ARM: diagnose unpredictable IT instructions

IT instructions are allowed to have the 'AL' predicate, but it must never
result in an 'NV' predicated instruction. Essentially this means that all
branches must be 't' rather than 'e' if the predicate is 'AL'.

This patch adds a diagnostic for this during assembly (error because parsing
hits an assertion if allowed to continue) and an annotation during disassembly.

llvm-svn: 335593

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;