Import vtable_ptr lazily (#5762)

Ensure `vtable_ptr`s(and the vtables they refer to) aren't
imported if the type is imported but the vtable isn't
needed (no initialization of a value of that type is required).

toolchain/check/convert.cpp

@@ -18,6 +18,7 @@
 #include "toolchain/check/diagnostic_helpers.h"
 #include "toolchain/check/eval.h"
 #include "toolchain/check/impl_lookup.h"
+#include "toolchain/check/import_ref.h"
 #include "toolchain/check/inst.h"
 #include "toolchain/check/operator.h"
 #include "toolchain/check/pattern_match.h"
@@ -586,12 +587,19 @@ static auto ConvertStructToClass(
         SemIR::LocId(value_id), {.type_id = target.type_id});
   }
 
+  if (!dest_vtable_ptr_inst_id.has_value()) {
+    dest_vtable_ptr_inst_id = dest_class_info.vtable_ptr_id;
+  }
+
+  if (dest_vtable_ptr_inst_id.has_value()) {
+    LoadImportRef(context, dest_vtable_ptr_inst_id);
+  }
+
   auto result_id = ConvertStructToStructOrClass<SemIR::ClassElementAccess>(
       context, src_type, dest_struct_type, value_id, target,
       // TODO: Pass down the specific_id of the passed in
       // dest_vtable_ptr_inst_id, or from the dest_type.specific_id.
-      dest_vtable_ptr_inst_id.has_value() ? dest_vtable_ptr_inst_id
-                                          : dest_class_info.vtable_ptr_id);
+      dest_vtable_ptr_inst_id);
 
   if (need_temporary) {
     target_block.InsertHere();

toolchain/check/import_ref.cpp

@@ -1717,8 +1717,8 @@ static auto TryResolveTypedInst(ImportRefResolver& resolver,
   // it's needed, not for every use of the class.
   auto vtable_ptr_const_id =
       import_class.vtable_ptr_id.has_value()
-          ? GetLocalConstantId(resolver, import_class.vtable_ptr_id)
-          : SemIR::ConstantId::None;
+          ? AddImportRef(resolver, import_class.vtable_ptr_id)
+          : SemIR::InstId::None;
 
   if (resolver.HasNewWork()) {
     return ResolveResult::Retry(class_const_id, new_class.first_decl_id());
@@ -1742,17 +1742,9 @@ static auto TryResolveTypedInst(ImportRefResolver& resolver,
         GetSingletonType(resolver.local_context(),
                          SemIR::WitnessType::TypeInstId),
         import_class.complete_type_witness_id, complete_type_witness_const_id);
-    auto vtable_ptr_id =
-        vtable_ptr_const_id.has_value()
-            ? AddLoadedImportRef(
-                  resolver,
-                  GetSingletonType(resolver.local_context(),
-                                   SemIR::WitnessType::TypeInstId),
-                  import_class.vtable_ptr_id, vtable_ptr_const_id)
-            : SemIR::InstId::None;
     AddClassDefinition(resolver, import_class, new_class,
                        complete_type_witness_id, base_id, adapt_id,
-                       vtable_ptr_id);
+                       vtable_ptr_const_id);
   }
 
   return ResolveResult::Done(class_const_id, new_class.first_decl_id());

toolchain/check/testdata/class/virtual_modifiers.carbon

@@ -339,6 +339,14 @@ base class T1(T:! type) {
   virtual fn F[self: Self](t: T);
 }
 
+// --- vtable_import_unneeded.carbon
+
+library "[[@TEST_NAME]]";
+
+import Modifiers;
+
+fn F(b: Modifiers.Base);
+
 
