/interop/cpp/reference.carbon
+++ b/toolchain/lower/testdata/interop/cpp/reference.carbon
@@ -222,24 +222,27 @@ fn GetRefs() {
 // CHECK:STDOUT:   %c3.var = alloca ptr, align 8, !dbg !12
 // CHECK:STDOUT:   %n1.var = alloca ptr, align 8, !dbg !13
 // CHECK:STDOUT:   %n2.var = alloca i32, align 4, !dbg !14
-// CHECK:STDOUT:   %n3.var = alloca ptr, align 8, !dbg !15
+// CHECK:STDOUT:   %.loc22_35.1.temp = alloca i32, align 4, !dbg !15
+// CHECK:STDOUT:   %n3.var = alloca ptr, align 8, !dbg !16
 // CHECK:STDOUT:   call void @llvm.lifetime.start.p0(ptr %c1.var), !dbg !10
-// CHECK:STDOUT:   %ReturnCRef.call = call ptr @_Z10ReturnCRefv(), !dbg !16
+// CHECK:STDOUT:   %ReturnCRef.call = call ptr @_Z10ReturnCRefv(), !dbg !17
 // CHECK:STDOUT:   store ptr %ReturnCRef.call, ptr %c1.var, align 8, !dbg !10
 // CHECK:STDOUT:   call void @llvm.lifetime.start.p0(ptr %c2.var), !dbg !11
-// CHECK:STDOUT:   call void @_Z11ReturnCRRefv.carbon_thunk(ptr %c2.var), !dbg !17
+// CHECK:STDOUT:   call void @_Z11ReturnCRRefv.carbon_thunk(ptr %c2.var), !dbg !18
 // CHECK:STDOUT:   call void @llvm.lifetime.start.p0(ptr %c3.var), !dbg !12
-// CHECK:STDOUT:   %ReturnConstCRef.call = call ptr @_Z15ReturnConstCRefv(), !dbg !18
+// CHECK:STDOUT:   %ReturnConstCRef.call = call ptr @_Z15ReturnConstCRefv(), !dbg !19
 // CHECK:STDOUT:   store ptr %ReturnConstCRef.call, ptr %c3.var, align 8, !dbg !12
 // CHECK:STDOUT:   call void @llvm.lifetime.start.p0(ptr %n1.var), !dbg !13
-// CHECK:STDOUT:   %ReturnIntRef.call = call ptr @_Z12ReturnIntRefv(), !dbg !19
+// CHECK:STDOUT:   %ReturnIntRef.call = call ptr @_Z12ReturnIntRefv(), !dbg !20
 // CHECK:STDOUT:   store ptr %ReturnIntRef.call, ptr %n1.var, align 8, !dbg !13
 // CHECK:STDOUT:   call void @llvm.lifetime.start.p0(ptr %n2.var), !dbg !14
-// CHECK:STDOUT:   %ReturnIntRRef.call = call i32 @_Z13ReturnIntRRefv(), !dbg !20
-// CHECK:STDOUT:   store i32 %ReturnIntRRef.call, ptr %n2.var, align 4, !dbg !14
-// CHECK:STDOUT:   call void @llvm.lifetime.start.p0(ptr %n3.var), !dbg !15
+// CHECK:STDOUT:   call void @llvm.lifetime.start.p0(ptr %.loc22_35.1.temp), !dbg !15
+// CHECK:STDOUT:   call void @_Z13ReturnIntRRefv.carbon_thunk(ptr %.loc22_35.1.temp), !dbg !15
+// CHECK:STDOUT:   %.loc22_35.2 = load i32, ptr %.loc22_35.1.temp, align 4, !dbg !15
+// CHECK:STDOUT:   store i32 %.loc22_35.2, ptr %n2.var, align 4, !dbg !14
+// CHECK:STDOUT:   call void @llvm.lifetime.start.p0(ptr %n3.var), !dbg !16
 // CHECK:STDOUT:   %ReturnConstIntRef.call = call ptr @_Z17ReturnConstIntRefv(), !dbg !21
-// CHECK:STDOUT:   store ptr %ReturnConstIntRef.call, ptr %n3.var, align 8, !dbg !15
+// CHECK:STDOUT:   store ptr %ReturnConstIntRef.call, ptr %n3.var, align 8, !dbg !16
 // CHECK:STDOUT:   ret void, !dbg !22
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
@@ -249,8 +252,6 @@ fn GetRefs() {
 // CHECK:STDOUT:
 // CHECK:STDOUT: declare ptr @_Z12ReturnIntRefv()
 // CHECK:STDOUT:
