[llvm] [X86][MC] Support encoding of EGPR for APX (PR #71909)

[Shengchen Kan via llvm-commits]

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@@ -139,7 +147,20 @@ class X86OpcodePrefixHelper {
   }
 
   void setR(unsigned Encoding) { R = Encoding >> 3 & 1; }
-  void setR2(unsigned Encoding) { EVEX_R2 = Encoding >> 4 & 1; }
+  void setR2(unsigned Encoding) {
+    R2 = Encoding >> 4 & 1;
+    assert((!R2 || (Kind <= REX2 || Kind == EVEX)) && "invalid setting");
+  }
+  void setX(unsigned Encoding) { X = Encoding >> 3 & 1; }
+  void setX2(unsigned Encoding) {
+    assert((Kind <= REX2 || Kind == EVEX) && "invalid setting");
+    X2 = Encoding >> 4 & 1;
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KanRobert wrote:

Not a mistake. `R2` ,`B2` and `X2` are used to add 16 to the reg number, so the offset is 4. 

https://github.com/llvm/llvm-project/pull/71909