 // CHECK:STDOUT: --- modifiers.carbon
 // CHECK:STDOUT:
@@ -447,17 +455,17 @@ base class T1(T:! type) {
 // CHECK:STDOUT: constants {
 // CHECK:STDOUT:   %Derived: type = class_type @Derived [concrete]
 // CHECK:STDOUT:   %Base: type = class_type @Base [concrete]
-// CHECK:STDOUT:   %H.type.09e: type = fn_type @H.1 [concrete]
-// CHECK:STDOUT:   %empty_tuple.type: type = tuple_type () [concrete]
-// CHECK:STDOUT:   %H.c1b: %H.type.09e = struct_value () [concrete]
 // CHECK:STDOUT:   %ptr.454: type = ptr_type <vtable> [concrete]
-// CHECK:STDOUT:   %Base.vtable_ptr: ref %ptr.454 = vtable_ptr @Base.vtable [concrete]
 // CHECK:STDOUT:   %struct_type.vptr: type = struct_type {.<vptr>: %ptr.454} [concrete]
 // CHECK:STDOUT:   %complete_type.513: <witness> = complete_type_witness %struct_type.vptr [concrete]
 // CHECK:STDOUT:   %Derived.elem: type = unbound_element_type %Derived, %Base [concrete]
 // CHECK:STDOUT:   %pattern_type.fb9: type = pattern_type %Derived [concrete]
-// CHECK:STDOUT:   %H.type.dba: type = fn_type @H.2 [concrete]
+// CHECK:STDOUT:   %H.type.dba: type = fn_type @H.1 [concrete]
+// CHECK:STDOUT:   %empty_tuple.type: type = tuple_type () [concrete]
 // CHECK:STDOUT:   %H.bce: %H.type.dba = struct_value () [concrete]
+// CHECK:STDOUT:   %H.type.09e: type = fn_type @H.2 [concrete]
+// CHECK:STDOUT:   %H.c1b: %H.type.09e = struct_value () [concrete]
+// CHECK:STDOUT:   %Base.vtable_ptr: ref %ptr.454 = vtable_ptr @Base.vtable [concrete]
 // CHECK:STDOUT:   %Derived.vtable_ptr: ref %ptr.454 = vtable_ptr @Derived.vtable [concrete]
 // CHECK:STDOUT:   %struct_type.base.96c: type = struct_type {.base: %Base} [concrete]
 // CHECK:STDOUT:   %complete_type.0e2: <witness> = complete_type_witness %struct_type.base.96c [concrete]
@@ -492,8 +500,8 @@ base class T1(T:! type) {
 // CHECK:STDOUT:     import Modifiers//default
 // CHECK:STDOUT:   }
 // CHECK:STDOUT:   %Modifiers.Base: type = import_ref Modifiers//default, Base, loaded [concrete = constants.%Base]
+// CHECK:STDOUT:   %Modifiers.import_ref.be7: ref %ptr.454 = import_ref Modifiers//default, loc6_1, loaded [concrete = constants.%Base.vtable_ptr]
 // CHECK:STDOUT:   %Modifiers.import_ref.05e: <witness> = import_ref Modifiers//default, loc6_1, loaded [concrete = constants.%complete_type.513]
-// CHECK:STDOUT:   %Modifiers.import_ref.4e7: ref <witness> = import_ref Modifiers//default, loc6_1, loaded [concrete = constants.%Base.vtable_ptr]
 // CHECK:STDOUT:   %Modifiers.import_ref.1f3 = import_ref Modifiers//default, inst18 [no loc], unloaded
 // CHECK:STDOUT:   %Modifiers.import_ref.2cc = import_ref Modifiers//default, loc5_29, unloaded
 // CHECK:STDOUT:   %Core.Destroy: type = import_ref Core//prelude/parts/destroy, Destroy, loaded [concrete = constants.%Destroy.type]
@@ -518,7 +526,7 @@ base class T1(T:! type) {
 // CHECK:STDOUT:   %Modifiers.ref: <namespace> = name_ref Modifiers, imports.%Modifiers [concrete = imports.%Modifiers]
 // CHECK:STDOUT:   %Base.ref: type = name_ref Base, imports.%Modifiers.Base [concrete = constants.%Base]
 // CHECK:STDOUT:   %.loc7: %Derived.elem = base_decl %Base.ref, element0 [concrete]
-// CHECK:STDOUT:   %H.decl: %H.type.dba = fn_decl @H.2 [concrete = constants.%H.bce] {
+// CHECK:STDOUT:   %H.decl: %H.type.dba = fn_decl @H.1 [concrete = constants.%H.bce] {
 // CHECK:STDOUT:     %self.patt: %pattern_type.fb9 = binding_pattern self [concrete]
 // CHECK:STDOUT:     %self.param_patt: %pattern_type.fb9 = value_param_pattern %self.patt, call_param0 [concrete]
 // CHECK:STDOUT:   } {
