Switch cross references to a ValueStore. (#3414)

As I get ready to add imports, I need an API to add values and this
offers a consistent way of doing so.

toolchain/check/context.cpp

@@ -658,9 +658,9 @@ class TypeCompleter {
   auto BuildCrossReferenceValueRepresentation(SemIR::TypeId type_id,
                                               SemIR::CrossReference xref) const
       -> SemIR::ValueRepresentation {
-    auto xref_inst = context_.sem_ir()
-                         .GetCrossReferenceIR(xref.ir_id)
-                         .insts()
+    auto xref_inst = context_.cross_reference_irs()
+                         .Get(xref.ir_id)
+                         ->insts()
                          .Get(xref.inst_id);
 
     // The canonical description of a type should only have cross-references

toolchain/check/context.h

@@ -300,6 +300,10 @@ class Context {
   auto classes() -> ValueStore<SemIR::ClassId, SemIR::Class>& {
     return sem_ir().classes();
   }
+  auto cross_reference_irs()
+      -> ValueStore<SemIR::CrossReferenceIRId, const SemIR::File*>& {
+    return sem_ir().cross_reference_irs();
+  }
   auto names() -> SemIR::NameStoreWrapper { return sem_ir().names(); }
   auto name_scopes() -> SemIR::NameScopeStore& {
     return sem_ir().name_scopes();

toolchain/sem_ir/file.cpp

@@ -44,10 +44,12 @@ auto TypeInfo::Print(llvm::raw_ostream& out) const -> void {
 File::File(SharedValueStores& value_stores)
     : value_stores_(&value_stores),
       filename_("<builtins>"),
-      // Builtins are always the first IR, even when self-referential.
-      cross_reference_irs_({this}),
       type_blocks_(allocator_),
       inst_blocks_(allocator_) {
+  auto builtins_id = cross_reference_irs_.Add(this);
+  CARBON_CHECK(builtins_id == CrossReferenceIRId::Builtins)
+      << "Builtins must be the first IR, even if self-referential";
+
   // Default entry for InstBlockId::Empty.
   inst_blocks_.AddDefaultValue();
 
@@ -72,13 +74,12 @@ File::File(SharedValueStores& value_stores, std::string filename,
            const File* builtins)
     : value_stores_(&value_stores),
       filename_(std::move(filename)),
-      // Builtins are always the first IR.
-      cross_reference_irs_({builtins}),
       type_blocks_(allocator_),
       inst_blocks_(allocator_) {
   CARBON_CHECK(builtins != nullptr);
-  CARBON_CHECK(builtins->cross_reference_irs_[0] == builtins)
-      << "Not called with builtins!";
+  auto builtins_id = cross_reference_irs_.Add(builtins);
+  CARBON_CHECK(builtins_id == CrossReferenceIRId::Builtins)
+      << "Builtins must be the first IR";
 
   // Default entry for InstBlockId::Empty.
   inst_blocks_.AddDefaultValue();
@@ -482,7 +483,7 @@ auto GetExprCategory(const File& file, InstId inst_id) -> ExprCategory {
 
       case CrossReference::Kind: {
         auto xref = inst.As<CrossReference>();
-        ir = &ir->GetCrossReferenceIR(xref.ir_id);
+        ir = ir->cross_reference_irs().Get(xref.ir_id);
         inst_id = xref.inst_id;
         continue;
       }

toolchain/sem_ir/file.h

@@ -204,11 +204,6 @@ class File : public Printable<File> {
         .getZExtValue();
   }
 
-  // Returns the requested IR.
-  auto GetCrossReferenceIR(CrossReferenceIRId xref_id) const -> const File& {
-    return *cross_reference_irs_[xref_id.index];
-  }
-
   // Marks a type as complete, and sets its value representation.
   auto CompleteType(TypeId object_type_id,
                     ValueRepresentation value_representation) -> void {
@@ -300,6 +295,13 @@ class File : public Printable<File> {
   }
   auto classes() -> ValueStore<ClassId, Class>& { return classes_; }
   auto classes() const -> const ValueStore<ClassId, Class>& { return classes_; }
+  auto cross_reference_irs() -> ValueStore<CrossReferenceIRId, const File*>& {
+    return cross_reference_irs_;
+  }
+  auto cross_reference_irs() const
+      -> const ValueStore<CrossReferenceIRId, const File*>& {
+    return cross_reference_irs_;
+  }
   auto names() const -> NameStoreWrapper {
     return NameStoreWrapper(&identifiers());
   }
@@ -360,7 +362,7 @@ class File : public Printable<File> {
   // Related IRs. There will always be at least 2 entries, the builtin IR (used
   // for references of builtins) followed by the current IR (used for references
   // crossing instruction blocks).
-  llvm::SmallVector<const File*> cross_reference_irs_;
+  ValueStore<CrossReferenceIRId, const File*> cross_reference_irs_;
 
   // Storage for name scopes.
   NameScopeStore name_scopes_;

toolchain/sem_ir/ids.h

@@ -83,6 +83,7 @@ constexpr ClassId ClassId::Invalid = ClassId(ClassId::InvalidIndex);
 // The ID of a cross-referenced IR.
 struct CrossReferenceIRId : public IndexBase,
                             public Printable<CrossReferenceIRId> {
+  static const CrossReferenceIRId Builtins;
   using IndexBase::IndexBase;
   auto Print(llvm::raw_ostream& out) const -> void {
     out << "ir";
@@ -90,6 +91,9 @@ struct CrossReferenceIRId : public IndexBase,
   }
 };
 
+constexpr CrossReferenceIRId CrossReferenceIRId::Builtins =
+    CrossReferenceIRId(0);
+
 // A boolean value.
 struct BoolValue : public IndexBase, public Printable<BoolValue> {
   static const BoolValue False;