[llvm] [SandboxVec][Scheduler] Boilerplate and initial implementation. (PR #112449)

[Sriraman Tallam via llvm-commits]

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+//===- Scheduler.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
+//
+//===----------------------------------------------------------------------===//
+//
+// This is the bottom-up list scheduler used by the vectorizer. It is used for
+// checking the legality of vectorization and for scheduling instructions in
+// such a way that makes vectorization possible, if legal.
+//
+// The legality check is performed by `trySchedule(Instrs)`, which will try to
+// schedule the IR until all instructions in `Instrs` can be scheduled together
+// back-to-back. If this fails then it is illegal to vectorize `Instrs`.
+//
+// Internally the scheduler uses the vectorizer-specific DependencyGraph class.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_TRANSFORMS_VECTORIZE_SANDBOXVECTORIZER_SCHEDULER_H
+#define LLVM_TRANSFORMS_VECTORIZE_SANDBOXVECTORIZER_SCHEDULER_H
+
+#include "llvm/SandboxIR/Instruction.h"
+#include "llvm/Transforms/Vectorize/SandboxVectorizer/DependencyGraph.h"
+#include <queue>
+
+namespace llvm::sandboxir {
+
+class PriorityCmp {
+public:
+  bool operator()(const DGNode *N1, const DGNode *N2) {
+    // TODO: This should be a hierarchical comparator.
+    return N1->getInstruction()->comesBefore(N2->getInstruction());
+  }
+};
+
+/// The list holding nodes that are ready to schedule. Used by the scheduler.
+class ReadyListContainer {
+  PriorityCmp Cmp;
+  /// Control/Other dependencies are not modeled by the DAG to save memory.
+  /// These have to be modeled in the ready list for correctness.
+  /// This means that the list will hold back nodes that need to meet such
+  /// unmodeled dependencies.
+  std::priority_queue<DGNode *, std::vector<DGNode *>, PriorityCmp> List;
+
+public:
+  ReadyListContainer() : List(Cmp) {}
+  void insert(DGNode *N) { List.push(N); }
+  DGNode *pop() {
+    auto *Back = List.top();
+    List.pop();
+    return Back;
+  }
+  bool empty() const { return List.empty(); }
+#ifndef NDEBUG
+  void dump(raw_ostream &OS) const;
+  LLVM_DUMP_METHOD void dump() const;
+#endif // NDEBUG
+};
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tmsri wrote:

ReadyListContainer just reads like a wrapper around priority_queue.  Do you really need it, why not just use the priority_queue?

https://github.com/llvm/llvm-project/pull/112449