@@ -542,7 +550,7 @@ base class T1(T:! type) {
 // CHECK:STDOUT:
 // CHECK:STDOUT: class @Base [from "modifiers.carbon"] {
 // CHECK:STDOUT:   complete_type_witness = imports.%Modifiers.import_ref.05e
-// CHECK:STDOUT:   vtable_ptr = imports.%Modifiers.import_ref.4e7
+// CHECK:STDOUT:   vtable_ptr = imports.%Modifiers.import_ref.be7
 // CHECK:STDOUT:
 // CHECK:STDOUT: !members:
 // CHECK:STDOUT:   .Self = imports.%Modifiers.import_ref.1f3
@@ -557,9 +565,9 @@ base class T1(T:! type) {
 // CHECK:STDOUT:   @Derived.%H.decl
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
-// CHECK:STDOUT: fn @H.1 [from "modifiers.carbon"];
+// CHECK:STDOUT: impl fn @H.1(%self.param: %Derived);
 // CHECK:STDOUT:
-// CHECK:STDOUT: impl fn @H.2(%self.param: %Derived);
+// CHECK:STDOUT: fn @H.2 [from "modifiers.carbon"];
 // CHECK:STDOUT:
 // CHECK:STDOUT: fn @Use() {
 // CHECK:STDOUT: !entry:
@@ -597,13 +605,13 @@ base class T1(T:! type) {
 // CHECK:STDOUT:   %F.type: type = fn_type @F [concrete]
 // CHECK:STDOUT:   %F: %F.type = struct_value () [concrete]
 // CHECK:STDOUT:   %Base: type = class_type @Base [concrete]
-// CHECK:STDOUT:   %H.type: type = fn_type @H [concrete]
-// CHECK:STDOUT:   %H: %H.type = struct_value () [concrete]
 // CHECK:STDOUT:   %ptr.454: type = ptr_type <vtable> [concrete]
-// CHECK:STDOUT:   %Base.vtable_ptr: ref %ptr.454 = vtable_ptr @Base.vtable [concrete]
 // CHECK:STDOUT:   %struct_type.vptr: type = struct_type {.<vptr>: %ptr.454} [concrete]
 // CHECK:STDOUT:   %complete_type.513: <witness> = complete_type_witness %struct_type.vptr [concrete]
 // CHECK:STDOUT:   %Derived.elem: type = unbound_element_type %Derived, %Base [concrete]
+// CHECK:STDOUT:   %H.type: type = fn_type @H [concrete]
+// CHECK:STDOUT:   %H: %H.type = struct_value () [concrete]
+// CHECK:STDOUT:   %Base.vtable_ptr: ref %ptr.454 = vtable_ptr @Base.vtable [concrete]
 // CHECK:STDOUT:   %Derived.vtable_ptr: ref %ptr.454 = vtable_ptr @Derived.vtable [concrete]
 // CHECK:STDOUT:   %struct_type.base: type = struct_type {.base: %Base} [concrete]
 // CHECK:STDOUT:   %complete_type.0e2: <witness> = complete_type_witness %struct_type.base [concrete]
@@ -619,8 +627,8 @@ base class T1(T:! type) {
 // CHECK:STDOUT:     import Modifiers//default
 // CHECK:STDOUT:   }
 // CHECK:STDOUT:   %Modifiers.Base: type = import_ref Modifiers//default, Base, loaded [concrete = constants.%Base]
+// CHECK:STDOUT:   %Modifiers.import_ref.be7: ref %ptr.454 = import_ref Modifiers//default, loc6_1, loaded [concrete = constants.%Base.vtable_ptr]
 // CHECK:STDOUT:   %Modifiers.import_ref.05e: <witness> = import_ref Modifiers//default, loc6_1, loaded [concrete = constants.%complete_type.513]
-// CHECK:STDOUT:   %Modifiers.import_ref.4e7: ref <witness> = import_ref Modifiers//default, loc6_1, loaded [concrete = constants.%Base.vtable_ptr]
 // CHECK:STDOUT:   %Modifiers.import_ref.1f3 = import_ref Modifiers//default, inst18 [no loc], unloaded
 // CHECK:STDOUT:   %Modifiers.import_ref.2cc = import_ref Modifiers//default, loc5_29, unloaded
 // CHECK:STDOUT: }
@@ -664,7 +672,7 @@ base class T1(T:! type) {
 // CHECK:STDOUT:
 // CHECK:STDOUT: class @Base [from "modifiers.carbon"] {
 // CHECK:STDOUT:   complete_type_witness = imports.%Modifiers.import_ref.05e
-// CHECK:STDOUT:   vtable_ptr = imports.%Modifiers.import_ref.4e7
+// CHECK:STDOUT:   vtable_ptr = imports.%Modifiers.import_ref.be7
 // CHECK:STDOUT:
 // CHECK:STDOUT: !members:
 // CHECK:STDOUT:   .Self = imports.%Modifiers.import_ref.1f3
@@ -691,14 +699,14 @@ base class T1(T:! type) {
