[AMDGPU][MC] Disassembler warning for v_cmpx instructions (llvm#127925)

For GFX10+ the destination reg of v_cmpx instructions is implicitly EXEC,
which is encoded as 0x7E. However, the disassembler does not check this
field, thus allowing any value. With this patch, if the field is not
EXEC a warning is issued.

Author: jwanggit86

llvm/lib/Target/AMDGPU/Disassembler/AMDGPUDisassembler.cpp

@@ -664,6 +664,8 @@ DecodeStatus AMDGPUDisassembler::getInstruction(MCInst &MI, uint64_t &Size,
     return MCDisassembler::Fail;
   } while (false);
 
+  DecodeStatus Status = MCDisassembler::Success;
+
   if (MCII->get(MI.getOpcode()).TSFlags & SIInstrFlags::DPP) {
     if (isMacDPP(MI))
       convertMacDPPInst(MI);
@@ -802,8 +804,18 @@ DecodeStatus AMDGPUDisassembler::getInstruction(MCInst &MI, uint64_t &Size,
   if (ImmLitIdx != -1 && !IsSOPK)
     convertFMAanyK(MI, ImmLitIdx);
 
+  // Some VOPC instructions, e.g., v_cmpx_f_f64, use VOP3 encoding and
+  // have EXEC as implicit destination. Issue a warning if encoding for
+  // vdst is not EXEC.
+  if ((MCII->get(MI.getOpcode()).TSFlags & SIInstrFlags::VOP3) &&
+      MCII->get(MI.getOpcode()).hasImplicitDefOfPhysReg(AMDGPU::EXEC)) {
+    auto ExecEncoding = MRI.getEncodingValue(AMDGPU::EXEC_LO);
+    if (Bytes_[0] != ExecEncoding)
+      Status = MCDisassembler::SoftFail;
+  }
+
   Size = MaxInstBytesNum - Bytes.size();
-  return MCDisassembler::Success;
+  return Status;
 }
 
 void AMDGPUDisassembler::convertEXPInst(MCInst &MI) const {