[llvm] [SandboxVec] Reapply "Add barebones Region class. (#108899)" (PR #109059)

[via llvm-commits]

================
@@ -0,0 +1,106 @@
+//===- Region.h -------------------------------------------------*- C++ -*-===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_TRANSFORMS_VECTORIZE_SANDBOXVECTORIZER_REGION_H
+#define LLVM_TRANSFORMS_VECTORIZE_SANDBOXVECTORIZER_REGION_H
+
+#include "llvm/ADT/SetVector.h"
+#include "llvm/ADT/iterator_range.h"
+#include "llvm/SandboxIR/SandboxIR.h"
+#include "llvm/Support/InstructionCost.h"
+#include "llvm/Support/raw_ostream.h"
+
+namespace llvm::sandboxir {
+
+/// The main job of the Region is to point to new instructions generated by
+/// vectorization passes. It is the unit that RegionPasses operate on with their
+/// runOnRegion() function.
+///
+/// The region allows us to stack transformations horizontally, meaning that
+/// each transformation operates on a single region and the resulting region is
+/// the input to the next transformation, as opposed to vertically, which is the
+/// common way of applying a transformation across the whole BB. This enables us
+/// to check for profitability and decide whether we accept or rollback at a
+/// region granularity, which is much better than doing this at the BB level.
+///
+//  Traditional approach: transformations applied vertically for the whole BB
+//    BB
+//  +----+
+//  |    |
+//  |    |
+//  |    | -> Transform1 ->  ... -> TransformN -> Check Cost
+//  |    |
+//  |    |
+//  +----+
+//
+//  Region-based approach: transformations applied horizontally, for each Region
+//    BB
+//  +----+
+//  |Rgn1| -> Transform1 ->  ... -> TransformN -> Check Cost
+//  |    |
+//  |Rgn2| -> Transform1 ->  ... -> TransformN -> Check Cost
+//  |    |
+//  |Rgn3| -> Transform1 ->  ... -> TransformN -> Check Cost
+//  +----+
+
+class Region {
+  /// All the instructions in the Region. Only new instructions generated during
+  /// vectorization are part of the Region.
+  SetVector<Instruction *> Insts;
+
+  /// A unique ID, used for debugging.
+  unsigned RegionID = 0;
+
+  Context &Ctx;
+
+  /// The basic block containing this region.
+  BasicBlock &BB;
----------------
vporpo wrote:

Let's drop this for now, we can add it in if/when needed.

https://github.com/llvm/llvm-project/pull/109059