[RISCV] Fix disassembling of fence instruction with invalid field

Summary:
Instruction with 0 in fence field being disassembled as fence , iorw.
Printing "unknown" to match GAS behavior.

This bug was uncovered by a LLVM MC Disassembler Protocol Buffer Fuzzer
for the RISC-V assembly language.

Reviewers: asb

Subscribers: rbar, johnrusso, simoncook, sabuasal, niosHD, kito-cheng, shiva0217, zzheng, edward-jones, mgrang, rogfer01, MartinMosbeck, brucehoult, the_o, rkruppe, jfb, PkmX, jocewei, asb

Differential Revision: https://reviews.llvm.org/D51828

llvm-svn: 344309

1 files changed, 4 insertions, 0 deletions

diff --git a/llvm/lib/Target/RISCV/InstPrinter/RISCVInstPrinter.cpp b/llvm/lib/Target/RISCV/InstPrinter/RISCVInstPrinter.cpp
index aa21cf0e6b4..979c8f4e2fa 100644
--- a/llvm/lib/Target/RISCV/InstPrinter/RISCVInstPrinter.cpp
+++ b/llvm/lib/Target/RISCV/InstPrinter/RISCVInstPrinter.cpp
@@ -93,6 +93,8 @@ void RISCVInstPrinter::printFenceArg(const MCInst *MI, unsigned OpNo,
                                      const MCSubtargetInfo &STI,
                                      raw_ostream &O) {
   unsigned FenceArg = MI->getOperand(OpNo).getImm();
+  assert (((FenceArg >> 4) == 0) && "Invalid immediate in printFenceArg");
+
   if ((FenceArg & RISCVFenceField::I) != 0)
     O << 'i';
   if ((FenceArg & RISCVFenceField::O) != 0)
@@ -101,6 +103,8 @@ void RISCVInstPrinter::printFenceArg(const MCInst *MI, unsigned OpNo,
     O << 'r';
   if ((FenceArg & RISCVFenceField::W) != 0)
     O << 'w';
+  if (FenceArg == 0)
+    O << "unknown";
 }
 
 void RISCVInstPrinter::printFRMArg(const MCInst *MI, unsigned OpNo,