[llvm] [CFIInserter] Improve `CSRSavedLocation` struct. (PR #168869)

[Sergei Barannikov via llvm-commits]

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@@ -88,11 +88,60 @@ class CFIInstrInserter : public MachineFunctionPass {
 #define INVALID_REG UINT_MAX
 #define INVALID_OFFSET INT_MAX
   /// contains the location where CSR register is saved.
-  struct CSRSavedLocation {
-    CSRSavedLocation(std::optional<unsigned> R, std::optional<int> O)
-        : Reg(R), Offset(O) {}
-    std::optional<unsigned> Reg;
-    std::optional<int> Offset;
+  class CSRSavedLocation {
+  public:
+    enum Kind { INVALID, REGISTER, CFA_OFFSET };
+    Kind K;
+
+  private:
+    union {
+      // Dwarf register number
+      unsigned Reg;
+      // CFA offset
+      int64_t Offset;
+    } U;
+
+  public:
+    CSRSavedLocation() { K = Kind::INVALID; }
+
+    bool isValid() const { return K != Kind::INVALID; }
+
+    unsigned getRegister() const {
+      assert(K == REGISTER);
+      return U.Reg;
+    }
+
+    void setRegister(unsigned _Reg) {
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s-barannikov wrote:

```suggestion
    void setRegister(unsigned Reg) {
```
`_Reg` is reserved identifier

https://github.com/llvm/llvm-project/pull/168869