[AMDGPU][MC] Disassembler warning for v_cmpx instructions

For GFX10+ the destination reg of v_cmpx instructions and
others 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 0x7E a warning
is issued.

Author: jwanggit86

llvm/include/llvm/MC/MCDisassembler/MCDisassembler.h

@@ -194,6 +194,7 @@ class MCDisassembler {
 
 private:
   MCContext &Ctx;
+  std::string ErrorOrWarningMsg;
 
 protected:
   // Subtarget information, for instruction decoding predicates if required.
@@ -222,6 +223,9 @@ class MCDisassembler {
   // Marked mutable because we cache it inside the disassembler, rather than
   // having to pass it around as an argument through all the autogenerated code.
   mutable raw_ostream *CommentStream = nullptr;
+
+  const std::string& getErrorOrWarningMsg() const { return ErrorOrWarningMsg; }
+  void setErrorOrWarningMsg(const std::string &Msg) { ErrorOrWarningMsg = Msg; }
 };
 
 } // end namespace llvm

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

@@ -663,6 +663,8 @@ 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);
@@ -801,8 +803,17 @@ DecodeStatus AMDGPUDisassembler::getInstruction(MCInst &MI, uint64_t &Size,
   if (ImmLitIdx != -1 && !IsSOPK)
     convertFMAanyK(MI, ImmLitIdx);
 
+  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) {
+      const_cast<AMDGPUDisassembler*>(this)->setErrorOrWarningMsg("invalid vdst encoding");
+      Status = MCDisassembler::SoftFail;
+    }
+  }
+
   Size = MaxInstBytesNum - Bytes.size();
-  return MCDisassembler::Success;
+  return Status;
 }
 
 void AMDGPUDisassembler::convertEXPInst(MCInst &MI) const {