2 files changed, 19 insertions, 1 deletions
diff --git a/llvm/lib/Target/ARM/AsmParser/ARMAsmParser.cpp b/llvm/lib/Target/ARM/AsmParser/ARMAsmParser.cpp
index f91397aca22..8a725876b68 100644
--- a/llvm/lib/Target/ARM/AsmParser/ARMAsmParser.cpp
+++ b/llvm/lib/Target/ARM/AsmParser/ARMAsmParser.cpp
@@ -6348,6 +6348,21 @@ bool ARMAsmParser::validateInstruction(MCInst &Inst,
 
   const unsigned Opcode = Inst.getOpcode();
   switch (Opcode) {
+  case ARM::t2IT: {
+    // Encoding is unpredictable if it ever results in a notional 'NV'
+    // predicate. Since we don't parse 'NV' directly this means an 'AL'
+    // predicate with an "else" mask bit.
+    unsigned Cond = Inst.getOperand(0).getImm();
+    unsigned Mask = Inst.getOperand(1).getImm();
+
+    // Mask hasn't been modified to the IT instruction encoding yet so
+    // conditions only allowing a 't' are a block of 1s starting at bit 3
+    // followed by all 0s. Easiest way is to just list the 4 possibilities.
+    if (Cond == ARMCC::AL && Mask != 8 && Mask != 12 && Mask != 14 &&
+        Mask != 15)
+      return Error(Loc, "unpredictable IT predicate sequence");
+    break;
+  }
   case ARM::LDRD:
   case ARM::LDRD_PRE:
   case ARM::LDRD_POST: {
diff --git a/llvm/lib/Target/ARM/Disassembler/ARMDisassembler.cpp b/llvm/lib/Target/ARM/Disassembler/ARMDisassembler.cpp
index 3ffa702462c..f21dce3f3ad 100644
--- a/llvm/lib/Target/ARM/Disassembler/ARMDisassembler.cpp
+++ b/llvm/lib/Target/ARM/Disassembler/ARMDisassembler.cpp
@@ -729,10 +729,13 @@ DecodeStatus ThumbDisassembler::getInstruction(MCInst &MI, uint64_t &Size,
     // code and mask operands so that we can apply them correctly
     // to the subsequent instructions.
     if (MI.getOpcode() == ARM::t2IT) {
-
       unsigned Firstcond = MI.getOperand(0).getImm();
       unsigned Mask = MI.getOperand(1).getImm();
       ITBlock.setITState(Firstcond, Mask);
+
+      // An IT instruction that would give a 'NV' predicate is unpredictable.
+      if (Firstcond == ARMCC::AL && !isPowerOf2_32(Mask))
+        CS << "unpredictable IT predicate sequence";
     }
 
     return Result;