 // CHECK:STDOUT:   %empty_tuple.type: type = tuple_type () [concrete]
 // CHECK:STDOUT:   %F: %F.type = struct_value () [concrete]
 // CHECK:STDOUT:   %Base: type = class_type @Base [concrete]
-// CHECK:STDOUT:   %H.type: type = fn_type @H [concrete]
-// CHECK:STDOUT:   %H: %H.type = struct_value () [concrete]
-// CHECK:STDOUT:   %pattern_type.80f: type = pattern_type %Base [concrete]
 // CHECK:STDOUT:   %ptr.454: type = ptr_type <vtable> [concrete]
-// CHECK:STDOUT:   %Base.vtable_ptr: ref %ptr.454 = vtable_ptr @Base.vtable [concrete]
 // CHECK:STDOUT:   %struct_type.vptr: type = struct_type {.<vptr>: %ptr.454} [concrete]
 // CHECK:STDOUT:   %complete_type.513: <witness> = complete_type_witness %struct_type.vptr [concrete]
+// CHECK:STDOUT:   %pattern_type.80f: type = pattern_type %Base [concrete]
 // CHECK:STDOUT:   %empty_struct_type: type = struct_type {} [concrete]
+// CHECK:STDOUT:   %H.type: type = fn_type @H [concrete]
+// CHECK:STDOUT:   %H: %H.type = struct_value () [concrete]
+// CHECK:STDOUT:   %Base.vtable_ptr: ref %ptr.454 = vtable_ptr @Base.vtable [concrete]
 // CHECK:STDOUT:   %Base.val: %Base = struct_value (%Base.vtable_ptr) [concrete]
 // CHECK:STDOUT:   %Destroy.type: type = facet_type <@Destroy> [concrete]
 // CHECK:STDOUT:   %Op.type.bae: type = fn_type @Op.1 [concrete]
@@ -725,8 +733,8 @@ base class T1(T:! type) {
 // CHECK:STDOUT:     import Modifiers//default
 // CHECK:STDOUT:   }
 // CHECK:STDOUT:   %Modifiers.Base: type = import_ref Modifiers//default, Base, loaded [concrete = constants.%Base]
+// CHECK:STDOUT:   %Modifiers.import_ref.be7: ref %ptr.454 = import_ref Modifiers//default, loc6_1, loaded [concrete = constants.%Base.vtable_ptr]
 // CHECK:STDOUT:   %Modifiers.import_ref.05e: <witness> = import_ref Modifiers//default, loc6_1, loaded [concrete = constants.%complete_type.513]
-// CHECK:STDOUT:   %Modifiers.import_ref.4e7: ref <witness> = import_ref Modifiers//default, loc6_1, loaded [concrete = constants.%Base.vtable_ptr]
 // CHECK:STDOUT:   %Modifiers.import_ref.1f3 = import_ref Modifiers//default, inst18 [no loc], unloaded
 // CHECK:STDOUT:   %Modifiers.import_ref.2cc = import_ref Modifiers//default, loc5_29, unloaded
 // CHECK:STDOUT:   %Core.Destroy: type = import_ref Core//prelude/parts/destroy, Destroy, loaded [concrete = constants.%Destroy.type]
@@ -747,7 +755,7 @@ base class T1(T:! type) {
 // CHECK:STDOUT:
 // CHECK:STDOUT: class @Base [from "modifiers.carbon"] {
 // CHECK:STDOUT:   complete_type_witness = imports.%Modifiers.import_ref.05e
-// CHECK:STDOUT:   vtable_ptr = imports.%Modifiers.import_ref.4e7
+// CHECK:STDOUT:   vtable_ptr = imports.%Modifiers.import_ref.be7
 // CHECK:STDOUT:
 // CHECK:STDOUT: !members:
 // CHECK:STDOUT:   .Self = imports.%Modifiers.import_ref.1f3
@@ -767,7 +775,7 @@ base class T1(T:! type) {
 // CHECK:STDOUT:   %v.var: ref %Base = var %v.var_patt
 // CHECK:STDOUT:   %.loc7_28.1: %empty_struct_type = struct_literal ()
 // CHECK:STDOUT:   %.loc7_28.2: ref %ptr.454 = class_element_access %v.var, element0
-// CHECK:STDOUT:   %.loc7_28.3: init %ptr.454 = initialize_from imports.%Modifiers.import_ref.4e7 to %.loc7_28.2 [concrete = constants.%Base.vtable_ptr]
+// CHECK:STDOUT:   %.loc7_28.3: init %ptr.454 = initialize_from imports.%Modifiers.import_ref.be7 to %.loc7_28.2 [concrete = constants.%Base.vtable_ptr]
 // CHECK:STDOUT:   %.loc7_28.4: init %Base = class_init (%.loc7_28.3), %v.var [concrete = constants.%Base.val]
 // CHECK:STDOUT:   %.loc7_3: init %Base = converted %.loc7_28.1, %.loc7_28.4 [concrete = constants.%Base.val]
