[llvm] [LV][VPlan] Add initial support for CSA vectorization (PR #106560)

[Graham Hunter via llvm-commits]

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@@ -0,0 +1,78 @@
+//===------------- CSADescriptors.h - CSA Descriptors -----------*- 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 file "describes" conditional scalar assignments (CSA).
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/Analysis/LoopInfo.h"
+#include "llvm/IR/Instructions.h"
+#include "llvm/IR/Value.h"
+
+#ifndef LLVM_ANALYSIS_CSADESCRIPTORS_H
+#define LLVM_ANALYSIS_CSADESCRIPTORS_H
+
+namespace llvm {
+
+/// A Conditional Scalar Assignment (CSA) is an assignment from an initial
+/// scalar that may or may not occur.
+class CSADescriptor {
+  /// If the conditional assignment occurs inside a loop, then Phi chooses
+  /// the value of the assignment from the entry block or the loop body block.
+  PHINode *Phi = nullptr;
+
+  /// The initial value of the CSA. If the condition guarding the assignment is
+  /// not met, then the assignment retains this value.
+  Value *InitScalar = nullptr;
+
+  /// The Instruction that conditionally assigned to inside the loop.
+  Instruction *Assignment = nullptr;
+
+  /// Create a CSA Descriptor that models an invalid CSA.
+  CSADescriptor() = default;
+
+  /// Create a CSA Descriptor that models a valid CSA with its members
+  /// initialized correctly.
+  CSADescriptor(PHINode *Phi, Instruction *Assignment, Value *InitScalar)
+      : Phi(Phi), InitScalar(InitScalar), Assignment(Assignment) {}
+
+public:
+  /// If Phi is the root of a CSA, return the CSADescriptor of the CSA rooted by
+  /// Phi. Otherwise, return a CSADescriptor with IsValidCSA set to false.
+  static CSADescriptor isCSAPhi(PHINode *Phi, Loop *TheLoop);
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huntergr-arm wrote:

I think returning a std::optional<CSADescriptor> instead of having valid/invalid descriptors would work better.

https://github.com/llvm/llvm-project/pull/106560