-// CHECK:STDOUT: declare i32 @_Z13ReturnIntRRefv()
-// CHECK:STDOUT:
 // CHECK:STDOUT: declare ptr @_Z17ReturnConstIntRefv()
 // CHECK:STDOUT:
 // CHECK:STDOUT: ; Function Attrs: nocallback nofree nosync nounwind willreturn memory(argmem: readwrite)
@@ -268,8 +269,22 @@ fn GetRefs() {
 // CHECK:STDOUT:
 // CHECK:STDOUT: declare nonnull align 1 dereferenceable(1) ptr @_Z11ReturnCRRefv() #2
 // CHECK:STDOUT:
+// CHECK:STDOUT: ; Function Attrs: alwaysinline mustprogress
+// CHECK:STDOUT: define dso_local void @_Z13ReturnIntRRefv.carbon_thunk(ptr %return) #1 {
+// CHECK:STDOUT: entry:
+// CHECK:STDOUT:   %return.addr = alloca ptr, align 8
+// CHECK:STDOUT:   store ptr %return, ptr %return.addr, align 8
+// CHECK:STDOUT:   %0 = load ptr, ptr %return.addr, align 8
+// CHECK:STDOUT:   %call = call nonnull align 4 dereferenceable(4) ptr @_Z13ReturnIntRRefv()
+// CHECK:STDOUT:   %1 = load i32, ptr %call, align 4
+// CHECK:STDOUT:   store i32 %1, ptr %0, align 4
+// CHECK:STDOUT:   ret void
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: declare nonnull align 4 dereferenceable(4) ptr @_Z13ReturnIntRRefv() #2
+// CHECK:STDOUT:
 // CHECK:STDOUT: ; uselistorder directives
-// CHECK:STDOUT: uselistorder ptr @llvm.lifetime.start.p0, { 5, 4, 3, 2, 1, 0 }
+// CHECK:STDOUT: uselistorder ptr @llvm.lifetime.start.p0, { 6, 5, 4, 3, 2, 1, 0 }
 // CHECK:STDOUT:
 // CHECK:STDOUT: attributes #0 = { nocallback nofree nosync nounwind willreturn memory(argmem: readwrite) }
 // CHECK:STDOUT: attributes #1 = { alwaysinline mustprogress "min-legal-vector-width"="0" "no-trapping-math"="true" "stack-protector-buffer-size"="0" "target-cpu"="x86-64" "target-features"="+cmov,+cx8,+fxsr,+mmx,+sse,+sse2,+x87" "tune-cpu"="generic" }
@@ -293,11 +308,11 @@ fn GetRefs() {
 // CHECK:STDOUT: !12 = !DILocation(line: 19, column: 3, scope: !7)
 // CHECK:STDOUT: !13 = !DILocation(line: 21, column: 3, scope: !7)
 // CHECK:STDOUT: !14 = !DILocation(line: 22, column: 3, scope: !7)
-// CHECK:STDOUT: !15 = !DILocation(line: 23, column: 3, scope: !7)
-// CHECK:STDOUT: !16 = !DILocation(line: 17, column: 20, scope: !7)
-// CHECK:STDOUT: !17 = !DILocation(line: 18, column: 19, scope: !7)
-// CHECK:STDOUT: !18 = !DILocation(line: 19, column: 26, scope: !7)
-// CHECK:STDOUT: !19 = !DILocation(line: 21, column: 18, scope: !7)
-// CHECK:STDOUT: !20 = !DILocation(line: 22, column: 17, scope: !7)
+// CHECK:STDOUT: !15 = !DILocation(line: 22, column: 17, scope: !7)
+// CHECK:STDOUT: !16 = !DILocation(line: 23, column: 3, scope: !7)
+// CHECK:STDOUT: !17 = !DILocation(line: 17, column: 20, scope: !7)
+// CHECK:STDOUT: !18 = !DILocation(line: 18, column: 19, scope: !7)
+// CHECK:STDOUT: !19 = !DILocation(line: 19, column: 26, scope: !7)
+// CHECK:STDOUT: !20 = !DILocation(line: 21, column: 18, scope: !7)
 // CHECK:STDOUT: !21 = !DILocation(line: 23, column: 24, scope: !7)
 // CHECK:STDOUT: !22 = !DILocation(line: 16, column: 1, scope: !7)