 // CHECK:STDOUT:   assign %v.var, %.loc7_3
@@ -2727,3 +2735,63 @@ base class T1(T:! type) {
 // CHECK:STDOUT:   %pattern_type.loc5_28 => constants.%pattern_type.7dc
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
+// CHECK:STDOUT: --- vtable_import_unneeded.carbon
+// CHECK:STDOUT:
+// CHECK:STDOUT: constants {
+// CHECK:STDOUT:   %Base: type = class_type @Base [concrete]
+// CHECK:STDOUT:   %ptr: type = ptr_type <vtable> [concrete]
+// CHECK:STDOUT:   %struct_type.vptr: type = struct_type {.<vptr>: %ptr} [concrete]
+// CHECK:STDOUT:   %complete_type: <witness> = complete_type_witness %struct_type.vptr [concrete]
+// CHECK:STDOUT:   %pattern_type: type = pattern_type %Base [concrete]
+// CHECK:STDOUT:   %F.type: type = fn_type @F [concrete]
+// CHECK:STDOUT:   %F: %F.type = struct_value () [concrete]
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: imports {
+// CHECK:STDOUT:   %Core: <namespace> = namespace file.%Core.import, [concrete] {
+// CHECK:STDOUT:     import Core//prelude
+// CHECK:STDOUT:     import Core//prelude/...
+// CHECK:STDOUT:   }
+// CHECK:STDOUT:   %Modifiers: <namespace> = namespace file.%Modifiers.import, [concrete] {
+// CHECK:STDOUT:     .Base = %Modifiers.Base
+// CHECK:STDOUT:     import Modifiers//default
+// CHECK:STDOUT:   }
+// CHECK:STDOUT:   %Modifiers.Base: type = import_ref Modifiers//default, Base, loaded [concrete = constants.%Base]
+// CHECK:STDOUT:   %Modifiers.import_ref.39e92f.2 = import_ref Modifiers//default, loc6_1, unloaded
+// CHECK:STDOUT:   %Modifiers.import_ref.05e: <witness> = import_ref Modifiers//default, loc6_1, loaded [concrete = constants.%complete_type]
+// CHECK:STDOUT:   %Modifiers.import_ref.1f3 = import_ref Modifiers//default, inst18 [no loc], unloaded
+// CHECK:STDOUT:   %Modifiers.import_ref.2cc = import_ref Modifiers//default, loc5_29, unloaded
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: file {
+// CHECK:STDOUT:   package: <namespace> = namespace [concrete] {
+// CHECK:STDOUT:     .Core = imports.%Core
+// CHECK:STDOUT:     .Modifiers = imports.%Modifiers
+// CHECK:STDOUT:     .F = %F.decl
+// CHECK:STDOUT:   }
+// CHECK:STDOUT:   %Core.import = import Core
+// CHECK:STDOUT:   %Modifiers.import = import Modifiers
+// CHECK:STDOUT:   %F.decl: %F.type = fn_decl @F [concrete = constants.%F] {
+// CHECK:STDOUT:     %b.patt: %pattern_type = binding_pattern b [concrete]
+// CHECK:STDOUT:     %b.param_patt: %pattern_type = value_param_pattern %b.patt, call_param0 [concrete]
+// CHECK:STDOUT:   } {
+// CHECK:STDOUT:     %b.param: %Base = value_param call_param0
+// CHECK:STDOUT:     %.loc6: type = splice_block %Base.ref [concrete = constants.%Base] {
+// CHECK:STDOUT:       %Modifiers.ref: <namespace> = name_ref Modifiers, imports.%Modifiers [concrete = imports.%Modifiers]
+// CHECK:STDOUT:       %Base.ref: type = name_ref Base, imports.%Modifiers.Base [concrete = constants.%Base]
+// CHECK:STDOUT:     }
+// CHECK:STDOUT:     %b: %Base = bind_name b, %b.param
+// CHECK:STDOUT:   }
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: class @Base [from "modifiers.carbon"] {
+// CHECK:STDOUT:   complete_type_witness = imports.%Modifiers.import_ref.05e
+// CHECK:STDOUT:   vtable_ptr = imports.%Modifiers.import_ref.39e92f.2
+// CHECK:STDOUT:
+// CHECK:STDOUT: !members:
+// CHECK:STDOUT:   .Self = imports.%Modifiers.import_ref.1f3
+// CHECK:STDOUT:   .H = imports.%Modifiers.import_ref.2cc
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: fn @F(%b.param: %Base);
+// CHECK:STDOUT: