[llvm] [X86][MC] Support decoding of EGPR for APX (PR #72102)

[Phoebe Wang via llvm-commits]

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@@ -474,6 +483,23 @@ static int readPrefixes(struct InternalInstruction *insn) {
                                   insn->vectorExtensionPrefix[1],
                                   insn->vectorExtensionPrefix[2]));
     }
+  } else if (isREX2(insn, byte)) {
+    uint8_t byte1;
+    if (peek(insn, byte1)) {
+      LLVM_DEBUG(dbgs() << "Couldn't read second byte of REX2");
+      return -1;
+    }
+    insn->rex2ExtensionPrefix[0] = byte;
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phoebewang wrote:

Can we directly compare payload with 0? I don't think there's rex2 prefix with all payload 0.

https://github.com/llvm/llvm-project/pull/72102