[llvm] [SLP] Make getSameOpcode support interchangeable instructions. (PR #127450)

[Alexey Bataev via llvm-commits]

================
@@ -810,6 +810,285 @@ static std::optional<unsigned> getExtractIndex(Instruction *E) {
 
 namespace {
 
+/// Base class for representing instructions that can be interchanged with other
+/// equivalent forms. For example, multiplication by a power of 2 can be
+/// interchanged with a left shift.
+///
+/// Derived classes implement specific interchange patterns by overriding the
+/// virtual methods to define their interchange logic.
+///
+/// The class maintains a reference to the main instruction (MainOp) and
+/// provides methods to:
+/// - Check if another instruction is interchangeable (isSame)
+/// - Get the opcode for the interchangeable form (getOpcode)
+/// - Get the operands for the interchangeable form (getOperand)
+class InterchangeableInstruction {
+protected:
+  Instruction *const MainOp = nullptr;
+
+public:
+  InterchangeableInstruction(Instruction *MainOp) : MainOp(MainOp) {}
+  virtual bool isSame(Instruction *I) const {
+    return MainOp->getOpcode() == I->getOpcode();
+  }
+  virtual unsigned getOpcode() const { return MainOp->getOpcode(); }
+  virtual SmallVector<Value *> getOperand(Instruction *I) const {
+    assert(MainOp->getOpcode() == I->getOpcode() &&
+           "Cannot convert the instruction.");
+    return SmallVector<Value *>(MainOp->operands());
+  }
+  virtual ~InterchangeableInstruction() = default;
+};
+
+class InterchangeableBinOp final : public InterchangeableInstruction {
+  using MaskType = std::uint_fast8_t;
+  // Sort SupportedOp because it is used by binary_search.
+  constexpr static std::initializer_list<unsigned> SupportedOp = {
+      Instruction::Add,  Instruction::Sub, Instruction::Mul, Instruction::Shl,
+      Instruction::AShr, Instruction::And, Instruction::Or,  Instruction::Xor};
+  enum : MaskType {
+    SHL_BIT = 0b1,
+    AShr_BIT = 0b10,
+    Mul_BIT = 0b100,
+    Add_BIT = 0b1000,
+    Sub_BIT = 0b10000,
+    And_BIT = 0b100000,
+    Or_BIT = 0b1000000,
+    Xor_BIT = 0b10000000,
+  };
+  // The bit it sets represents whether MainOp can be converted to.
+  mutable MaskType Mask = Xor_BIT | Or_BIT | And_BIT | Sub_BIT | Add_BIT |
+                          Mul_BIT | AShr_BIT | SHL_BIT;
+  // We cannot create an interchangeable instruction that does not exist in VL.
+  // For example, VL [x + 0, y * 1] can be converted to [x << 0, y << 0], but
+  // 'shl' does not exist in VL. In the end, we convert VL to [x * 1, y * 1].
+  // SeenBefore is used to know what operations have been seen before.
+  mutable MaskType SeenBefore = 0;
----------------
alexey-bataev wrote:

Avoid using mutable

https://github.com/llvm/llvm-project/pull/127450