[llvm-branch-commits] [flang] [flang] translate derived type array init to attribute if possible (PR #140268)

[via llvm-branch-commits]

================
@@ -0,0 +1,204 @@
+//===-- LLVMInsertChainFolder.cpp -----------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "flang/Optimizer/CodeGen/LLVMInsertChainFolder.h"
+#include "mlir/Dialect/LLVMIR/LLVMAttrs.h"
+#include "mlir/Dialect/LLVMIR/LLVMDialect.h"
+#include "mlir/IR/Builders.h"
+#include "llvm/Support/Debug.h"
+
+#define DEBUG_TYPE "flang-insert-folder"
+
+#include <deque>
+
+namespace {
+// Helper class to construct the attribute elements of an aggregate value being
+// folded without creating a full mlir::Attribute representation for each step
+// of the insert value chain, which would both be expensive in terms of
+// compilation time and memory (since the intermediate Attribute would survive,
+// unused, inside the mlir context).
+class InsertChainBackwardFolder {
+  // Type for the current value of an element of the aggregate value being
+  // constructed by the insert chain.
+  // At any point of the insert chain, the value of an element is either:
+  //  - nullptr: not yet known, the insert has not yet been seen.
+  //  - an mlir::Attribute: the element is fully defined.
+  //  - a nested InsertChainBackwardFolder: the element is itself an aggregate
+  //    and its sub-elements have been partially defined (insert with mutliple
+  //    indices have been seen).
+
+  // The insertion folder assumes backward walk of the insert chain. Once an
+  // element or sub-element has been defined, it is not overriden by new
+  // insertions (last insert wins).
+  using InFlightValue =
+      llvm::PointerUnion<mlir::Attribute, InsertChainBackwardFolder *>;
+
+public:
+  InsertChainBackwardFolder(
+      mlir::Type type, std::deque<InsertChainBackwardFolder> *folderStorage)
+      : values(getNumElements(type), mlir::Attribute{}),
+        folderStorage{folderStorage}, type{type} {}
+
+  /// Push
+  bool pushValue(mlir::Attribute val, llvm::ArrayRef<int64_t> at);
+
+  mlir::Attribute finalize(mlir::Attribute defaultFieldValue);
+
+private:
+  static int64_t getNumElements(mlir::Type type) {
+    if (auto structTy =
+            llvm::dyn_cast_if_present<mlir::LLVM::LLVMStructType>(type))
+      return structTy.getBody().size();
+    if (auto arrayTy =
+            llvm::dyn_cast_if_present<mlir::LLVM::LLVMArrayType>(type))
+      return arrayTy.getNumElements();
+    return 0;
+  }
+
+  static mlir::Type getSubElementType(mlir::Type type, int64_t field) {
+    if (auto arrayTy =
+            llvm::dyn_cast_if_present<mlir::LLVM::LLVMArrayType>(type))
+      return arrayTy.getElementType();
+    if (auto structTy =
+            llvm::dyn_cast_if_present<mlir::LLVM::LLVMStructType>(type))
+      return structTy.getBody()[field];
+    return {};
+  }
----------------
jeanPerier wrote:

That the sub element type could not be retrieved. I changed that to return `nullptr` (which is equivalent to `{}`), and to have the caller directly check the result so that it is clearer. Since mlir::Value, Attribute, Type have a null state, it quite commonly used to indicate failure in helpers (for instance, mlir::Value::getDefiningOp<T> returns a null T if the defining op is not a T).

I however changed the exposed API to return `FailureOr<Attribute>` to make that very clear to external users.

https://github.com/llvm/llvm-project/pull/140268