Factor out CopyOnWriteBlock. (#3785)

Generalize it to also support type blocks.

toolchain/check/convert.cpp

@@ -10,6 +10,7 @@
 #include "common/check.h"
 #include "llvm/ADT/STLExtras.h"
 #include "toolchain/check/context.h"
+#include "toolchain/sem_ir/copy_on_write_block.h"
 #include "toolchain/sem_ir/file.h"
 #include "toolchain/sem_ir/inst.h"
 
@@ -194,47 +195,6 @@ static auto ConvertAggregateElement(
   return Convert(context, node_id, src_elem_id, target);
 }
 
-namespace {
-// A handle to a new block that may be modified, with copy-on-write semantics.
-//
-// The constructor is given the ID of an existing block that provides the
-// initial contents of the new block. The new block is lazily allocated; if no
-// modifications have been made, the `id()` function will return the original
-// block ID.
-//
-// This is intended to avoid an unnecessary block allocation in the case where
-// the new block ends up being exactly the same as the original block.
-class CopyOnWriteBlock {
- public:
-  // Constructs the block. If `source_id` is valid, it is used as the initial
-  // value of the block. Otherwise, uninitialized storage for `size` elements
-  // is allocated.
-  CopyOnWriteBlock(SemIR::File& file, SemIR::InstBlockId source_id, size_t size)
-      : file_(file), source_id_(source_id) {
-    if (!source_id_.is_valid()) {
-      id_ = file_.inst_blocks().AddUninitialized(size);
-    }
-  }
-
-  auto id() const -> SemIR::InstBlockId { return id_; }
-
-  auto Set(int i, SemIR::InstId value) -> void {
-    if (source_id_.is_valid() && file_.inst_blocks().Get(id_)[i] == value) {
-      return;
-    }
-    if (id_ == source_id_) {
-      id_ = file_.inst_blocks().Add(file_.inst_blocks().Get(source_id_));
-    }
-    file_.inst_blocks().Get(id_)[i] = value;
-  }
-
- private:
-  SemIR::File& file_;
-  SemIR::InstBlockId source_id_;
-  SemIR::InstBlockId id_ = source_id_;
-};
-}  // namespace
-
 // Performs a conversion from a tuple to an array type. This function only
 // converts the type, and does not perform a final conversion to the requested
 // expression category.
@@ -370,7 +330,12 @@ static auto ConvertTupleToTuple(Context& context, SemIR::TupleType src_type,
   // Initialize each element of the destination from the corresponding element
   // of the source.
   // TODO: Annotate diagnostics coming from here with the element index.
-  CopyOnWriteBlock new_block(sem_ir, literal_elems_id, src_elem_types.size());
+  auto new_block =
+      literal_elems_id.is_valid()
+          ? SemIR::CopyOnWriteInstBlock(sem_ir, literal_elems_id)
+          : SemIR::CopyOnWriteInstBlock(
+                sem_ir, SemIR::CopyOnWriteInstBlock::UninitializedBlock{
+                            src_elem_types.size()});
   for (auto [i, src_type_id, dest_type_id] :
        llvm::enumerate(src_elem_types, dest_elem_types)) {
     // TODO: This call recurses back into conversion. Switch to an iterative
@@ -463,7 +428,12 @@ static auto ConvertStructToStructOrClass(Context& context,
   // Initialize each element of the destination from the corresponding element
   // of the source.
   // TODO: Annotate diagnostics coming from here with the element index.
-  CopyOnWriteBlock new_block(sem_ir, literal_elems_id, src_elem_fields.size());
+  auto new_block =
+      literal_elems_id.is_valid()
+          ? SemIR::CopyOnWriteInstBlock(sem_ir, literal_elems_id)
+          : SemIR::CopyOnWriteInstBlock(
+                sem_ir, SemIR::CopyOnWriteInstBlock::UninitializedBlock{
+                            src_elem_fields.size()});
   for (auto [i, dest_field_id] : llvm::enumerate(dest_elem_fields)) {
     auto dest_field =
         sem_ir.insts().GetAs<SemIR::StructTypeField>(dest_field_id);

toolchain/sem_ir/BUILD

@@ -82,6 +82,7 @@ cc_library(
     hdrs = [
         "class.h",
         "constant.h",
+        "copy_on_write_block.h",
         "file.h",
         "function.h",
         "impl.h",

toolchain/sem_ir/copy_on_write_block.h

@@ -0,0 +1,69 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+#ifndef CARBON_TOOLCHAIN_SEM_IR_COPY_ON_WRITE_BLOCK_H_
+#define CARBON_TOOLCHAIN_SEM_IR_COPY_ON_WRITE_BLOCK_H_
+
+#include "toolchain/sem_ir/file.h"
+#include "toolchain/sem_ir/ids.h"
+
+namespace Carbon::SemIR {
+
+// A handle to a new block that may be modified, with copy-on-write semantics.
+//
+// The constructor is given the ID of an existing block that provides the
+// initial contents of the new block. The new block is lazily allocated; if no
+// modifications have been made, the `id()` function will return the original
+// block ID.
+//
+// This is intended to avoid an unnecessary block allocation in the case where
+// the new block ends up being exactly the same as the original block.
+template <typename BlockIdType, auto (SemIR::File::*ValueStore)()>
+class CopyOnWriteBlock {
+ public:
+  struct UninitializedBlock {
+    size_t size;
+  };
+
+  // Constructs the block. `source_id` is used as the initial value of the
+  // block.
+  CopyOnWriteBlock(SemIR::File& file, BlockIdType source_id)
+      : file_(file), source_id_(source_id) {}
+
+  // Constructs the block, treating the original block as an uninitialized block
+  // with `size` elements.
+  CopyOnWriteBlock(SemIR::File& file, UninitializedBlock uninit)
+      : file_(file),
+        source_id_(BlockIdType::Invalid),
+        id_(file_.inst_blocks().AddUninitialized(uninit.size)) {}
+
+  // Gets a block ID containing the resulting elements. Note that further
+  // modifications may or may not allocate a new ID, so this should only be
+  // called once all modifications have been performed.
+  auto id() const -> BlockIdType { return id_; }
+
+  // Sets the element at index `i` within the block. Lazily allocates a new
+  // block when the value changes for the first time.
+  auto Set(int i, typename BlockIdType::ElementType value) -> void {
+    if (source_id_.is_valid() && (file_.*ValueStore)().Get(id_)[i] == value) {
+      return;
+    }
+    if (id_ == source_id_) {
+      id_ = (file_.*ValueStore)().Add((file_.*ValueStore)().Get(source_id_));
+    }
+    (file_.*ValueStore)().Get(id_)[i] = value;
+  }
+
+ private:
+  SemIR::File& file_;
+  BlockIdType source_id_;
+  BlockIdType id_ = source_id_;
+};
+
+using CopyOnWriteInstBlock = CopyOnWriteBlock<InstBlockId, &File::inst_blocks>;
+using CopyOnWriteTypeBlock = CopyOnWriteBlock<TypeBlockId, &File::type_blocks>;
+
+}  // namespace Carbon::SemIR
+
+#endif  // CARBON_TOOLCHAIN_SEM_IR_COPY_ON_WRITE_BLOCK_H_