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

[Alexey Bataev via llvm-commits]

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@@ -813,6 +835,300 @@ static std::optional<unsigned> getExtractIndex(const Instruction *E) {
 }
 
 namespace {
+/// \returns true if \p Opcode is allowed as part of the main/alternate
+/// instruction for SLP vectorization.
+///
+/// Example of unsupported opcode is SDIV that can potentially cause UB if the
+/// "shuffled out" lane would result in division by zero.
+bool isValidForAlternation(unsigned Opcode) {
+  return !Instruction::isIntDivRem(Opcode);
+}
+
+/// Helper class that determines VL can use the same opcode.
+/// Alternate instruction is supported. In addition, it supports interchangeable
+/// instruction. An interchangeable instruction is an instruction that can be
+/// converted to another instruction with same semantics. For example, x << 1 is
+/// equal to x * 2. x * 1 is equal to x | 0.
+class BinOpSameOpcodeHelper {
+  using MaskType = std::uint_fast16_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 {
+    ShlBIT = 0b1,
+    AShrBIT = 0b10,
+    MulBIT = 0b100,
+    AddBIT = 0b1000,
+    SubBIT = 0b10000,
+    AndBIT = 0b100000,
+    OrBIT = 0b1000000,
+    XorBIT = 0b10000000,
+    MainOpBIT = 0b100000000,
+    LLVM_MARK_AS_BITMASK_ENUM(MainOpBIT)
+  };
+  // Return a non-nullptr if either operand of I is a ConstantInt.
+  // The second return value represents the operand position. We check the
+  // right-hand side first (1). If the right hand side is not a ConstantInt and
+  // the instruction is neither Sub, Shl, nor AShr, we then check the left hand
+  // side (0).
+  static std::pair<ConstantInt *, unsigned>
+  isBinOpWithConstantInt(Instruction *I) {
+    unsigned Opcode = I->getOpcode();
+    assert(binary_search(SupportedOp, Opcode) && "Unsupported opcode.");
+    auto *BinOp = cast<BinaryOperator>(I);
+    if (auto *CI = dyn_cast<ConstantInt>(BinOp->getOperand(1)))
+      return {CI, 1};
+    if (Opcode == Instruction::Sub || Opcode == Instruction::Shl ||
+        Opcode == Instruction::AShr)
+      return {nullptr, 0};
+    if (auto *CI = dyn_cast<ConstantInt>(BinOp->getOperand(0)))
+      return {CI, 0};
+    return {nullptr, 0};
+  }
+  struct InterchangeableInfo {
+    Instruction *I = nullptr;
+    // The bit it sets represents whether MainOp can be converted to.
+    MaskType Mask = MainOpBIT | XorBIT | OrBIT | AndBIT | SubBIT | AddBIT |
+                    MulBIT | AShrBIT | ShlBIT;
+    // We cannot create an interchangeable instruction that does not exist in
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alexey-bataev wrote:

```suggestion
    /// We cannot create an interchangeable instruction that does not exist in
```

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